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[SQUEAKING]

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[RUSTLING]

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[CLICKING]

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MATTHEW VANDER
HEIDEN: OK, hello.

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So last time, we discussed
the pentose phosphate pathway,

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which can serve as this
shunt from glycolysis

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where glucose can be converted
to five-carbon ribulose

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1, 5-bisphosphate
generating NADPH and ribose.

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And this shunt
from glycolysis can

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work with the non-oxidative
pathway, where

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those five-carbon
units can then be

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used by the non-oxidative
pathway to re-enter glycolysis,

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giving cells and ability
to generate NADPH.

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Now, we described that this
non-oxidative pathway can also

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operate in the
reverse direction,

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such that cells can take
products of glycolysis

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and instead use them to
generate ribose for nucleotides

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and avoid the
production of NADPH,

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really giving cells
the flexibility

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to either make NADPH
when they need it,

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make ribose when they
need it, with the ability

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to either operate in
this direction as a shunt

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and allow NADPH to be produced
and material to re-enter

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glycolysis for
further oxidation,

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or to just generate ribose
if they have enough NADPH

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and they simply need that
for nucleotide synthesis.

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And we've spent
some time discussing

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that NADPH is a key
molecule that allows cells

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to do reductive reactions.

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This includes the biosynthesis
of reduced carbon, which

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is, of course, the
way that nature

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stores energy for later use.

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We saw this used as a
way to make reduce carbon

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as carbohydrates
and photosynthesis.

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And of course, it
can also be used

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to generate fatty acids,
which is the most reduced

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form of carbon that
all organisms can

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have to store energy for later.

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And the topic of today is
really to go through the pathway

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that all organisms use to
produce those fatty acids

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and lipids as a way to
store this energy as reduced

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carbon for later.

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Now, you'll notice
I drew up there,

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as an introduction for the
reminder of what we talked

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about last time with the
pentose phosphate pathways,

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really layering
it on top of many

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of the other pathways in
central carbon metabolism

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that we've already discussed.

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And I drew it that way because
I want to show that you already

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know quite a bit.

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If you think back to
the very beginning

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where we had this complex
metabolic pathway chart, well,

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we've already been able to build
quite a complex network of how

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metabolism works.

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And really, at this point,
you know the basics.

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So today's lectures and the
three lectures after it,

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so the four remaining lectures
that we have in this course,

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we're going to cover an
awful lot of material.

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Because, of course, we
have to discuss how all

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of the remaining classes
of biomolecules-- lipids,

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nucleic acids, proteins--

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link into the rest
of metabolism,

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which is an awful lot to cover.

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But what you will see is
that, in understanding

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the basics and the complexity
of what we've already described,

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you actually know
already most of what

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you need to know to understand
these remaining pathways.

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That is nature continues to
repurpose the same relatively

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simple reactions over and over
and over again to really build

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this complex network that
is metabolism, including

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this diversity of
macromolecules.

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Now, of course, there's
a few bits of chemistry

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that we still have to discuss.

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We haven't discussed much in
terms of nitrogen metabolism.

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But most of what you
need to know you've

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already covered in understanding
glycolysis, the TCA cycle,

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pentose phosphate
pathway, et cetera.

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OK.

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So now onto the
topic of the day,

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which is really lipids
and fatty acid synthesis.

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And so, again, I want to
reiterate that organisms

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store energy as reduced carbon.

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Fat is the most reduced
form of carbon to store.

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And so if we're going
to generate fat,

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we need a source of electrons.

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Because if we're going
to reduce carbon,

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something else has
to be oxidized.

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That is those electrons
have to come from somewhere.

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And as hopefully
as clear to you,

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those electrons
will come from NADPH

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because it forms this useful
electron donor for cells.

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Now, it should also hopefully
be clear at this point

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that, if oxidation
of carbon releases

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energy, reduction of
carbon, therefore,

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it needs energy input.

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And so we're also going
to need a bunch of ATP

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if we're going to make fat.

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And so ADP and NADPH are
really the energetic drivers

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of how we're going to
take more oxidized carbon

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and reduce it to
build fatty acids.

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Now, all organisms use
similar pathways to do this.

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But, of course, the sources
of where the NADPH the

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ATP come from can be different
depending on the organism.

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And so we described
photosynthesis

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is this process used by
photosynthetic organisms, where

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they can use the light
reactions of photosynthesis

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to make both ATP and NADPH.

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And we described how
that ATP and NADPH

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could be used to drive
synthesis of glucose

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or other carbohydrates.

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And effectively,
you can also imagine

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that that same ATP and NADPH
from the light reactions

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could also be used to
form the electron donors

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and the ATP needed to
synthesize fats and lipids.

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But of course, as
animals, we also

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know that, if we eat too
much, we also have the ability

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to store excess energy as fat.

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And, therefore, we must also
have sources of NADPH and ATP

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that we can use that
are non-photosynthetic.

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And those, of course,
are the reactions

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that we've already talked about
with glycolysis and the TCA

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cycle, oxidative
phosphorylation as a way

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to make ATP, as well
as NADPH from reactions

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like the oxidative
pentose phosphate

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pathway as a source of NADPH.

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Now, eukaryotes will make
fatty acids in the cytosol.

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And if you think
about why that is,

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it's because mitochondria,
remember, is where

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we did fatty acid oxidation.

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And so fatty acid oxidation
is breaking down fatty acids.

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Fatty acid synthesis--
building fatty acids.

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One set of reactions is
in the cytosol, synthesis.

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One set is in the mitochondria,
breakdown, makes sense.

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Remember, compartmentalized
metabolism

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gives you this ability to
favor different pathways.

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Mitochondria is better
at oxidative reactions.

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Cytosol is going to be better
at reductive reactions.

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And also, this
compartmentalization

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will keep catabolism and
anabolism separate, so another

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example of a point that we've
been making over and over again

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throughout this course.

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Now, if we look up
here at our diagram,

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we're going to make fatty acids
from two carbon acetyl-CoA

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units, OK?

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And so, remember, when we
broke down fatty acids,

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most fatty acids
were even in number.

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That's allowed us to break them
down into acetyl-CoA units.

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Well, we're also going to build
them with two carbon acetyl-CoA

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units.

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And so this also contributes to
why most fatty acids in nature

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have even numbers of carbons.

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Now, you'll also recall
from our past lectures

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that fatty acids are often
esterified to alcohols.

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Those can be things
such as glycerol

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based alcohols, which
allow us to form lipids,

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either triacylglyerides, those
neutral lipids for energy

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storage, or phospholipids
to form membranes.

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And that, of course,
comes from a branch of--

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glycerol, of course, comes from
dihydroxyacetone phosphate,

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a molecule in glycolysis.

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Now, of course, I'm not
going to draw acetyl-CoA now.

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I'll draw it later.

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But remember, it is not reduced.

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And so that's where we're going
to need the electrons, NADPH

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as well as ATP for the
energetic requirements

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in order to take those
two carbon units,

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build the acyl chain,
and ultimate reduce

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them to make fat.

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Now, when we do
this, you're going

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to see that carbon dioxide,
as well as biotin--

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OK, so you might be thinking,
oh, what's biotin used for?

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It's a carboxylation reaction.

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And so CO2 in a carboxylation
reaction involving biotin

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is going to be required
for fatty acid synthesis.

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However, you will also see
that that CO2 carbon is not

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incorporated into
the fatty acyl chain.

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And so you'll see that the CO2
is required, but ultimately

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is added and released,
which is exactly

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the same analogy to what we saw
when we did gluconeogenesis.

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Remember, the pyruvate
carboxylase PEPCK reaction

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in gluconeogenesis,
if you look back,

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a CO2 was added to pyruvate
to make oxaloacetate and then

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release later to generate PEP.

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That helped drive the energetics
of that reaction, no net CO2

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incorporation,
very similar thing

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happening in fatty
acid synthesis.

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Now, in animals, mammals,
much of fatty acid synthesis

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is catalyzed by a giant single
polypeptide enzyme called

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FASN or Fatty Acid
Synthase Complex,

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which is, unlike
other complexes we

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talked about where you might
have different polypeptide

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subunits coming together
to form a complex,

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this is one gigantic
polypeptide that

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is able to catalyze many
of the enzymatic steps

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of fatty acid synthesis.

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Now, a bit of
trivia is that this

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is different from plants
and bacteria, which

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will carry out the exact
same enzymatic reactions.

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However, it, rather
than using a giant fatty

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acid synthase
complex, will have all

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the different
enzymatic activities

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broken up and catalyzed by
different proteins, separately

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encoded polypeptides.

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Now, all organisms will do
this using a protein called

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acyl carrier protein.

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And so acyl carrier
protein, you'll see,

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is really analogous
to acetyl-CoA.

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So it's a way to
add a thioester,

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mark a separate pool, and will
carry the growing fatty acyl

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00:12:28,720 --> 00:12:31,840
chain as we synthesize it, OK?

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So acyl carrier protein is part
of FASN, the big polypeptide

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00:12:42,880 --> 00:12:45,700
FASN in mammals that
catalyze this thing.

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00:12:45,700 --> 00:12:47,560
The acyl carrier
protein is not separate.

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It's actually built in
to the sequence of FASN,

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whereas acyl carrier protein
in plants and bacteria

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is a small 9 kilodalton
protein, all right?

217
00:13:04,720 --> 00:13:07,240
So in mammals, acyl
carrier protein-- part

218
00:13:07,240 --> 00:13:09,820
of the fatty acids
synthase polypeptide

219
00:13:09,820 --> 00:13:11,350
in plants and bacteria.

220
00:13:11,350 --> 00:13:15,520
Acyl carrier protein is a
small 9 kilodalton protein.

221
00:13:15,520 --> 00:13:18,820
And so what acyl
carrier protein looks

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00:13:18,820 --> 00:13:23,740
like is that as, part of
this 9 kilodalton protein

223
00:13:23,740 --> 00:13:31,060
or sequence within FASN,
there is a serine residue.

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Remember, serine has an
alcohol on its side chain.

225
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And that alcohol on its side
chain has a phosphodiester bond

226
00:13:40,270 --> 00:13:45,280
to a phosphopantothenate
group just

227
00:13:45,280 --> 00:14:03,930
like we saw with coenzyme A.

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00:14:03,930 --> 00:14:06,000
So if you look
back in your notes

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00:14:06,000 --> 00:14:10,770
when I drew out coenzyme A, the
business end of the molecule

230
00:14:10,770 --> 00:14:16,020
was right here with
the sulfur on the end

231
00:14:16,020 --> 00:14:19,800
there being the one that
form the phosphodiester bond.

232
00:14:19,800 --> 00:14:24,330
This one, rather than being
linked to a nucleotide,

233
00:14:24,330 --> 00:14:28,410
in this case is linked to a
serine as part of a peptide.

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00:14:28,410 --> 00:14:32,910
And so this is why it's
very, very similar.

235
00:14:32,910 --> 00:14:43,910
So this here is same as end
of coenzyme A. And so really

236
00:14:43,910 --> 00:14:47,720
you can think of ACP,
Acyl Carrier Protein,

237
00:14:47,720 --> 00:14:52,280
and coenzyme A, in some
ways analogous to how

238
00:14:52,280 --> 00:14:55,100
we talked about NAD and NADPH--

239
00:14:55,100 --> 00:14:57,050
same functionality,
in this case,

240
00:14:57,050 --> 00:15:03,800
providing a sulfur to make
these thioester bonds as a way

241
00:15:03,800 --> 00:15:09,620
to activate the acid on
the end of the fatty acid.

242
00:15:09,620 --> 00:15:12,730
One used for synthesis,
one use for breakdown--

243
00:15:12,730 --> 00:15:14,720
acyl carrier protein
used for synthesis.

244
00:15:14,720 --> 00:15:21,290
Coenzyme A, acetyl-CoA is used
for oxidation, whereas in,

245
00:15:21,290 --> 00:15:25,640
remember, NAD in NADP, one
was used to create an NAD,

246
00:15:25,640 --> 00:15:29,750
NADH ratio that favored
oxidative reactions, the other

247
00:15:29,750 --> 00:15:33,680
used to create an NADP,
NADPH ratio that favored

248
00:15:33,680 --> 00:15:41,100
reductive reactions even
though the functionality

249
00:15:41,100 --> 00:15:44,090
of the electron donor, in
the case of NAD and NADP

250
00:15:44,090 --> 00:15:49,880
or, in this case, the carrier
function with the thioester

251
00:15:49,880 --> 00:15:56,630
bond in ACP and coenzyme
A are analogous,

252
00:15:56,630 --> 00:15:59,480
really allows you to mark
different pools to carry out,

253
00:15:59,480 --> 00:16:03,890
in this case, anabolic and
catabolic reactions in cells.

254
00:16:03,890 --> 00:16:09,140
So often, we will abbreviate
this just as we abbreviate CoA

255
00:16:09,140 --> 00:16:12,830
as sort of CoASH or SCoA.

256
00:16:12,830 --> 00:16:17,030
In this case, we're
going to use ACPSH

257
00:16:17,030 --> 00:16:21,570
as an abbreviation for
Acyl Carrier Protein.

258
00:16:21,570 --> 00:16:23,930
However, remember, just
like coenzyme A was

259
00:16:23,930 --> 00:16:26,270
this giant molecule, it kind
of is a little misleading

260
00:16:26,270 --> 00:16:27,440
to write it this way.

261
00:16:27,440 --> 00:16:30,800
Acyl carrier protein is an even
bigger molecule, a 9 kilodalton

262
00:16:30,800 --> 00:16:32,870
peptide in plants and bacteria.

263
00:16:32,870 --> 00:16:36,020
And so, again, misleading
to write it this way

264
00:16:36,020 --> 00:16:39,890
because it's really this
giant group linked to

265
00:16:39,890 --> 00:16:44,270
and carrying the acyl chain.

266
00:16:44,270 --> 00:16:44,770
OK.

267
00:16:48,240 --> 00:16:51,750
Now, the first step in
fatty acid synthesis

268
00:16:51,750 --> 00:16:55,830
is carboxylation of acetyl-CoA.

269
00:16:55,830 --> 00:16:58,890
This is a very important
step of the process.

270
00:16:58,890 --> 00:17:02,820
And it is not catalyzed
by the fatty acid synthase

271
00:17:02,820 --> 00:17:07,770
complex, the fatty acid
synthase protein in mammals.

272
00:17:07,770 --> 00:17:11,460
And so, in all organisms,
it is catalyzed

273
00:17:11,460 --> 00:17:15,390
by a separate enzyme
abbreviated ACC, which stands

274
00:17:15,390 --> 00:17:19,890
for Acetyl-CoA Carboxylase.

275
00:17:24,310 --> 00:17:27,400
So ACC is a pretty
famous enzyme.

276
00:17:27,400 --> 00:17:31,000
It's argued by many to
be the rate limiting

277
00:17:31,000 --> 00:17:34,785
step in fatty acid synthesis.

278
00:17:34,785 --> 00:17:36,160
And it makes sense
because you're

279
00:17:36,160 --> 00:17:38,170
going to see this as we're
doing a carboxylation

280
00:17:38,170 --> 00:17:41,500
reaction, big energetics here.

281
00:17:41,500 --> 00:17:44,110
And that is, obviously,
as we learned

282
00:17:44,110 --> 00:17:45,970
before, the steps
that you regulate

283
00:17:45,970 --> 00:17:51,350
are the ones with the biggest
change in free energy.

284
00:17:51,350 --> 00:18:00,400
So here's our old
friend, acetyl-CoA.

285
00:18:00,400 --> 00:18:08,500
And what ACC carries out
is it carries out the

286
00:18:08,500 --> 00:18:13,420
as biotin found in the
active site, which,

287
00:18:13,420 --> 00:18:16,600
of course, can contain a CO2.

288
00:18:16,600 --> 00:18:20,230
Just as a quick reminder,
how do we put CO2 on biotin

289
00:18:20,230 --> 00:18:22,550
for carboxylic reactions?

290
00:18:22,550 --> 00:18:30,490
And so, remember, CO2 is in
equilibrium with bicarbonate.

291
00:18:30,490 --> 00:18:38,170
That bicarbonate can be
phosphorylated by ATP

292
00:18:38,170 --> 00:18:51,230
to give this phosphobicarbonate.

293
00:18:51,230 --> 00:18:56,630
And then that phosphate
can be released

294
00:18:56,630 --> 00:19:05,450
to add the CO2 onto the active
biotin part of the enzyme.

295
00:19:05,450 --> 00:19:08,210
That then carries
that CO2 and can

296
00:19:08,210 --> 00:19:13,400
be used to transfer
the CO2 to carboxylate,

297
00:19:13,400 --> 00:19:26,240
in this example,
acetyl-CoA to generate

298
00:19:26,240 --> 00:19:30,530
this carboxylated acetyl-CoA
three-carbon molecule, which

299
00:19:30,530 --> 00:19:35,550
is referred to as
malonyl CoA, all right?

300
00:19:35,550 --> 00:19:39,000
So this is exactly
the same mechanism

301
00:19:39,000 --> 00:19:42,720
that we described before
for pyruvate carboxylase

302
00:19:42,720 --> 00:19:45,510
in gluconeogenesis, all right.

303
00:19:45,510 --> 00:19:48,040
So I drew a part of it here.

304
00:19:48,040 --> 00:19:56,840
Remember, if we draw
acetyl-CoA, in the enol form

305
00:19:56,840 --> 00:19:59,000
of the molecule--

306
00:19:59,000 --> 00:20:07,560
here we have our biotin, CO2.

307
00:20:07,560 --> 00:20:14,370
If you look back at your notes,
we drew a mechanism like that.

308
00:20:14,370 --> 00:20:18,330
And that allowed us
to, in that case,

309
00:20:18,330 --> 00:20:21,270
take pyruvate and turn
it into oxaloacetate

310
00:20:21,270 --> 00:20:23,970
and gluconeogenesis by
pyruvate carboxylase.

311
00:20:23,970 --> 00:20:26,970
In this case--
identical reaction,

312
00:20:26,970 --> 00:20:29,040
but this time you're
carboxylating acetyl-CoA

313
00:20:29,040 --> 00:20:31,890
to make malonyl-CoA, right?

314
00:20:31,890 --> 00:20:35,310
And it's malonyl-CoA
that ends up

315
00:20:35,310 --> 00:20:39,510
being the substrate for
fatty acid synthase,

316
00:20:39,510 --> 00:20:44,820
either the single fatty acid
synthase protein in mammals

317
00:20:44,820 --> 00:20:50,070
or the same sets of
individually encoded activities

318
00:20:50,070 --> 00:20:54,120
on different polypeptide
in plants and bacteria.

319
00:20:54,120 --> 00:20:58,510
All right, so let's
draw this out.

320
00:20:58,510 --> 00:21:01,290
So here's fatty acid synthase.

321
00:21:01,290 --> 00:21:05,040
And so fatty acid
synthase has on it

322
00:21:05,040 --> 00:21:08,640
two different acyl
carrier protein sites,

323
00:21:08,640 --> 00:21:11,070
so two different
acyl carrier protein

324
00:21:11,070 --> 00:21:15,360
encoded within the fatty acid
synthase polypeptide, each

325
00:21:15,360 --> 00:21:19,290
of which have attached to them
this phosphopantothenate group

326
00:21:19,290 --> 00:21:21,450
to make it an acyl
carrier protein.

327
00:21:21,450 --> 00:21:23,760
Or there would be two
separate acyl carrier proteins

328
00:21:23,760 --> 00:21:26,550
in plants and bacteria as
part of the complex that

329
00:21:26,550 --> 00:21:28,740
synthesizes fatty acids.

330
00:21:28,740 --> 00:21:37,020
And each of them can
pick up a acyl-CoA.

331
00:21:39,840 --> 00:21:47,130
So here's a malonyl-CoA
and an acetyl-CoA.

332
00:21:47,130 --> 00:21:54,030
Each of those can basically,
onto the acyl carrier protein,

333
00:21:54,030 --> 00:21:57,750
exchange the thioester
bond with the CoA

334
00:21:57,750 --> 00:22:02,835
to be a thioester bond with
the acyl carrier protein.

335
00:22:16,820 --> 00:22:19,640
And so to get this
started, on one site

336
00:22:19,640 --> 00:22:22,190
you'll end up going an
acetyl-CoA, exchange

337
00:22:22,190 --> 00:22:25,070
the thioester bond, such
that you release the CoA.

338
00:22:25,070 --> 00:22:28,010
And now, you have an acetyl ACP.

339
00:22:28,010 --> 00:22:29,240
And on the other side--

340
00:22:29,240 --> 00:22:32,300
a malonyl-CoA exchange
that thioester bond

341
00:22:32,300 --> 00:22:33,530
releasing the CoA.

342
00:22:33,530 --> 00:22:37,130
So you have a malonyl
ACP, all right?

343
00:22:37,130 --> 00:22:39,030
So what does that look like?

344
00:22:39,030 --> 00:22:40,895
Well, it'll look like this.

345
00:23:09,340 --> 00:23:13,660
OK, so here we're going
to have basically--

346
00:23:36,115 --> 00:23:38,740
I'll give you some color, so
you can see what's going on.

347
00:23:38,740 --> 00:23:41,940
But here you have, on an
upper acyl carrier protein

348
00:23:41,940 --> 00:23:46,230
site, the malonyl
group, so malonyl ACP.

349
00:23:46,230 --> 00:23:52,800
And on this lower site, I
put on an acetyl ACP, OK?

350
00:23:52,800 --> 00:23:57,980
So what happens
is you now release

351
00:23:57,980 --> 00:24:07,970
the CO2 that was added by
acetyl-CoA carboxylase.

352
00:24:07,970 --> 00:24:11,720
And that allows forming
carbon bond from here to here,

353
00:24:11,720 --> 00:24:22,930
releasing the ACP
on the lower site

354
00:24:22,930 --> 00:24:54,340
and generating this
longer four-carbon chain

355
00:24:54,340 --> 00:25:02,410
on the upper acyl
carrier protein site.

356
00:25:02,410 --> 00:25:03,670
OK.

357
00:25:03,670 --> 00:25:09,475
Once you have this, now,
four-carbon chain, obviously,

358
00:25:09,475 --> 00:25:11,140
if we're going to
make a fatty acid,

359
00:25:11,140 --> 00:25:14,480
we have to reduce this carbon.

360
00:25:14,480 --> 00:25:17,900
And so that's
exactly what happens.

361
00:25:17,900 --> 00:25:20,560
And so how do we reduce it?

362
00:25:20,560 --> 00:25:23,350
We've now seen this
many, many times.

363
00:25:23,350 --> 00:25:28,180
And so if we use NADPH
as an electron donor,

364
00:25:28,180 --> 00:25:37,960
oxidizing it to NADP+, that
generates this hydride ion

365
00:25:37,960 --> 00:25:39,070
to electron carrier.

366
00:25:44,690 --> 00:26:11,650
And now, we net reduce this
ketone to this alcohol, OK?

367
00:26:11,650 --> 00:26:15,400
Next step is we're
going to remove water

368
00:26:15,400 --> 00:26:20,200
across that bond,
just a dehydration.

369
00:26:56,550 --> 00:26:57,940
All right.

370
00:26:57,940 --> 00:27:04,750
Now, we can further reduce
that carbon-carbon bond.

371
00:27:04,750 --> 00:27:11,620
The electrons, again, come
from NADPH as it's oxidized

372
00:27:11,620 --> 00:27:13,210
to NADP+.

373
00:27:21,090 --> 00:27:41,330
And that leaves us with a
reduced four-carbon fatty

374
00:27:41,330 --> 00:27:44,210
acid, a four zero
fatty acid here

375
00:27:44,210 --> 00:27:50,150
on this, esterified
by this thioester

376
00:27:50,150 --> 00:27:56,570
to ACP, on the upper site
of fatty acid synthase.

377
00:27:56,570 --> 00:27:57,890
OK.

378
00:27:57,890 --> 00:28:01,680
All right, so what happens next?

379
00:28:01,680 --> 00:28:04,400
Well, we can repeat this cycle.

380
00:28:04,400 --> 00:28:13,790
So, now, we can have basically
this S-ACP on the lower site

381
00:28:13,790 --> 00:28:33,030
pick up a malonyl-CoA,
release the CoA.

382
00:28:33,030 --> 00:28:39,140
And then we have, down
here, another malonyl-CoA.

383
00:29:04,840 --> 00:29:11,590
Now, they're esterified to
the ACP on the lower side.

384
00:29:11,590 --> 00:29:17,510
All right, now, we can run this
exact same series of reactions

385
00:29:17,510 --> 00:29:18,010
again.

386
00:29:34,560 --> 00:29:35,060
OK.

387
00:29:42,550 --> 00:29:57,970
That releases CO2, also releases
a free S-ACP on the upper site.

388
00:29:57,970 --> 00:30:01,675
And now, we are left with this.

389
00:30:37,310 --> 00:30:41,600
This now-- six-carbon
molecule with a thioester

390
00:30:41,600 --> 00:30:46,160
now on the ACP on
the lower side.

391
00:30:46,160 --> 00:30:48,020
Hopefully, you're
getting the point.

392
00:30:48,020 --> 00:30:56,090
I can now take this through
the same cycles I took before.

393
00:30:56,090 --> 00:30:59,030
But first, we're
going to use NADPH

394
00:30:59,030 --> 00:31:06,590
to reduce this ketone to the
alcohol, the dehydration,

395
00:31:06,590 --> 00:31:13,980
use NADPH again to reduce the
carbon-carbon double bond.

396
00:31:13,980 --> 00:31:44,760
And we are left with now
this S-APP on the lower site.

397
00:31:44,760 --> 00:31:52,455
I can now run this
cycle five times.

398
00:31:52,455 --> 00:31:56,620
We'll add another malonyl-CoA,
this time to the upper site,

399
00:31:56,620 --> 00:31:57,400
combine them.

400
00:31:57,400 --> 00:32:00,310
Now, you have this growing
chain on the upper site,

401
00:32:00,310 --> 00:32:04,340
reduction, another
malonyl-CoA to the lower site,

402
00:32:04,340 --> 00:32:05,480
blah, blah, blah.

403
00:32:05,480 --> 00:32:06,710
Run it five times.

404
00:32:06,710 --> 00:32:14,540
Eventually, I'm going to end up
with a 16, 0, fatty acyl ACP.

405
00:32:14,540 --> 00:32:23,550
And then I can take that,
exchange it back for a CoA.

406
00:32:23,550 --> 00:32:31,320
And I end up with a 16
carbon fatty acyl-CoA

407
00:32:31,320 --> 00:32:38,230
or palmitoyl-CoA CoA molecule,
such that all of the carbons

408
00:32:38,230 --> 00:32:43,330
added came from
acetyl-CoA except for one.

409
00:32:43,330 --> 00:32:46,720
All of that acetyl-CoA
was added by, first,

410
00:32:46,720 --> 00:32:50,110
adding a CO2 via
acetyl-CoA carboxylase,

411
00:32:50,110 --> 00:32:54,340
generating a malonyl-CoA
that's spending ATP

412
00:32:54,340 --> 00:32:55,990
to put the carboxyl group on.

413
00:32:55,990 --> 00:32:58,120
Because, of course, we
need it to spend an ATP

414
00:32:58,120 --> 00:33:01,070
in the acetyl-CoA reaction.

415
00:33:01,070 --> 00:33:05,290
And so seven cycles
were needed to generate

416
00:33:05,290 --> 00:33:07,870
this 16, 0 fatty acyl-CoA.

417
00:33:07,870 --> 00:33:13,090
And so those seven cycles
were seven ATP molecules.

418
00:33:13,090 --> 00:33:19,690
I also needed 2 times 7,
or 14, NADPH molecules.

419
00:33:22,260 --> 00:33:27,530
And that allowed me to
run this cycle 17 times

420
00:33:27,530 --> 00:33:32,360
and make a fully reduced
16-carbon fatty acid.

421
00:33:32,360 --> 00:33:36,880
Now, it turns out that
all of this activity

422
00:33:36,880 --> 00:33:42,220
stops on the fatty acid
synthase molecule when it gets

423
00:33:42,220 --> 00:33:45,010
to a 16-carbon palmitoyl-CoA.

424
00:33:45,010 --> 00:33:47,020
But of course, as
we described before,

425
00:33:47,020 --> 00:33:53,900
organisms have longer fatty
acids than 16 carbons.

426
00:33:53,900 --> 00:34:04,360
And so 18-carbon and
longer fatty acids

427
00:34:04,360 --> 00:34:10,000
are made in exactly the same
chemistry, same reactions

428
00:34:10,000 --> 00:34:11,650
that we just described.

429
00:34:11,650 --> 00:34:14,020
The only difference
is that they are not

430
00:34:14,020 --> 00:34:17,570
made on the fatty
acid synthase complex.

431
00:34:17,570 --> 00:34:19,699
So they're made in a
different location.

432
00:34:19,699 --> 00:34:25,280
And so they're made
at the ER membrane

433
00:34:25,280 --> 00:34:31,760
in eukaryotes, all right,
so the ER in eukaryotes.

434
00:34:36,800 --> 00:34:38,330
Even though the
enzymes themselves

435
00:34:38,330 --> 00:34:42,080
carry out the same reaction,
the enzymes that carry out this

436
00:34:42,080 --> 00:34:52,350
are encoded by
different polypeptides,

437
00:34:52,350 --> 00:34:55,949
so-called fatty acid
elongase enzymes.

438
00:34:55,949 --> 00:34:57,660
Although the chemistry
is exactly what

439
00:34:57,660 --> 00:35:00,300
we already described.

440
00:35:00,300 --> 00:35:04,620
But those enzymes
act on the thioester

441
00:35:04,620 --> 00:35:12,510
with the CoA, not the thioester
with the acyl carrier protein.

442
00:35:12,510 --> 00:35:14,140
But it's still the same.

443
00:35:14,140 --> 00:35:19,950
There's still one
ATP per acetylcholine

444
00:35:19,950 --> 00:35:23,502
for two carbon unit
added because that

445
00:35:23,502 --> 00:35:24,960
is needed to
carboxylate it to make

446
00:35:24,960 --> 00:35:28,770
the malonyl-CoA because that's
what's driving the addition

447
00:35:28,770 --> 00:35:37,150
and two NADPHs per
two-carbon unit

448
00:35:37,150 --> 00:35:43,600
added to carry out the
reduction to take that carbonyl

449
00:35:43,600 --> 00:35:46,300
from the acetyl CoA or
the malonyl-CoA that's

450
00:35:46,300 --> 00:35:51,100
added as a two-carbon unit and
reduce it to the fully reduced

451
00:35:51,100 --> 00:35:52,610
carbon.

452
00:35:52,610 --> 00:35:53,720
OK.

453
00:35:53,720 --> 00:35:58,650
And so that is how you
make saturated fatty acids.

454
00:35:58,650 --> 00:36:01,160
So what about
unsaturated fatty acids?

455
00:36:01,160 --> 00:36:05,330
Well, it is not that
you just stop and don't

456
00:36:05,330 --> 00:36:06,710
make it fully saturated.

457
00:36:06,710 --> 00:36:10,550
Nature, first, makes a
fully saturated fatty acid.

458
00:36:10,550 --> 00:36:13,940
And then it comes back and
reoxidizes the fully saturated

459
00:36:13,940 --> 00:36:20,180
fatty acid to introduce double
bonds in the right positions.

460
00:36:20,180 --> 00:36:40,050
And so for unsaturated, so you
start with a saturated fatty

461
00:36:40,050 --> 00:36:44,310
acid of the desired length.

462
00:36:48,530 --> 00:36:50,870
So you make that first.

463
00:36:50,870 --> 00:37:00,810
And then you use so-called
desaturase enzymes

464
00:37:00,810 --> 00:37:19,830
to introduce double bonds at
the desired location, all right?

465
00:37:19,830 --> 00:37:22,100
So it's a little
counterintuitive,

466
00:37:22,100 --> 00:37:25,430
but this is effectively
why you end up

467
00:37:25,430 --> 00:37:28,040
with double bonds at
the stereotyped places.

468
00:37:28,040 --> 00:37:30,590
So, remember, the
delta 9 position

469
00:37:30,590 --> 00:37:32,990
is the first place,
when we describe

470
00:37:32,990 --> 00:37:36,050
the nomenclature of fatty
acids, where we always

471
00:37:36,050 --> 00:37:39,270
put the first double bond.

472
00:37:39,270 --> 00:37:40,930
Why does nature do it this way?

473
00:37:40,930 --> 00:37:42,630
Well, it's very
hard to say that,

474
00:37:42,630 --> 00:37:45,300
but you can imagine
evolution of how you would

475
00:37:45,300 --> 00:37:47,070
get these enzyme activities.

476
00:37:47,070 --> 00:37:50,280
It's hard to evolve a fatty
acid synthase complex that

477
00:37:50,280 --> 00:37:56,430
would stop doing the
reduction reaction only

478
00:37:56,430 --> 00:37:57,810
at specific locations.

479
00:37:57,810 --> 00:38:00,810
And so you probably
first generate

480
00:38:00,810 --> 00:38:04,320
these fatty acids fully
saturated because that's

481
00:38:04,320 --> 00:38:07,230
the way an enzyme could
do it and then, later,

482
00:38:07,230 --> 00:38:09,030
have a different enzyme
that can pick out

483
00:38:09,030 --> 00:38:11,610
a location on a
saturated fatty acid

484
00:38:11,610 --> 00:38:14,760
to introduce a double bond.

485
00:38:14,760 --> 00:38:17,580
All right, so how can we
introduce a double bond?

486
00:38:17,580 --> 00:38:20,440
Well, that's an
oxidation reaction.

487
00:38:20,440 --> 00:38:24,420
And so if we're going to
do an oxidation reaction,

488
00:38:24,420 --> 00:38:27,840
we need a place for the
electrons that we move.

489
00:38:27,840 --> 00:38:31,890
So if we oxidize the fatty
acid, that's removing electrons.

490
00:38:31,890 --> 00:38:34,230
Those electrons have
to go somewhere.

491
00:38:34,230 --> 00:38:38,190
You might imagine, if we're
oxidizing a carbon-carbon bond,

492
00:38:38,190 --> 00:38:41,790
we've seen that
reaction before when

493
00:38:41,790 --> 00:38:44,370
we did succinate to fumarate.

494
00:38:44,370 --> 00:38:47,610
Succinate dehydrogenase, that
was an oxidation reaction.

495
00:38:47,610 --> 00:38:48,960
We used FAD.

496
00:38:48,960 --> 00:38:52,770
We saw it with the
fatty acid breakdown

497
00:38:52,770 --> 00:38:55,370
when we first put
the double bond in.

498
00:38:55,370 --> 00:38:57,120
We used FAD.

499
00:38:57,120 --> 00:39:01,600
And so, indeed, FAD is
used in this reaction,

500
00:39:01,600 --> 00:39:05,460
but it does not work
like other FAD reactions

501
00:39:05,460 --> 00:39:08,850
to carry out this oxidation.

502
00:39:08,850 --> 00:39:15,400
For whatever reason, the way
desaturases work is different.

503
00:39:15,400 --> 00:39:16,210
OK.

504
00:39:16,210 --> 00:39:22,120
So these desaturase
reactions use oxygen

505
00:39:22,120 --> 00:39:27,190
as a final electron acceptor.

506
00:39:27,190 --> 00:39:37,090
And they work via a
mini-electron transport chain

507
00:39:37,090 --> 00:39:41,000
in the ER in eukaryotes OK.

508
00:39:41,000 --> 00:39:44,420
So it's a little bit of a
different weird mechanism.

509
00:39:44,420 --> 00:39:53,200
But if we think here's
our carbon-carbon bond

510
00:39:53,200 --> 00:39:56,110
that we're going to oxidize
to introduce a double bond

511
00:39:56,110 --> 00:40:01,330
and desaturate our
lipid, so what is that?

512
00:40:01,330 --> 00:40:05,720
Well, it's basically
this reaction.

513
00:40:05,720 --> 00:40:06,220
OK.

514
00:40:06,220 --> 00:40:12,810
So this is, rather than just
remove this as a hydride ion,

515
00:40:12,810 --> 00:40:15,540
it turns out that
those electrons

516
00:40:15,540 --> 00:40:22,620
go to oxygen, which, of
course, will generate one water

517
00:40:22,620 --> 00:40:23,550
molecule.

518
00:40:23,550 --> 00:40:25,380
But there's two
oxygen there, so you

519
00:40:25,380 --> 00:40:27,750
need to generate a
second water molecule.

520
00:40:27,750 --> 00:40:30,340
Those electrons have to
come from somewhere else.

521
00:40:30,340 --> 00:40:36,090
And those electrons come from
this little mini-electron

522
00:40:36,090 --> 00:40:37,210
transport chain.

523
00:40:39,760 --> 00:40:44,400
So if we oxidize iron,
we can get electrons

524
00:40:44,400 --> 00:40:45,940
from that reaction.

525
00:40:45,940 --> 00:40:59,030
And so this ends up generating
a double bond in the fatty acid

526
00:40:59,030 --> 00:41:02,300
with two of the electrons going
to oxygen and the other two

527
00:41:02,300 --> 00:41:05,810
electrons coming from this
mini-electron transport chain

528
00:41:05,810 --> 00:41:10,760
in this desaturase complex
in the endoplasmic reticulum,

529
00:41:10,760 --> 00:41:11,300
all right?

530
00:41:11,300 --> 00:41:12,950
And so, of course,
those electrons

531
00:41:12,950 --> 00:41:15,900
have to come from somewhere.

532
00:41:15,900 --> 00:41:18,290
So we have to re-reduce.

533
00:41:18,290 --> 00:41:21,540
This iron we oxidize to
get the electrons from.

534
00:41:21,540 --> 00:41:26,510
And so this works via
an electron transport

535
00:41:26,510 --> 00:41:28,490
chain similar to
what we saw before,

536
00:41:28,490 --> 00:41:32,480
where it's this whole series
of oxidation and reduction

537
00:41:32,480 --> 00:41:39,230
reactions that does happen
to involve FAD and FADH2.

538
00:41:39,230 --> 00:41:43,220
But ultimately, the electrons
are coming from NADH.

539
00:41:46,710 --> 00:41:51,000
So oxidation of
NADH ultimately is

540
00:41:51,000 --> 00:41:55,200
providing electrons that
together with oxidation here

541
00:41:55,200 --> 00:41:59,350
lead to the reduction
of oxygen to water.

542
00:41:59,350 --> 00:42:02,790
And you can, of course,
read more about this

543
00:42:02,790 --> 00:42:05,310
if you're interested
in it, but it's

544
00:42:05,310 --> 00:42:08,250
important to point out that
this process works by a slightly

545
00:42:08,250 --> 00:42:14,490
different mechanism than
you might predict from sort

546
00:42:14,490 --> 00:42:17,580
of the general principles of
the way most double bonds are

547
00:42:17,580 --> 00:42:22,320
introduced in carbon-carbon
molecules in metabolism.

548
00:42:22,320 --> 00:42:24,630
Why it works this
way is something

549
00:42:24,630 --> 00:42:27,720
we can only speculate about.

550
00:42:27,720 --> 00:42:29,400
OK.

551
00:42:29,400 --> 00:42:41,900
Now, mammals only use
a disaccharase enzyme

552
00:42:41,900 --> 00:42:45,830
that, in a fully
saturated fatty acid,

553
00:42:45,830 --> 00:42:57,100
can only introduce a double
bond at the delta 9 position.

554
00:42:57,100 --> 00:43:06,335
That is between carbons 9 and
10 of a fully saturated fatty

555
00:43:06,335 --> 00:43:06,835
acid.

556
00:43:11,180 --> 00:43:14,450
And that's because they only
have an enzyme complex that

557
00:43:14,450 --> 00:43:15,120
does that.

558
00:43:15,120 --> 00:43:21,050
So that is they can take an 18,
0 newly synthesized fatty acid

559
00:43:21,050 --> 00:43:22,940
and they can make an 18, 1--

560
00:43:22,940 --> 00:43:28,980
or we can make an 18, 1
delta 9 fatty acid, OK?

561
00:43:28,980 --> 00:43:31,170
That's the only
desaturase enzyme

562
00:43:31,170 --> 00:43:34,410
we have to put into a
saturated fatty acid.

563
00:43:34,410 --> 00:43:39,330
That is to make completely
a de novo unsaturated fatty

564
00:43:39,330 --> 00:43:42,810
acid, which is one
of the reasons why

565
00:43:42,810 --> 00:43:45,940
we don't have a lot of
polyunsaturated fatty acids.

566
00:43:45,940 --> 00:43:48,840
Now, we do have enzyme
complexes that work similarly

567
00:43:48,840 --> 00:43:52,290
to these that can add double
bonds at other locations,

568
00:43:52,290 --> 00:43:55,260
but they have to start having
a double bond already present

569
00:43:55,260 --> 00:43:56,850
in those locations.

570
00:43:56,850 --> 00:43:59,220
Many of those lipids have
to come from the diet.

571
00:43:59,220 --> 00:44:02,160
And that's that concept of
essential lipids, something

572
00:44:02,160 --> 00:44:04,740
that we have to eat from a plant
or bacteria that already has

573
00:44:04,740 --> 00:44:06,270
a double bond there
in order to add

574
00:44:06,270 --> 00:44:10,105
double bonds to make some of
our other polyunsaturated fatty

575
00:44:10,105 --> 00:44:10,605
acids.

576
00:44:13,390 --> 00:44:18,460
So that's how fatty
acids are made.

577
00:44:18,460 --> 00:44:22,030
But as I alluded to
earlier, all of this

578
00:44:22,030 --> 00:44:25,810
is taking place in the cytosol.

579
00:44:25,810 --> 00:44:30,760
And if we're doing all of
this work in the cytosol,

580
00:44:30,760 --> 00:44:32,920
maybe you notice
that we actually

581
00:44:32,920 --> 00:44:38,320
have another problem
that nature has to solve.

582
00:44:38,320 --> 00:44:43,590
And that is, because we're
doing all this from acetyl-CoA,

583
00:44:43,590 --> 00:44:46,560
and so we have to have
acetyl-CoA in the cytosol

584
00:44:46,560 --> 00:44:48,150
to have this work.

585
00:44:48,150 --> 00:44:50,470
But acetyl-CoA, all
the ways we've talked

586
00:44:50,470 --> 00:44:52,830
or most of the ways we've
talked about producing it,

587
00:44:52,830 --> 00:44:54,690
happen in the mitochondria.

588
00:44:54,690 --> 00:44:56,430
Remember, acetyl-CoA
can't get across

589
00:44:56,430 --> 00:44:57,560
the mitochondrial membrane.

590
00:44:57,560 --> 00:44:59,160
So we need a way
to get acetyl-CoA

591
00:44:59,160 --> 00:45:02,370
out of the mitochondria into
the cytosol for this whole thing

592
00:45:02,370 --> 00:45:03,390
to work.

593
00:45:03,390 --> 00:45:05,190
Put another way--
so if we draw here,

594
00:45:05,190 --> 00:45:06,900
here's glucose to pyruvate.

595
00:45:06,900 --> 00:45:09,070
That's glycolysis.

596
00:45:09,070 --> 00:45:17,200
Remember to turn that
pyruvate into acetyl-CoA.

597
00:45:17,200 --> 00:45:21,100
We had to move the pyruvate
into the mitochondria.

598
00:45:21,100 --> 00:45:27,520
That's where the PDH complex
was to generate the acetyl-CoA.

599
00:45:27,520 --> 00:45:33,820
But if we need acetyl-CoA in the
cytosol for fatty acid synthase

600
00:45:33,820 --> 00:45:40,530
to make fatty
acids, we basically

601
00:45:40,530 --> 00:45:43,680
need a way to get that
acetyl-CoA back out

602
00:45:43,680 --> 00:45:47,510
of the mitochondria in order
to generate fatty acids.

603
00:45:47,510 --> 00:45:50,670
Other big source
of, remember, was

604
00:45:50,670 --> 00:45:54,690
we start with a fatty
acid in the mitochondria,

605
00:45:54,690 --> 00:45:59,250
run fatty acid oxidation.

606
00:45:59,250 --> 00:46:01,860
Again, fatty acid oxidation
in the mitochondria

607
00:46:01,860 --> 00:46:03,690
generates acetyl-CoA
in the mitochondria.

608
00:46:03,690 --> 00:46:05,730
We're going to use it
to rebuild a fatty acid,

609
00:46:05,730 --> 00:46:10,070
need to get it out of the
mitochondria into the cytosol.

610
00:46:10,070 --> 00:46:14,360
Coenzyme A is not
membrane permeable.

611
00:46:14,360 --> 00:46:17,150
Remember, we had shuttles
to get fatty acids

612
00:46:17,150 --> 00:46:19,250
in the mitochondria.

613
00:46:19,250 --> 00:46:22,280
We did the pyruvate
dehydrogenase reaction here

614
00:46:22,280 --> 00:46:23,340
to begin with.

615
00:46:23,340 --> 00:46:28,660
And so it's a problem
to get this giant CoA,

616
00:46:28,660 --> 00:46:31,940
the CoA from the acetyl-CoA,
out of the mitochondria

617
00:46:31,940 --> 00:46:32,765
into the cytosol.

618
00:46:36,290 --> 00:46:37,130
OK.

619
00:46:37,130 --> 00:46:42,680
So one solution to this,
to getting acetyl CoA

620
00:46:42,680 --> 00:46:47,600
in the cytosol, is simply
to start with acetate, OK?

621
00:46:47,600 --> 00:46:50,960
That is take the CoA
group off and just have

622
00:46:50,960 --> 00:46:53,030
an acetate molecule.

623
00:46:53,030 --> 00:46:57,500
And of course, acetate or
acetic acid, well, that's food.

624
00:46:57,500 --> 00:46:58,490
That's vinegar.

625
00:46:58,490 --> 00:47:02,480
All right, the salad you ate
has acetate in it, acetyl-CoA.

626
00:47:02,480 --> 00:47:03,890
It can be in the cytosol.

627
00:47:03,890 --> 00:47:06,740
We can make acetate if you
look back at your notes

628
00:47:06,740 --> 00:47:08,850
about how we
metabolized alcohol.

629
00:47:08,850 --> 00:47:11,990
So alcohol gets
metabolized to acetate.

630
00:47:11,990 --> 00:47:15,770
And so that acetate
in the cytosol,

631
00:47:15,770 --> 00:47:19,880
we can just add a CoA
group to it in the cytosol.

632
00:47:19,880 --> 00:47:20,960
And how do we do that?

633
00:47:20,960 --> 00:47:23,270
Well, we saw the reaction
to do this already.

634
00:47:31,990 --> 00:47:32,490
OK.

635
00:47:32,490 --> 00:47:40,320
And this is basically when
we added any fatty acid

636
00:47:40,320 --> 00:47:42,000
to make a fatty acyl-CoA.

637
00:47:42,000 --> 00:47:45,150
Remember, the free fatty acid
turned into a fatty acyl-CoA.

638
00:47:45,150 --> 00:47:48,120
We add this reaction
where we used ATP.

639
00:47:48,120 --> 00:47:51,600
We added the AMP to it.

640
00:47:51,600 --> 00:47:53,460
And then that
[INAUDIBLE] pyrophosphate

641
00:47:53,460 --> 00:47:56,040
can drive it forward with the
two pyrophosphate molecules

642
00:47:56,040 --> 00:47:58,470
and take the AMP off
and adds a CoA group.

643
00:47:58,470 --> 00:47:59,970
If you look back
in your notes, it's

644
00:47:59,970 --> 00:48:03,240
the identical reaction to
how we made a fatty acyl-CoA.

645
00:48:03,240 --> 00:48:06,720
Well, there's also an enzyme
that can act on acetate

646
00:48:06,720 --> 00:48:09,840
and do this to make acetyl-CoA.

647
00:48:09,840 --> 00:48:10,680
And that's great.

648
00:48:10,680 --> 00:48:13,590
It's a way to make
acetyl-CoA in the cytosol

649
00:48:13,590 --> 00:48:16,210
if you start with acetate.

650
00:48:16,210 --> 00:48:20,190
However, this is
not the way it works

651
00:48:20,190 --> 00:48:26,460
if you already have an
acetyl-CoA in the mitochondria.

652
00:48:26,460 --> 00:48:31,080
And so if in the
mitochondria you already have

653
00:48:31,080 --> 00:48:35,990
an acetyl-CoA--

654
00:48:35,990 --> 00:48:45,600
so this can come from
pyruvate via the PDH reaction.

655
00:48:45,600 --> 00:48:50,640
It can come from fatty acid
oxidation, this acetyl-CoA

656
00:48:50,640 --> 00:48:52,770
in the mitochondria, all right?

657
00:48:52,770 --> 00:48:57,150
Well, what do we talk about
doing with it in the TCA cycle?

658
00:48:57,150 --> 00:49:00,390
Well, the TCA cycle, we
can use citrate synthase

659
00:49:00,390 --> 00:49:05,610
to generate this
citrate molecule.

660
00:49:05,610 --> 00:49:10,860
And it turns out citrate
itself can be used as a carrier

661
00:49:10,860 --> 00:49:19,650
to export the acetyl-CoA from
the mitochondria to the side

662
00:49:19,650 --> 00:49:24,780
cytosol, such that now, when
you have citrate in the side

663
00:49:24,780 --> 00:49:34,830
cytosol, that citrate can now be
used where you basically carry

664
00:49:34,830 --> 00:49:38,850
out the opposite of the
citrate synthase reaction

665
00:49:38,850 --> 00:49:45,800
to regenerate acetyl-CoA
and oxaloacetate.

666
00:49:45,800 --> 00:49:47,990
Well, if one direction
is favorable,

667
00:49:47,990 --> 00:49:50,400
the other direction is
going to be not favorable.

668
00:49:50,400 --> 00:49:53,420
And so you need energy input
in one of the directions.

669
00:49:53,420 --> 00:49:57,725
And so this costs you ATP.

670
00:50:01,650 --> 00:50:03,750
And this is carried
out by an enzyme

671
00:50:03,750 --> 00:50:13,050
called ATP citrate lyase,
often abbreviated ACLY.

672
00:50:17,010 --> 00:50:21,330
So ATP citrate lyase allows
you to basically reverse

673
00:50:21,330 --> 00:50:23,730
the citrate synthase
reaction, such

674
00:50:23,730 --> 00:50:28,140
that you can shuttle acetyl-CoA
out of the mitochondria

675
00:50:28,140 --> 00:50:32,760
to the cytosol using
citrate as a molecule.

676
00:50:32,760 --> 00:50:36,720
And then, of course,
that oxaloacetate

677
00:50:36,720 --> 00:50:40,573
can be shuttled back into the
mitochondria via something

678
00:50:40,573 --> 00:50:42,240
like the malate-aspartate
shuttle, which

679
00:50:42,240 --> 00:50:45,330
we describe several
lectures ago,

680
00:50:45,330 --> 00:50:49,960
as a way to make this
a complete cycle.

681
00:50:49,960 --> 00:50:52,300
I want to spend a little bit
of time talking about this

682
00:50:52,300 --> 00:50:55,180
because, here, a
lot of metabolism

683
00:50:55,180 --> 00:50:58,600
begins to come
together, all right?

684
00:50:58,600 --> 00:51:04,860
And so when do you want
to produce fatty acids?

685
00:51:04,860 --> 00:51:08,740
We want to produce fatty acids
when you have a lot of ATP.

686
00:51:08,740 --> 00:51:11,310
And so if you have
a lot of ATP, this

687
00:51:11,310 --> 00:51:14,250
is a situation
where the TCA cycle

688
00:51:14,250 --> 00:51:15,480
isn't going to want to run.

689
00:51:15,480 --> 00:51:20,760
And so that favors citrate
export from the TCA cycle

690
00:51:20,760 --> 00:51:22,530
into the cytosol.

691
00:51:22,530 --> 00:51:25,920
Remember, citrate was
this important regulator

692
00:51:25,920 --> 00:51:27,420
of glycolysis.

693
00:51:27,420 --> 00:51:29,520
We talked about you
have a lot of citrate.

694
00:51:29,520 --> 00:51:31,590
Let's slow down glycolysis.

695
00:51:31,590 --> 00:51:33,630
Well, that citrate
in the side cytosol

696
00:51:33,630 --> 00:51:37,170
also now provides a
source of acetyl-CoA

697
00:51:37,170 --> 00:51:42,030
to take all that extra carbon
and turn it into fatty acids,

698
00:51:42,030 --> 00:51:44,310
all right?

699
00:51:44,310 --> 00:51:47,070
We also, in the process
of moving this out,

700
00:51:47,070 --> 00:51:49,650
we generate oxaloacetate.

701
00:51:49,650 --> 00:51:52,030
Remember, oxaloacetate,
of course,

702
00:51:52,030 --> 00:51:53,760
is part of the
malate-aspartate shuttle.

703
00:51:53,760 --> 00:51:56,040
We'll come back to
that in a minute.

704
00:51:56,040 --> 00:51:58,980
But oxaloacetate in
the cytosol, that

705
00:51:58,980 --> 00:52:02,100
was the product of the
pyruvate carboxylase reaction

706
00:52:02,100 --> 00:52:06,330
to get oxaloacetate in the
cytosol so PEPCK can make

707
00:52:06,330 --> 00:52:09,120
PEP and do gluconeogenesis.

708
00:52:09,120 --> 00:52:13,200
And so the same thing is
also taking oxaloacetate

709
00:52:13,200 --> 00:52:15,540
and putting that in this
cytosol where it's a good thing

710
00:52:15,540 --> 00:52:17,880
to do gluconeogenesis, which
is something else that you

711
00:52:17,880 --> 00:52:22,150
want to do if you have a lot
of excess ATP to store carbon.

712
00:52:22,150 --> 00:52:25,800
And so this is really
set up in a way

713
00:52:25,800 --> 00:52:27,870
that, now, you have your
citrate and oxaloacetate

714
00:52:27,870 --> 00:52:30,690
in the cytosol, which
are the starting points

715
00:52:30,690 --> 00:52:34,980
to generate either fat
or carbohydrate as a way

716
00:52:34,980 --> 00:52:41,070
to store excess energy if you
have plenty of ATP around.

717
00:52:41,070 --> 00:52:44,460
Now, it turns out that
oxaloacetate in the cytosol

718
00:52:44,460 --> 00:52:48,250
is also beneficial
in another way

719
00:52:48,250 --> 00:52:51,630
because it can be part of a
series of enzyme reactions

720
00:52:51,630 --> 00:52:58,110
that also benefits
fatty acid synthesis.

721
00:52:58,110 --> 00:53:02,730
And that's because it's a
substrate for an enzyme.

722
00:53:02,730 --> 00:53:03,900
It can create a substrate.

723
00:53:03,900 --> 00:53:07,980
It can create malate that's a
substrate for an enzyme called

724
00:53:07,980 --> 00:53:12,750
malic enzyme, which is
another way to generate NADPH.

725
00:53:12,750 --> 00:53:15,220
All right, so let's
go through this.

726
00:53:15,220 --> 00:53:21,510
So remember, oxaloacetate
differs from malate

727
00:53:21,510 --> 00:53:23,715
by an oxidation
reduction reaction.

728
00:53:23,715 --> 00:53:28,770
So remember, we described malate
dehydrogenase in the TCA cycle

729
00:53:28,770 --> 00:53:32,040
to turn malate into oxaloacetate
in the malate-aspartate

730
00:53:32,040 --> 00:53:32,670
shuttle.

731
00:53:32,670 --> 00:53:35,790
We pointed out that this
enzyme can run in reverse,

732
00:53:35,790 --> 00:53:43,530
and so can be used to
regenerate NAD in the cytosol.

733
00:53:43,530 --> 00:53:46,230
And this oxaloacetate
back to malate

734
00:53:46,230 --> 00:53:48,270
as part of the
malate-aspartate shuttle

735
00:53:48,270 --> 00:53:53,010
was a way to regenerate NAD
to help keep glycolysis going

736
00:53:53,010 --> 00:53:54,750
as an alternative
to fermentation,

737
00:53:54,750 --> 00:53:56,590
get those electrons
into the mitochondria,

738
00:53:56,590 --> 00:53:59,500
so we give them to oxygen.

739
00:53:59,500 --> 00:54:04,270
Well, this malate that's made
is the substrate not just

740
00:54:04,270 --> 00:54:06,460
for the malate-aspartate
shuttle to bring electrons

741
00:54:06,460 --> 00:54:11,200
into the mitochondria, but is
also a substrate for an enzyme

742
00:54:11,200 --> 00:54:12,835
called malic enzyme.

743
00:54:15,490 --> 00:54:19,060
And malic enzyme is
a way to make NADPH.

744
00:54:19,060 --> 00:54:20,410
Well, how does this work?

745
00:54:32,170 --> 00:54:32,670
OK.

746
00:54:32,670 --> 00:54:33,435
So here's malate.

747
00:54:36,120 --> 00:55:00,230
All right, so if we reoxidize
this alcohol to bring malate

748
00:55:00,230 --> 00:55:09,270
back to oxaloacetate, so
that generates hydride ion,

749
00:55:09,270 --> 00:55:14,640
which can, of course, be
given to a nicotinamide group,

750
00:55:14,640 --> 00:55:15,840
the malic enzyme.

751
00:55:15,840 --> 00:55:21,810
The nicotinamide group is
NAD+ to generate NADPH.

752
00:55:21,810 --> 00:55:25,320
NAPDH is, of course,
useful for reducing power

753
00:55:25,320 --> 00:55:26,920
to make fatty acids.

754
00:55:26,920 --> 00:55:28,140
So what does this generate?

755
00:55:28,140 --> 00:55:32,270
Well, this generates--
again, all I've done

756
00:55:32,270 --> 00:55:34,820
is regenerate oxaloacetate.

757
00:55:37,440 --> 00:55:38,430
OK.

758
00:55:38,430 --> 00:55:40,800
This is oxaloacetate.

759
00:55:40,800 --> 00:55:45,330
It turns out this oxaloacetate
is retained on the enzyme.

760
00:55:45,330 --> 00:55:52,050
And remember, oxaloacetate is a
beta-keto acid, so acid group,

761
00:55:52,050 --> 00:55:58,670
alpha, beta, beta-keto acid.

762
00:55:58,670 --> 00:56:05,261
And beta carboxylation
is favorable.

763
00:56:05,261 --> 00:56:08,150
You've now seen this
many, many times.

764
00:56:08,150 --> 00:56:10,670
This is going to
generate enolpyruvate.

765
00:56:16,520 --> 00:56:21,330
Enolpyruvate will want
to rearrange to pyruvate.

766
00:56:30,360 --> 00:56:34,260
And so, effectively, I can
turn malate into pyruvate

767
00:56:34,260 --> 00:56:36,990
and generate NADPH.

768
00:56:36,990 --> 00:56:40,020
And that's what
malic enzyme does.

769
00:56:40,020 --> 00:56:43,740
And if you look back at the
malate-aspartate shuttle,

770
00:56:43,740 --> 00:56:45,960
we used oxaloacetate
to make malate.

771
00:56:45,960 --> 00:56:49,590
And it was malate that was
sent back to the mitochondria.

772
00:56:49,590 --> 00:56:53,160
Well, here, we can also
use it to make pyruvate.

773
00:56:53,160 --> 00:56:56,010
And then the pyruvate can
go back to the mitochondria

774
00:56:56,010 --> 00:56:59,910
and use pyruvate carboxylase
to generate acetate

775
00:56:59,910 --> 00:57:02,550
via pyruvate
carboxylase as a way

776
00:57:02,550 --> 00:57:06,880
to do anaplerosis for this
mini-cycle if you want.

777
00:57:06,880 --> 00:57:08,880
A lot of moving
parts here, let me

778
00:57:08,880 --> 00:57:11,190
be explicit about
what's going on

779
00:57:11,190 --> 00:57:16,410
and show you how I can build
a series of reactions here

780
00:57:16,410 --> 00:57:22,050
that is very useful if
I want to generate that.

781
00:57:22,050 --> 00:57:22,550
OK.

782
00:57:22,550 --> 00:57:26,120
So here's glycolysis.

783
00:57:26,120 --> 00:57:29,570
Remember to run glycolysis.

784
00:57:29,570 --> 00:57:33,050
I need a source of NAD+
plus for the GAPDH reaction.

785
00:57:33,050 --> 00:57:35,000
If I'm not going to
ferment the pyruvate,

786
00:57:35,000 --> 00:57:37,760
I need to deal with that NAD+.

787
00:57:37,760 --> 00:57:41,780
Well, if I send now
that pyruvate here

788
00:57:41,780 --> 00:57:51,150
into the mitochondria,
that pyruvate

789
00:57:51,150 --> 00:57:54,500
can go through the pyruvate
dehydrogenase reaction,

790
00:57:54,500 --> 00:57:57,820
make acetyl-CoA.

791
00:57:57,820 --> 00:58:03,785
Acetyl-CoA combine with
oxaloacetate to make citrate.

792
00:58:08,980 --> 00:58:09,480
OK.

793
00:58:13,370 --> 00:58:21,590
Export that citrate from the
mitochondria to the cytosol.

794
00:58:21,590 --> 00:58:26,135
Run the ATP citrate
lyase reaction.

795
00:58:37,100 --> 00:58:41,150
Now, I have acetyl-CoA
in the cytosol

796
00:58:41,150 --> 00:58:46,310
and can use that to
generate fatty acids.

797
00:58:46,310 --> 00:58:54,450
And of course,
that requires NADPH

798
00:58:54,450 --> 00:59:00,360
as reducing power to make
those fatty acids, OK?

799
00:59:00,360 --> 00:59:07,110
So all series of reactions that
you've seen many, many times,

800
00:59:07,110 --> 00:59:13,500
this is glycolysis,
pyruvate dehydrogenase,

801
00:59:13,500 --> 00:59:24,090
citrate synthase, ATP citrate
lyase that we just described

802
00:59:24,090 --> 00:59:29,400
and, of course, this over
here acetyl-CoA carboxylase,

803
00:59:29,400 --> 00:59:33,760
of course, and fatty
acid synthase to do that.

804
00:59:33,760 --> 00:59:42,240
All right, now, that's great,
but we have this NAD+ to deal

805
00:59:42,240 --> 00:59:43,530
with.

806
00:59:43,530 --> 00:59:47,550
And we need sources of NADPH
to balance all the electrons

807
00:59:47,550 --> 00:59:49,265
to make this work.

808
00:59:49,265 --> 00:59:50,640
Let's show how we
can incorporate

809
00:59:50,640 --> 00:59:53,580
malate dehydrogenase and
malic enzyme as a way

810
00:59:53,580 --> 00:59:56,620
to make all this balanced.

811
00:59:56,620 --> 01:00:11,920
So if we take oxaloacetate
and make malate,

812
01:00:11,920 --> 01:00:19,640
this is our malate
dehydrogenase reaction.

813
01:00:19,640 --> 01:00:23,140
I've now regenerated the
NAD I need in the cytosol

814
01:00:23,140 --> 01:00:29,990
to keep that carbon flowing
in from glucose to make

815
01:00:29,990 --> 01:00:32,770
acetylcholine, right?

816
01:00:32,770 --> 01:00:34,930
Now, I have malate.

817
01:00:34,930 --> 01:00:40,450
I can use malic enzyme to
turn malate into pyruvate.

818
01:00:48,700 --> 01:00:56,440
That's going to serve
as a source of NADPH

819
01:00:56,440 --> 01:00:59,980
that I can use, again,
NADPH in the cytosol

820
01:00:59,980 --> 01:01:03,940
to drive fatty acid
synthase in the cytosol.

821
01:01:03,940 --> 01:01:07,010
Obviously, you need more
than one NADPH to do this,

822
01:01:07,010 --> 01:01:10,330
but at least it generates
NADPH's reducing power

823
01:01:10,330 --> 01:01:11,920
to make the fatty acids.

824
01:01:14,750 --> 01:01:20,680
And of course, take
that pyruvate, bring it

825
01:01:20,680 --> 01:01:24,490
back in the mitochondria.

826
01:01:24,490 --> 01:01:34,915
I can run the pyruvate
carboxylase reaction.

827
01:01:40,320 --> 01:01:45,960
And I'll regenerate oxaloacetate
in the mitochondria.

828
01:01:45,960 --> 01:01:48,360
And now, I have a
balanced cycle where

829
01:01:48,360 --> 01:01:52,530
I can run this cycle to turn
glucose carbon into acetyl-CoA

830
01:01:52,530 --> 01:01:56,460
for fat and, in the process,
also make some NADPH's reducing

831
01:01:56,460 --> 01:02:01,170
power to support my fatty
acid synthase reaction.

832
01:02:01,170 --> 01:02:05,880
This is just one way that I
can take all these reactions we

833
01:02:05,880 --> 01:02:08,310
talked about and
build a pathway that's

834
01:02:08,310 --> 01:02:13,530
balanced, at least for the
NAD-NADH oxidation piece,

835
01:02:13,530 --> 01:02:15,225
and gives us something useful.

836
01:02:15,225 --> 01:02:17,340
Well, it gives us
ATP from glycolysis

837
01:02:17,340 --> 01:02:21,180
and also gives NADPH from
the malic enzyme reaction

838
01:02:21,180 --> 01:02:21,870
to do this.

839
01:02:21,870 --> 01:02:23,160
And what does it cost me?

840
01:02:23,160 --> 01:02:24,990
Well, it costs me a
bunch of ATP, right?

841
01:02:24,990 --> 01:02:26,355
It cost me ATP here.

842
01:02:26,355 --> 01:02:30,270
It costs me ATP there, of
course a lot of ATP up here.

843
01:02:30,270 --> 01:02:33,660
But you're doing this when
there's energy excess.

844
01:02:33,660 --> 01:02:35,670
You have plenty of ATP around.

845
01:02:35,670 --> 01:02:38,340
And it's a way that
nature then can

846
01:02:38,340 --> 01:02:44,220
use this high-energy, high-ATP
state as a way to store carbon,

847
01:02:44,220 --> 01:02:47,280
ultimately, reduce carbon
is fatty acids that

848
01:02:47,280 --> 01:02:53,660
can be used later when
times are not so good.

849
01:02:53,660 --> 01:02:59,910
All right, so let's
spend a couple

850
01:02:59,910 --> 01:03:01,760
of minutes talking
about the regulation

851
01:03:01,760 --> 01:03:03,530
of fatty acid synthesis.

852
01:03:03,530 --> 01:03:05,930
It's very straightforward,
not really

853
01:03:05,930 --> 01:03:07,330
a whole lot to talk about.

854
01:03:07,330 --> 01:03:14,190
You really only want to make fat
if you have high ATP-ADP ratio.

855
01:03:14,190 --> 01:03:17,900
This is a situation excess
citrate in the cytosol.

856
01:03:17,900 --> 01:03:26,830
And so high citrate, which,
remember, inhibits glycolysis--

857
01:03:26,830 --> 01:03:39,450
well, high citrate is going to
activate fatty acid synthase.

858
01:03:39,450 --> 01:03:41,850
OK.

859
01:03:41,850 --> 01:03:45,210
The big step, though, is
acetyl-CoA carboxylase.

860
01:03:45,210 --> 01:03:47,190
That's what makes
the malonyl-CoA.

861
01:03:47,190 --> 01:03:50,760
That's the big
change in delta g.

862
01:03:50,760 --> 01:03:52,950
By all the reasons we
talked about before, that's

863
01:03:52,950 --> 01:03:54,540
a step you want to regulate.

864
01:03:54,540 --> 01:04:03,390
And so it turns out that high
levels of palmitoyl-CoA--

865
01:04:03,390 --> 01:04:07,050
that is the 16, 0
fatty acid, the product

866
01:04:07,050 --> 01:04:09,720
of the fatty acid
synthase reaction.

867
01:04:09,720 --> 01:04:21,460
This is going to inhibit
acetyl-CoA carboxylase.

868
01:04:21,460 --> 01:04:26,290
It makes sense-- have a
lot of product around.

869
01:04:26,290 --> 01:04:28,060
Stop making it.

870
01:04:28,060 --> 01:04:30,880
Stop the step that costs
the most, that's hardest

871
01:04:30,880 --> 01:04:33,540
to go back to generate it.

872
01:04:33,540 --> 01:04:34,540
OK?

873
01:04:34,540 --> 01:04:35,500
What's the other thing?

874
01:04:35,500 --> 01:04:42,550
Well, you only want to do this
if cells have enough energy

875
01:04:42,550 --> 01:04:43,780
to do it.

876
01:04:43,780 --> 01:04:44,960
What's their energy charge?

877
01:04:44,960 --> 01:04:50,500
ATP-AMP ratio-- if AMP is high,
you also want to inhibit ACC.

878
01:04:50,500 --> 01:04:52,840
Low energy-- don't
try to make fat.

879
01:04:52,840 --> 01:04:56,440
That's basically what you need
to know about the regulation

880
01:04:56,440 --> 01:04:58,810
of fatty acid synthase.

881
01:04:58,810 --> 01:05:02,240
Now, of course, that's
making fatty acids.

882
01:05:02,240 --> 01:05:04,420
But of course, most
fatty acids are not

883
01:05:04,420 --> 01:05:06,220
floating around free in nature.

884
01:05:06,220 --> 01:05:08,170
They're stored as lipids.

885
01:05:08,170 --> 01:05:10,630
And so just to remind
you, here is an image

886
01:05:10,630 --> 01:05:11,770
from an earlier lecture.

887
01:05:11,770 --> 01:05:15,290
These were the image of a fat
cell, as well as a plant cell

888
01:05:15,290 --> 01:05:15,790
here.

889
01:05:15,790 --> 01:05:18,820
They both have
these lipid droplets

890
01:05:18,820 --> 01:05:20,920
that are filled with
these neutral lipids,

891
01:05:20,920 --> 01:05:22,990
these triacylglycerides.

892
01:05:22,990 --> 01:05:27,235
And so you want to store
the fatty acid for energy.

893
01:05:27,235 --> 01:05:29,590
You put it into a
triacylglyceride,

894
01:05:29,590 --> 01:05:31,180
pop it into a lipid droplet.

895
01:05:31,180 --> 01:05:32,680
And now, you have
this efficient way

896
01:05:32,680 --> 01:05:36,520
to store all this reduced carbon
without having to carry around

897
01:05:36,520 --> 01:05:38,680
the weight of the water.

898
01:05:38,680 --> 01:05:42,610
Or maybe you want to use this
to generate membranes, right?

899
01:05:42,610 --> 01:05:45,370
Phospholipids, you need
to generate phospholipids.

900
01:05:45,370 --> 01:05:47,440
Both triacylglycerides,
phospholipids

901
01:05:47,440 --> 01:05:50,110
are built on this
glycerol backbone.

902
01:05:50,110 --> 01:05:51,910
And I just want
to mention briefly

903
01:05:51,910 --> 01:05:53,920
the pathway that
cells really use

904
01:05:53,920 --> 01:05:57,820
or a pathway the cells use
to make these glycerol based

905
01:05:57,820 --> 01:05:59,050
lipids.

906
01:05:59,050 --> 01:06:03,220
And so as we talked
about before, here's

907
01:06:03,220 --> 01:06:10,090
our old friend dihydroxyacetone
phosphate from glycolysis.

908
01:06:10,090 --> 01:06:21,270
Remember, if we reduce this
ketone to the alcohol, that's

909
01:06:21,270 --> 01:06:24,570
how we made glycerol.

910
01:06:24,570 --> 01:06:28,320
It turns out, first,
before doing this,

911
01:06:28,320 --> 01:06:32,330
the way nature does it is it
first adds the fatty acid.

912
01:06:32,330 --> 01:06:38,220
So it takes the fatty
acyl-CoA, releases the CoA.

913
01:06:38,220 --> 01:06:48,970
And you end up with
this intermediate, OK?

914
01:06:48,970 --> 01:06:52,990
It's this molecule
that then you reduce.

915
01:06:55,670 --> 01:06:57,770
Of course, you can
reduce it within NADH.

916
01:06:57,770 --> 01:07:01,270
You can also reduce
it with NADPH.

917
01:07:01,270 --> 01:07:08,500
And that gives us this
phospho-monoacylglycerol, OK?

918
01:07:08,500 --> 01:07:11,320
So this molecule, with that
being the alcohol instead

919
01:07:11,320 --> 01:07:12,430
of the ketone--

920
01:07:12,430 --> 01:07:16,300
phospho-monoacylglycerol.

921
01:07:16,300 --> 01:07:21,580
Now, come on, add
another fatty acyl-CoA.

922
01:07:21,580 --> 01:07:27,250
Releasing the CoA, that gives
us a phospho-diacylglycerol,

923
01:07:27,250 --> 01:07:32,740
so, now, a fatty acid esterified
to that middle carbon.

924
01:07:32,740 --> 01:07:36,550
And then this
phospho-diacylglycerol,

925
01:07:36,550 --> 01:07:44,770
we can remove the phosphate
to get just a diacylglycerol

926
01:07:44,770 --> 01:07:50,710
and then generate a
triacylglycerol by putting

927
01:07:50,710 --> 01:07:57,660
on a third fatty acid
from a fatty acyl-CoA,

928
01:07:57,660 --> 01:07:59,100
releasing the CoA.

929
01:07:59,100 --> 01:08:01,110
And now, we have this
triacylglyceride,

930
01:08:01,110 --> 01:08:05,310
this neutral lipid that can be
packed into the lipid droplet

931
01:08:05,310 --> 01:08:11,650
here and store energy as
reduced carbon for later.

932
01:08:11,650 --> 01:08:12,490
That's great.

933
01:08:12,490 --> 01:08:17,740
But what if we want to
make a phospholipid?

934
01:08:17,740 --> 01:08:20,560
Well, I'll show you
briefly how this works.

935
01:08:20,560 --> 01:08:22,359
Don't worry about
the details of this.

936
01:08:22,359 --> 01:08:26,109
I just want you to have a flavor
for how this happens because it

937
01:08:26,109 --> 01:08:29,439
illustrates another way
that nature repurposes

938
01:08:29,439 --> 01:08:32,090
the same reactions
over and over again.

939
01:08:32,090 --> 01:08:36,729
And so as one releases
this phosphate

940
01:08:36,729 --> 01:08:43,910
from the diacylglycerol, it now
picks up head group on there.

941
01:08:43,910 --> 01:08:47,890
And so one that can be
picked up is ethanolamine.

942
01:08:47,890 --> 01:08:51,340
And the ethanolamine
comes from a molecule

943
01:08:51,340 --> 01:08:55,060
called CDP ethanolamine.

944
01:08:55,060 --> 01:08:57,310
If you're interested in the
structure of ethanolamine,

945
01:08:57,310 --> 01:08:59,740
be reminded of that, you
can, of course, look it up.

946
01:08:59,740 --> 01:09:05,979
But basically, it's ethanolamine
attached to a CDP group

947
01:09:05,979 --> 01:09:11,860
releasing a CMP group, which
makes a phospho-ethanolamine.

948
01:09:17,520 --> 01:09:20,130
And then that
phospho-ethanolamine

949
01:09:20,130 --> 01:09:25,210
can be turned into
phosphatidylethanolamine.

950
01:09:31,319 --> 01:09:33,569
A phospholipid
phosphatidylethanolamine

951
01:09:33,569 --> 01:09:36,290
can be turned into
phosphatidylcholine.

952
01:09:39,005 --> 01:09:40,380
And if you look
up the difference

953
01:09:40,380 --> 01:09:42,569
between choline
and ethanolamine,

954
01:09:42,569 --> 01:09:44,310
it's adding three methyl groups.

955
01:09:44,310 --> 01:09:47,670
And we'll talk about how to
do that in one of the upcoming

956
01:09:47,670 --> 01:09:49,410
lectures, all right?

957
01:09:49,410 --> 01:09:51,960
And so here's two of
the major phospholipids,

958
01:09:51,960 --> 01:09:54,570
phosphatidylcholine,
phosphatidylethanolamine.

959
01:09:54,570 --> 01:09:58,440
They are added to
a diacylglyceride

960
01:09:58,440 --> 01:10:01,680
by basically taking the
phosphate off and adding

961
01:10:01,680 --> 01:10:07,140
a phosphate from CDP
ethanolamine releasing CMP.

962
01:10:07,140 --> 01:10:09,250
What's CDP ethanolamine?

963
01:10:09,250 --> 01:10:12,570
Well, it's very similar
to how we already

964
01:10:12,570 --> 01:10:18,030
talked about with UDP glucose
in glycogen metabolism.

965
01:10:18,030 --> 01:10:27,030
And so if you start with the
amino alcohol ethanolamine,

966
01:10:27,030 --> 01:10:28,980
which itself is
made from serine,

967
01:10:28,980 --> 01:10:33,110
but we don't have time
to talk about how,

968
01:10:33,110 --> 01:10:47,820
this can basically be
phosphorylated by ATP

969
01:10:47,820 --> 01:10:49,500
to put a phosphate
on the alcohol

970
01:10:49,500 --> 01:10:51,260
to make phosphoethanolamine.

971
01:10:55,150 --> 01:10:58,810
So you have a
phosphoethanolamine.

972
01:10:58,810 --> 01:11:02,260
And then that
phosphoethanolamine

973
01:11:02,260 --> 01:11:13,390
can react with a CTP,
releasing a pyrophosphate such

974
01:11:13,390 --> 01:11:22,450
that the phospho from the
ATP is replaced by the CDP.

975
01:11:22,450 --> 01:11:25,360
You end up getting
a CDP ethanolamine

976
01:11:25,360 --> 01:11:30,688
with two pyrophosphate coming
off by a series of reactions

977
01:11:30,688 --> 01:11:32,230
that, if you look
back in your notes,

978
01:11:32,230 --> 01:11:36,460
will look identical to how
we made UDP glucose in making

979
01:11:36,460 --> 01:11:39,370
glycogen. And then
the CDP ethanolamine

980
01:11:39,370 --> 01:11:41,860
transfers the
phosphoethanolamine

981
01:11:41,860 --> 01:11:44,290
onto the diacylglyceride
to give you

982
01:11:44,290 --> 01:11:46,960
the phosphatidylethanolmine.

983
01:11:46,960 --> 01:11:48,040
Why do I mention this?

984
01:11:48,040 --> 01:11:51,490
Just because I want you to have
a flavor of how phospholipids

985
01:11:51,490 --> 01:11:56,980
are made, realize that here is
a repurposing of a same series

986
01:11:56,980 --> 01:11:59,920
and type of reactions as we
saw for glycogen metabolism,

987
01:11:59,920 --> 01:12:03,790
but now to make
phospholipids, and also

988
01:12:03,790 --> 01:12:06,250
to point out that it's
really expensive to make

989
01:12:06,250 --> 01:12:07,900
a phospholipid, all right?

990
01:12:07,900 --> 01:12:11,470
You need three ATPs just to
add this ethanolamine group

991
01:12:11,470 --> 01:12:12,050
to there.

992
01:12:12,050 --> 01:12:14,540
So that's actually
fairly expensive.

993
01:12:14,540 --> 01:12:17,680
And so a lot of
energy goes into cells

994
01:12:17,680 --> 01:12:21,280
building these phospholipids.

995
01:12:21,280 --> 01:12:24,790
Now, for the last
bit of time today, I

996
01:12:24,790 --> 01:12:29,620
want to return to a brief
topic of another lipid.

997
01:12:29,620 --> 01:12:32,500
And that's this guy
here, cholesterol.

998
01:12:32,500 --> 01:12:36,130
And so you'll remember
that cholesterol

999
01:12:36,130 --> 01:12:43,180
is this complex ring structure,
lots of reduced carbon there.

1000
01:12:43,180 --> 01:12:48,080
It's a molecule
that mammals use,

1001
01:12:48,080 --> 01:12:51,470
if you recall, to keep
their membranes fluid.

1002
01:12:51,470 --> 01:12:54,590
Now, mammals, of course,
can make cholesterol.

1003
01:12:54,590 --> 01:12:56,990
And you probably have
heard about cholesterol

1004
01:12:56,990 --> 01:12:59,473
because high levels
of cholesterol

1005
01:12:59,473 --> 01:13:00,890
have been linked
to heart disease.

1006
01:13:00,890 --> 01:13:03,920
And so a lot of people talk
about what their cholesterol

1007
01:13:03,920 --> 01:13:06,750
levels are, something
doctors check a lot.

1008
01:13:06,750 --> 01:13:10,790
And this has led the
recognition that cholesterol

1009
01:13:10,790 --> 01:13:13,580
can be associated
with vascular disease,

1010
01:13:13,580 --> 01:13:19,160
has led to the development of a
class of drugs called statins.

1011
01:13:19,160 --> 01:13:24,080
And statins are one of the
most commonly prescribed drugs

1012
01:13:24,080 --> 01:13:25,310
out there.

1013
01:13:25,310 --> 01:13:29,150
They're a drug that blocks an
enzyme in cholesterol synthesis

1014
01:13:29,150 --> 01:13:33,440
and have been very effective at
lowering risk of heart attacks

1015
01:13:33,440 --> 01:13:34,580
and strokes.

1016
01:13:34,580 --> 01:13:37,070
They've also been
a huge windfall,

1017
01:13:37,070 --> 01:13:40,550
a huge moneymaker for lots
of pharmaceutical companies.

1018
01:13:40,550 --> 01:13:42,170
And so if you care
about medicine,

1019
01:13:42,170 --> 01:13:43,420
they're important to medicine.

1020
01:13:43,420 --> 01:13:45,800
If you care about
biotech, they also

1021
01:13:45,800 --> 01:13:48,950
were very important
in supporting

1022
01:13:48,950 --> 01:13:52,910
the pharmaceutical industry.

1023
01:13:52,910 --> 01:13:54,890
All right, now,
we don't have time

1024
01:13:54,890 --> 01:13:57,950
to talk fully about how
cholesterol synthesis works.

1025
01:13:57,950 --> 01:13:59,510
It's a long pathway.

1026
01:13:59,510 --> 01:14:01,250
You can look it up
if you're interested.

1027
01:14:01,250 --> 01:14:02,750
You have all the
tools that you need

1028
01:14:02,750 --> 01:14:06,410
to understand all of the
steps to make cholesterol.

1029
01:14:06,410 --> 01:14:08,600
You'll see you build
it from acyl-CoA.

1030
01:14:08,600 --> 01:14:11,300
You need a bunch of NADPH.

1031
01:14:11,300 --> 01:14:13,580
I don't have time to go
through all those steps.

1032
01:14:13,580 --> 01:14:15,630
But I will discuss
the initial steps,

1033
01:14:15,630 --> 01:14:18,260
so you can understand
how statins work.

1034
01:14:18,260 --> 01:14:20,510
Because that's important in
medicine, and some of you,

1035
01:14:20,510 --> 01:14:22,190
I know, want to go
to medical school.

1036
01:14:22,190 --> 01:14:24,440
But apart from that,
these early steps

1037
01:14:24,440 --> 01:14:28,520
affect other aspects of
biology, making lipid tails

1038
01:14:28,520 --> 01:14:31,820
for signaling proteins,
also introduces

1039
01:14:31,820 --> 01:14:33,260
a discussion of
ketones, which are

1040
01:14:33,260 --> 01:14:35,030
another important
metabolic fuel that we

1041
01:14:35,030 --> 01:14:37,740
need to talk about as well.

1042
01:14:37,740 --> 01:14:44,510
Now, the way that one makes
these things is basically you

1043
01:14:44,510 --> 01:14:49,610
start with three
acetyl-CoA molecules.

1044
01:15:03,310 --> 01:15:03,810
OK.

1045
01:15:03,810 --> 01:15:07,500
So here's two
acetyl-CoA molecules.

1046
01:15:07,500 --> 01:15:17,100
We can combine those two
together, releasing a CoA.

1047
01:15:17,100 --> 01:15:25,260
And this is really
the identical reaction

1048
01:15:25,260 --> 01:15:32,745
that we just saw for the early
steps in fatty acid synthase.

1049
01:15:37,500 --> 01:15:38,370
OK.

1050
01:15:38,370 --> 01:15:44,010
That generates this molecule
called acetoacetyl-CoA.

1051
01:15:50,330 --> 01:16:08,380
And then this can combine
with yet another acetyl-CoA

1052
01:16:08,380 --> 01:16:42,530
to make this molecule, which is
called Hydroxy-Methyl-Glutaryl,

1053
01:16:42,530 --> 01:16:48,048
or HMG, CoA, all right?

1054
01:16:48,048 --> 01:16:50,340
I don't need to describe any
of these reactions to you.

1055
01:16:50,340 --> 01:16:52,410
As I said, this is just
what we showed earlier

1056
01:16:52,410 --> 01:16:54,180
for fatty acid synthesis.

1057
01:16:54,180 --> 01:16:58,020
And this reaction is
the identical reaction

1058
01:16:58,020 --> 01:17:00,900
to citrate synthase
in the TCA cycle

1059
01:17:00,900 --> 01:17:05,640
and takes these three, 1,
2, 3 acetyl-CoAs and makes

1060
01:17:05,640 --> 01:17:09,710
this molecule, HMG-CoA.

1061
01:17:09,710 --> 01:17:15,900
HMG-CoA is the
substrate for an enzyme

1062
01:17:15,900 --> 01:17:24,000
called HMG-CoA reductase.

1063
01:17:24,000 --> 01:17:30,300
HMG-CoA reductase
is the famous target

1064
01:17:30,300 --> 01:17:37,560
of statins to block
cholesterol synthesis.

1065
01:17:37,560 --> 01:17:45,860
What HMG-CoA reductase does
is it releases that CoA group

1066
01:17:45,860 --> 01:17:56,090
and uses two NADPH
oxidizing it to two NAD+.

1067
01:17:56,090 --> 01:18:01,340
That effectively
is, when you release

1068
01:18:01,340 --> 01:18:02,840
this CoA, what are
you're left with?

1069
01:18:02,840 --> 01:18:04,670
The acid, you reduce it twice.

1070
01:18:04,670 --> 01:18:07,310
And you end up with the alcohol.

1071
01:18:07,310 --> 01:18:35,490
And that gives
you this molecule,

1072
01:18:35,490 --> 01:18:37,370
which is called mevalonate.

1073
01:18:41,460 --> 01:18:52,020
And mevalonate is what you
use to build cholesterol.

1074
01:18:52,020 --> 01:18:56,250
And so statins, by
blocking HMG-CoA reductase,

1075
01:18:56,250 --> 01:18:58,620
basically block
the reactions that

1076
01:18:58,620 --> 01:19:02,460
are necessary to make the
precursor mevalonate, which is

1077
01:19:02,460 --> 01:19:04,540
necessary to make cholesterol.

1078
01:19:04,540 --> 01:19:07,740
And that's why statins
stop cholesterol synthesis.

1079
01:19:07,740 --> 01:19:12,060
Now, mevalonate, it
turns out, is also

1080
01:19:12,060 --> 01:19:14,820
used for other things in cells.

1081
01:19:14,820 --> 01:19:18,180
It's not just used
to make cholesterol.

1082
01:19:18,180 --> 01:19:20,730
It's also used to make
a class of molecules

1083
01:19:20,730 --> 01:19:21,730
called isoprenoids.

1084
01:19:21,730 --> 01:19:24,930
Isoprenoids are important.

1085
01:19:24,930 --> 01:19:27,660
And you'll encounter them if
you study signaling or anything

1086
01:19:27,660 --> 01:19:29,970
like that because
these make things

1087
01:19:29,970 --> 01:19:38,140
like the farnesylation
lipid modifications that

1088
01:19:38,140 --> 01:19:41,920
are oftentimes added to the
membrane associated proteins,

1089
01:19:41,920 --> 01:19:43,900
like signaling proteins.

1090
01:19:43,900 --> 01:19:47,530
And so statins will also
block the production

1091
01:19:47,530 --> 01:19:52,180
of these molecules that are
important for these various

1092
01:19:52,180 --> 01:19:54,050
signaling proteins.

1093
01:19:54,050 --> 01:19:57,940
And so you should have
some understanding

1094
01:19:57,940 --> 01:20:02,860
of what mevalonate is,
where it comes from,

1095
01:20:02,860 --> 01:20:05,170
that it's involved in
cholesterol and isoprenoid

1096
01:20:05,170 --> 01:20:09,140
synthesis, and that it's
the target of statins.

1097
01:20:09,140 --> 01:20:12,010
And of course, you
didn't need a whole lot.

1098
01:20:12,010 --> 01:20:14,840
We already have all the tools
to understand that pathway.

1099
01:20:14,840 --> 01:20:17,740
And if you look up isoprenoid
synthesis, cholesterol

1100
01:20:17,740 --> 01:20:21,730
synthesis, you'll see you'll
have the tools to understand

1101
01:20:21,730 --> 01:20:23,960
that as well.

1102
01:20:23,960 --> 01:20:25,660
The last thing I
want to talk about

1103
01:20:25,660 --> 01:20:31,450
is taking the acetoacetyl-CoA.

1104
01:20:31,450 --> 01:20:35,260
And if I take the CoA group
off, what I'm left with

1105
01:20:35,260 --> 01:20:39,880
is a molecule called
acetoacetate, OK?

1106
01:20:39,880 --> 01:20:52,990
This is basically two
acetyl-CoAs brought together

1107
01:20:52,990 --> 01:20:55,075
losing CoAs times 2.

1108
01:21:03,150 --> 01:21:03,900
OK.

1109
01:21:03,900 --> 01:21:06,270
So this here is acetoacetate.

1110
01:21:06,270 --> 01:21:13,080
And acetoacetate is a
canonical ketone body.

1111
01:21:16,190 --> 01:21:17,600
What is a ketone body?

1112
01:21:17,600 --> 01:21:20,480
Well, it turns out ketones
are an alternative fuel

1113
01:21:20,480 --> 01:21:24,500
that the body can use to
glucose, primarily the brain.

1114
01:21:24,500 --> 01:21:27,470
And so for reasons that
no one really understands,

1115
01:21:27,470 --> 01:21:32,660
a quirk of human and
mammalian physiology,

1116
01:21:32,660 --> 01:21:36,500
is that the brain prefers
to use glucose as its fuel.

1117
01:21:36,500 --> 01:21:40,230
If glucose is not available,
it doesn't use fatty acids.

1118
01:21:40,230 --> 01:21:42,690
It uses ketone bodies instead.

1119
01:21:42,690 --> 01:21:47,870
And so the ketone is acetate
is the canonical ketone body.

1120
01:21:47,870 --> 01:21:52,350
This underlies the keto diet,
which has become very popular.

1121
01:21:52,350 --> 01:21:54,140
And so what is the keto diet?

1122
01:21:54,140 --> 01:21:57,630
Well, the keto diet is you
don't eat any carbohydrates.

1123
01:21:57,630 --> 01:21:59,960
And so if you don't
eat carbohydrates,

1124
01:21:59,960 --> 01:22:03,420
you don't have a
source of glucose.

1125
01:22:03,420 --> 01:22:06,200
The liver, of course,
can do gluconeogenesis.

1126
01:22:06,200 --> 01:22:10,470
But if you run out of things
to make and turn into glucose,

1127
01:22:10,470 --> 01:22:11,960
now the liver gets into trouble.

1128
01:22:11,960 --> 01:22:15,290
Remember, we talked about with
anaplerosis of the TCA cycle.

1129
01:22:15,290 --> 01:22:17,780
There's no way to turn
two-carbon units that are made

1130
01:22:17,780 --> 01:22:21,140
from fat, acetyl-CoA from fat--

1131
01:22:21,140 --> 01:22:23,720
mammals cannot turn that
back into glucose because we

1132
01:22:23,720 --> 01:22:27,090
don't have the glyoxylate cycle.

1133
01:22:27,090 --> 01:22:30,170
And so we can't turn
fat into glucose.

1134
01:22:30,170 --> 01:22:33,030
And so when the liver can
no longer make glucose,

1135
01:22:33,030 --> 01:22:36,800
it will take acetyl-CoA
from the breakdown of fat

1136
01:22:36,800 --> 01:22:38,990
and make ketones instead.

1137
01:22:38,990 --> 01:22:44,020
And the ketone is
this acetoacetate.

1138
01:22:44,020 --> 01:22:47,500
People do the keto diet because
ketones, like acetoacetate,

1139
01:22:47,500 --> 01:22:49,900
will suppress appetite.

1140
01:22:49,900 --> 01:22:53,200
And that's at least how one
way it's thought to work.

1141
01:22:53,200 --> 01:22:56,440
But I want to talk about
what these ketones are

1142
01:22:56,440 --> 01:22:59,710
and how they fit
into metabolism.

1143
01:22:59,710 --> 01:23:01,600
So here's acetoacetate.

1144
01:23:01,600 --> 01:23:11,150
If you notice, acetoacetate
is a beta-keto acid.

1145
01:23:11,150 --> 01:23:17,300
Alpha, beta, the ketone
is beta to the acid group.

1146
01:23:17,300 --> 01:23:21,170
So a beta-keto acid,
as I've said now

1147
01:23:21,170 --> 01:23:26,030
many, many, many
times, beta-keto acids

1148
01:23:26,030 --> 01:23:33,710
can undergo decarboxylation
that generates

1149
01:23:33,710 --> 01:23:38,810
the enol, which can be
rearranged to the keto group.

1150
01:23:38,810 --> 01:23:41,240
And in that case,
what's the keto group?

1151
01:23:41,240 --> 01:23:45,750
Well, the keto group
would be this molecule,

1152
01:23:45,750 --> 01:23:49,862
which is acetone,
fingernail polish remover.

1153
01:23:49,862 --> 01:23:51,320
If you go to medical
school, you'll

1154
01:23:51,320 --> 01:23:55,460
learn that, if type
1 diabetics have

1155
01:23:55,460 --> 01:23:59,780
a state of a physiology
called ketoacidosis,

1156
01:23:59,780 --> 01:24:02,420
we don't have time to go
into what drives that state,

1157
01:24:02,420 --> 01:24:04,130
that physiological state.

1158
01:24:04,130 --> 01:24:08,300
But it's basically a state
with very high ketones.

1159
01:24:08,300 --> 01:24:10,460
They smell like acetone.

1160
01:24:10,460 --> 01:24:12,620
The reason is because
they have very high levels

1161
01:24:12,620 --> 01:24:14,630
of acetate, the ketone body.

1162
01:24:14,630 --> 01:24:19,310
And some of that spontaneously
carboxylates to make acetone,

1163
01:24:19,310 --> 01:24:20,740
all right?

1164
01:24:20,740 --> 01:24:22,030
The body does this, though.

1165
01:24:22,030 --> 01:24:24,790
It's making the
acetoacetate as a way

1166
01:24:24,790 --> 01:24:26,380
to give food to the brain.

1167
01:24:26,380 --> 01:24:30,120
But because acetoacetate
is unfavorable,

1168
01:24:30,120 --> 01:24:33,460
that's not the form that
is put into the blood.

1169
01:24:33,460 --> 01:24:39,610
Instead, it undergoes an
oxidation reduction reaction

1170
01:24:39,610 --> 01:24:46,180
where this ketone is
reduced to the alcohol

1171
01:24:46,180 --> 01:24:48,970
or interconverted between
the ketone and the alcohol.

1172
01:24:48,970 --> 01:24:50,180
Something gets oxidized.

1173
01:24:50,180 --> 01:24:51,347
Something else gets reduced.

1174
01:24:51,347 --> 01:24:53,290
NAD, NADH is the donor.

1175
01:24:53,290 --> 01:25:01,860
And that leads to
this molecule, which

1176
01:25:01,860 --> 01:25:03,820
is called beta-hydroxybutyrate.

1177
01:25:08,410 --> 01:25:12,310
And beta-hydroxybutyrate
is the canonical ketone

1178
01:25:12,310 --> 01:25:14,230
that circulates in your blood.

1179
01:25:14,230 --> 01:25:17,115
And measurement of
beta-hydroxybutyrate levels

1180
01:25:17,115 --> 01:25:18,490
really tells you
if you're really

1181
01:25:18,490 --> 01:25:22,220
doing the ketone diet properly.

1182
01:25:22,220 --> 01:25:28,110
So basically, your liver, if
it doesn't have enough glucose,

1183
01:25:28,110 --> 01:25:29,330
will have to break down fat.

1184
01:25:29,330 --> 01:25:31,950
It can't turn the acetyl-CoA
from fat into glucose.

1185
01:25:31,950 --> 01:25:34,340
So instead, it'll turn
it into acetoacetate,

1186
01:25:34,340 --> 01:25:37,760
which will convert to the more
stable beta-hydroxybutyrate.

1187
01:25:37,760 --> 01:25:40,670
And then the more stable
beta-hydroxybutyrate

1188
01:25:40,670 --> 01:25:45,590
can be oxidized by peripheral
tissues, predominantly

1189
01:25:45,590 --> 01:25:48,020
the brain, as an
alternative source of fuel

1190
01:25:48,020 --> 01:25:52,550
to glucose when the liver
can't maintain glucose.

1191
01:25:52,550 --> 01:25:56,060
To why this happens
this way, no one

1192
01:25:56,060 --> 01:25:58,640
really knows why it uses
ketones instead of that.

1193
01:25:58,640 --> 01:26:00,950
But it fits with
all the physiology,

1194
01:26:00,950 --> 01:26:03,410
and it also fits the
metabolism that we've already

1195
01:26:03,410 --> 01:26:08,252
learned about to see
why it works this way.

1196
01:26:08,252 --> 01:26:09,710
I just want to
point out, for those

1197
01:26:09,710 --> 01:26:12,620
of you who are doing the
keto diet, to do it properly.

1198
01:26:12,620 --> 01:26:14,630
Yes, you have to
limit carbohydrates.

1199
01:26:14,630 --> 01:26:16,710
That's the most
popular way to do it.

1200
01:26:16,710 --> 01:26:19,190
But to really make
this work, you also

1201
01:26:19,190 --> 01:26:22,190
have to eliminate other
sources of carbon that

1202
01:26:22,190 --> 01:26:25,720
can be used for gluconeogenesis,
mostly amino acid,

1203
01:26:25,720 --> 01:26:27,300
so lowering protein.

1204
01:26:27,300 --> 01:26:30,680
So a true ketogenic
diet is really all fat

1205
01:26:30,680 --> 01:26:35,960
because that is what basically
makes acetyl-CoA really

1206
01:26:35,960 --> 01:26:39,290
the only carbon source
that the liver can use.

1207
01:26:39,290 --> 01:26:44,030
And then it has to only generate
ketones, beta-hydroxybutyrate,

1208
01:26:44,030 --> 01:26:47,510
which then circulates as an
alternative oxidizable carbon

1209
01:26:47,510 --> 01:26:49,550
source for the brain.

1210
01:26:49,550 --> 01:26:50,660
OK.

1211
01:26:50,660 --> 01:26:52,660
Thanks so much.