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[SQUEAKING]
[RUSTLING] [CLICKING]

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PROFESSOR: OK, good morning.

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So today, we're going to begin
our discussion of nitrogen

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metabolism, which
is really going

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to continue over the final
three lectures in the class.

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Now, nitrogen, of course,
is critical for life.

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You need nitrogen to make RNA
and DNA, as well as protein.

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And so what we will see
is that as we discuss

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the metabolism of amino acids,
and nucleic acids, not so

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much how they contribute
to protein and RNA and DNA

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but rather how they
interface with other pathways

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and metabolism to lead to
how these molecules are

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synthesized, how
they're broken down--

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and what you will see as we
go through these discussions

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is that understanding the
metabolism of these molecules

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really is all about how
organisms handle nitrogen.

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Now, of course, we've spent
much of the class talking

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about how fat and sugar can
be oxidized as a way for cells

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to get energy.

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And of course, that's
very important.

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A major role of fat and
sugar is energy transduction.

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But of course, cells are
mostly made out of protein.

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And so roughly 50% of a
cell by mass is protein.

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And protein, therefore, is
a key part of our diets,

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and really, any animal's diet
that eats other organisms.

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So anything that's
nonphotosynthetic

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has to be taking
in a lot of protein

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because it's eating
other organisms in one

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way or another.

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Now, you will, of course,
know that nucleotides

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are central for and the
heritability of life, and so

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life couldn't exist, at
least in a heritable form,

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without those.

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And often, we equate
things like amino acids,

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and nucleic-acid metabolism,
nitrogen metabolism as all

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about energy and weight-lifting,
so weightlifters--

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all the supplements that are
sold to help people get huge--

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all of those things
usually involve

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heavy loads of nitrogen-- so
creatine-- we talked about

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that earlier in the course.

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Leucine is a very common
weightlifting supplement.

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These are all
nitrogen-containing compounds,

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and it makes sense because if
we're going to build new mass

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for cells, we certainly need
nitrogen to build the protein--

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that's what half the cells are--

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as well as the
nucleic acids that

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enable cells to go on
and build new cells.

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And in fact, roughly 50%, as
I said, of the mass of a cell

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is protein, and another
10% is nucleic acid.

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So really, a large chunk
of what our cells are

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comes from these
nitrogen-containing compounds.

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However may be something you
don't think about as much

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is that we also, because we
have to eat other organisms,

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consume a lot of
protein in our diet.

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And therefore, protein
and particularly,

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different amino
acids, also end up

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being a very important nutrient.

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And in fact, our
muscles and our liver

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are storage organs, if
you will, for amino acids.

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And so our muscles are always
getting bigger and smaller,

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breaking down,
rebuilding each other

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as we go through life as
they really participate

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in our body's metabolism.

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In fact, you have tons of energy
built up into your muscles.

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And if you starve
yourself, stop eating,

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you can live off of muscle
tissue for quite a lot of time.

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And that is really using
the amino-acid breakdown

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from our muscle tissue,
really, there as a way

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to keep glucose
in our blood high

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and keep our brain
thinking right.

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That's why we talked last
time about the keto diet.

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When we talked
about the keto diet,

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remember, if you're going to
do this properly and really

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be ketogenic, you can't
eat glucose, of course,

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but you also can't
eat-- or carbohydrates.

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But you also can't eat
protein, and the reason

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is because the liver will
turn your amino acids back

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into glucose.

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And that's true whether
those amino acids come

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from the diet or
those amino acids

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can come from the
breakdown of muscle.

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All right.

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So if we use amino acids
as a source of energy

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as a way to support
gluconeogenesis

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to keep our carbohydrates
available for our brain,

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we need a way to metabolize
those amino acids.

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And if we metabolize
those amino acids,

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we also then need a way to deal
with all the excess nitrogen.

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And so, indeed, we already
introduced how this works.

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And if you remember
back when we talked

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about the malate-aspartate
shuttle, which you'll

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recall was a way for us
to transfer electrons

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from the cytosol
into the mitochondria

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and involved this convoluted
swaps between aspartate,

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and oxalacetate, and glutamate,
and alpha-ketoglutarate,

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and at the time, we
introduced how amino acids are

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related to alpha-keto acids.

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And so I draw out how
oxalacetate, an amino acid,

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is related to the
alpha-keto acid--

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sorry-- how the amino
acid aspartame is related

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to the alpha-keto
acid oxalacetate,

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as well as how the
amino acid glutamate

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is related to the amino
acid alpha-ketoglutarate.

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Just to be very explicit remind
you what I meant for this,

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and so here is the
amino acid alanine.

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This is the amino acid alanine.

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It is related to the
alpha-keto acid pyruvate.

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And again the same relationship,
if you look back in your notes

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when we discussed the
malate-aspartate shuttle--

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glutamate to
alpha-ketoglutarate,

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aspartate to oxalacetate.

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Really, the keto group,
the alpha-keto group,

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is being replaced
with the amino group.

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And what should be clear to you
now is that if you can convert

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an amino acid into
an alpha-keto acid--

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that is dispose of the
nitrogen, get the ketone back--

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it should be very
clear how you can then

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oxidize pyruvate, oxidize
oxalacetate, oxidize

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alpha-ketoglutarate-- as a
way to get energy directly,

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burn it in the TCA cycle,
get ATP, NADH that way,

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or turn it into glucose
by gluconeogenesis.

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However, if we're
going to do this,

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obviously, these nitrogenous
have to go somewhere.

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And if you're going
to build amino acids,

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those nitrogenous have
to come from somewhere.

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And that's really why
the pathways that we're

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going to discuss really, are
about nitrogen metabolism

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and how cells deal with
either the excess nitrogen

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from breaking down amino
acids and nucleic acids

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or how cells obtain
that nitrogen as a way

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to build these molecules.

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Now, by way of introduction,
we have to discuss also,

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where is it then that nitrogen,
for biological systems,

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really comes from
in the first place?

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Now this is almost never
an issue for animals,

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and that's because we and other
animals eat other organisms.

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And so we eat other
organisms-- usually,

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we have a problem of dealing
with too much nitrogen.

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However, for photosynthetic
organisms, how to get nitrogen

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ends up being a real problem.

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And so any of you who
have done any gardening

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know that you need to add
fertilizer to your plants.

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What is fertilizer?

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Well, it's primarily
a source of nitrogen,

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as well as phosphorus.

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And really, it's the nitrogen
in the fertilizer that's

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helping solve the
problem of where

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the photosynthetic
organisms are going

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to get the nitrogen
that they need to grow.

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Now, of course, there's a lot
of nitrogen, nitrogen gas,

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in our atmosphere, all right?

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But nitrogen gas is
incredibly inert.

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You use nitrogen
gas is an inert gas

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to basically stop things that
are too reactive from reacting

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with the air in the lab, OK?

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Nitrogen gas as a
very stable molecule

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is not at all useful to biology.

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However, if we take this
nitrogen gas and we reduce it--

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that is we add a
bunch of electrons--

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what we end up with
now is we end up

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with all of these
reduced nitrogen groups,

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these amino groups that are
incredibly useful for biology.

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These come up over
and over again.

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These are the side chains
of our amino acids.

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They're found on
all the cofactors.

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They're forming
the hydrogen bonds

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between our different
nucleosides, OK?

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And so enzymes work.

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Genetics happen.

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Hydrogen bonds
form because of all

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of these nitrogen-containing
compounds.

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But you need a way to get
from this nitrogen gas

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down to this reduced
nitrogen. And that is really,

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so-called nitrogen
fixation, which is basically

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taking nitrogen
gas and reducing it

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to be a compound like
ammonia, which involves adding

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electrons, that ends up
being one of the things,

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the magic of biology, that
really makes life possible.

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Now, you don't often think about
nitrogen fixation very much,

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so obviously, we all
know that photosynthesis

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is really central to
what makes life happen.

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And we spent a lot of time
talking about photosynthesis.

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Apart from
photosynthesis, which,

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of course, takes the
energy from the Sun

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and gives us a way
to get reduced carbon

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that all life depends on--

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nitrogen fixation is a
way to take nitrogen gas

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from the atmosphere and turn
it into ammonia, a useful form

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of nitrogen for life to use.

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And what you may not realize
is nitrogen fixation really

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only occurs in a select group
of prokaryotic organisms.

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So these are really the
unsung heroes of life,

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or at least the unsung
heroes of metabolism.

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It's probably the most
central topic to life

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that we're not going
to spend a lot of time

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talking about in 705 simply
because we don't have time.

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But it's basically redox
chemistry to take nitrogen gas,

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add electrons to it, and end
up with reduced nitrogen.

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This is very energy intensive--
requires on the order of 16 ATP

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molecules.

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It's covered well
in various textbooks

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if you're interested in
reading about this further.

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But I really want
you to appreciate

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that this is something
that really has to happen,

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and it's really
relatively few organisms

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that allow this to happen for
all other life on the planet

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to be dependent on.

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This is really why
fertilizer ends up

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being the thing that
you add to your garden.

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So in the wild, nature would
set up symbiotic relationships

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with other organisms, with
these nitrogen-fixing organisms,

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in order to get the
nitrogen they can get.

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However, we can help
that process along

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by giving fertilizer.

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If there's a lot of fertilizer
runoff, you get algae blooms.

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That's really because
all this nitrogen now

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gets dumped into the water.

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That ends up being
rate-limiting, often,

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for photosynthetic organisms,
like algae, to grow.

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And so this ends up being
a really important topic

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if you're interested in
environmental science

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or if you're interested
in agriculture.

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And it's at least
something I want

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to make you aware of from
this introductory course.

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All right.

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Now, for most organisms, we
get, from either our diet

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or from the environment,
fixed nitrogen.

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And the two forms
of fixed nitrogen

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that we need to discuss
is ammonia, as well as

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the conjugate base of
ammonia, which is ammonium.

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Now, fertilizer is, of
course, very rich in ammonia.

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And animals that eat other
organisms get lots of nitrogen

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from breakdown of proteins.

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And that is a source of
ammonia or ammonium ions.

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And so photosynthetic
organisms have

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the problem of getting these.

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Animals off and have the problem
of, how do we get rid of this?

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Now, if you've ever
cleaned with ammonia,

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you know that it's pretty nasty.

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It smells bad.

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It's a cleaning agent.

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It's very toxic to
lots of organisms

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at high enough levels.

244
00:14:47,290 --> 00:14:51,700
And so animals really need
strategies-- or, all life,

245
00:14:51,700 --> 00:14:54,850
really, cells need
strategies about how they're

246
00:14:54,850 --> 00:15:02,530
going to handle and deal with
these nitrogen-containing

247
00:15:02,530 --> 00:15:06,610
compounds, either to incorporate
it or to get rid of it.

248
00:15:06,610 --> 00:15:09,280
Now, I'm going to spend a lot
of time today talking about how

249
00:15:09,280 --> 00:15:12,310
animals who take in a lot of--

250
00:15:12,310 --> 00:15:15,130
eat other organisms,
take in a lot of nitrogen

251
00:15:15,130 --> 00:15:17,410
have to deal with
the nitrogen excess.

252
00:15:17,410 --> 00:15:22,600
And it turns out that you can
deal with the ammonia that's

253
00:15:22,600 --> 00:15:26,320
generated from the breakdown
of your amino acids

254
00:15:26,320 --> 00:15:29,000
in lots of different ways.

255
00:15:29,000 --> 00:15:31,750
And basically, there
is no one solution

256
00:15:31,750 --> 00:15:35,080
that life uses that's
universal for how you deal

257
00:15:35,080 --> 00:15:37,180
with the excess
nitrogen. Really,

258
00:15:37,180 --> 00:15:39,430
different strategies
have been involved

259
00:15:39,430 --> 00:15:42,670
incorporating the
metabolism of amino acids

260
00:15:42,670 --> 00:15:45,880
and nucleic acids
that fit the lifestyle

261
00:15:45,880 --> 00:15:48,190
of different creatures.

262
00:15:48,190 --> 00:15:57,960
And so fish and
other aquatic life

263
00:15:57,960 --> 00:16:01,800
really deals with nitrogen
because they can just

264
00:16:01,800 --> 00:16:05,840
excrete ammonia directly.

265
00:16:08,660 --> 00:16:11,000
They live in the ocean--

266
00:16:11,000 --> 00:16:14,300
lots of excess space
into the ocean.

267
00:16:14,300 --> 00:16:15,950
Just take the ammonia.

268
00:16:15,950 --> 00:16:17,420
Send it off into the ocean.

269
00:16:17,420 --> 00:16:20,360
The large volume of
water in the ocean

270
00:16:20,360 --> 00:16:25,460
means dealing with ammonia
is not such a big problem--

271
00:16:25,460 --> 00:16:29,390
literally pissing into
the ocean to get rid

272
00:16:29,390 --> 00:16:34,700
of their excess ammonia,
their nitrogen waste.

273
00:16:34,700 --> 00:16:36,545
Now, terrestrial animals--

274
00:16:41,706 --> 00:16:44,870
I'll just say
terrestrial creatures--

275
00:16:44,870 --> 00:16:47,600
have a different
problem to deal with.

276
00:16:47,600 --> 00:16:51,470
And that is they can't tolerate
the high concentrations

277
00:16:51,470 --> 00:16:54,080
of ammonia that would be
necessary to just release it

278
00:16:54,080 --> 00:16:57,350
into the environment, so
they need another solution.

279
00:16:57,350 --> 00:17:02,690
And of course, our solution is
to excrete our excess nitrogen

280
00:17:02,690 --> 00:17:03,440
as urea.

281
00:17:15,950 --> 00:17:17,240
So that's urea.

282
00:17:17,240 --> 00:17:19,490
This is why we make urine.

283
00:17:19,490 --> 00:17:24,829
And really, our solution to
eliminate excess nitrogen

284
00:17:24,829 --> 00:17:27,740
is to concentrate that nitrogen,
rather than excreting it

285
00:17:27,740 --> 00:17:29,570
as ammonia--

286
00:17:29,570 --> 00:17:32,330
but to make urea, put
it into our urine,

287
00:17:32,330 --> 00:17:38,090
and eliminate it
through urination.

288
00:17:38,090 --> 00:17:42,350
Now, not all animals make urea.

289
00:17:42,350 --> 00:17:46,040
Things like reptiles
and birds, which--

290
00:17:46,040 --> 00:17:51,560
turns out making urea requires
access to lots of water.

291
00:17:51,560 --> 00:17:54,380
Reptiles don't always have
access to lots of water.

292
00:17:54,380 --> 00:17:56,180
They live in dry climates.

293
00:17:56,180 --> 00:17:57,710
Birds don't want
to have to carry

294
00:17:57,710 --> 00:18:01,340
around all the extra water
weight of dealing with urea.

295
00:18:01,340 --> 00:18:05,210
So instead of urea, it
turns out that they excrete

296
00:18:05,210 --> 00:18:08,460
their nitrogen as uric acid.

297
00:18:08,460 --> 00:18:09,545
So what's uric acid?

298
00:18:24,330 --> 00:18:25,815
So this here is uric acid.

299
00:18:35,950 --> 00:18:39,600
So uric acid is
basically a purine.

300
00:18:39,600 --> 00:18:43,230
So it should look very similar
to the adenine and guanine

301
00:18:43,230 --> 00:18:45,730
rings that you saw
earlier in the class.

302
00:18:45,730 --> 00:18:50,650
And so they basically
generate uric acid,

303
00:18:50,650 --> 00:18:56,230
which allows them to excrete
more nitrogen with less water.

304
00:18:56,230 --> 00:19:00,310
And so bird poop-- birds
don't urinate-- the bird poop

305
00:19:00,310 --> 00:19:03,040
that you get, that
slurry of material,

306
00:19:03,040 --> 00:19:06,580
is basically a mixture of
solid waste and uric acid.

307
00:19:06,580 --> 00:19:12,790
And that's how birds eliminate
their waste all in one

308
00:19:12,790 --> 00:19:17,980
go as a way to save
some water as they deal

309
00:19:17,980 --> 00:19:20,530
with their excess nitrogen.

310
00:19:20,530 --> 00:19:23,620
All right, the fact that
different terrestrial

311
00:19:23,620 --> 00:19:27,490
creatures-- some, like us, make
urea-- some make uric acid--

312
00:19:27,490 --> 00:19:30,250
really deals with,
shows that urea--

313
00:19:30,250 --> 00:19:32,590
you will see today is from
amino-acid metabolism--

314
00:19:32,590 --> 00:19:35,213
uric acid is, of
course, from purine,

315
00:19:35,213 --> 00:19:37,630
from purine metabolism-- we'll
talk about that in the last

316
00:19:37,630 --> 00:19:39,160
lecture of the course--

317
00:19:39,160 --> 00:19:42,580
they really illustrate
how the metabolism

318
00:19:42,580 --> 00:19:45,040
of amino acids and
nucleic acids is really

319
00:19:45,040 --> 00:19:50,920
about the metabolism of nitrogen
and how these pathways are

320
00:19:50,920 --> 00:19:56,890
really set up in a way that
allows exchange of ammonia

321
00:19:56,890 --> 00:20:11,310
with amino acids, as well
as with nucleic acids,

322
00:20:11,310 --> 00:20:14,610
as the nitrogen can
really flow between all

323
00:20:14,610 --> 00:20:18,150
of these different
biological molecules.

324
00:20:18,150 --> 00:20:19,860
And along the way,
well, this allows

325
00:20:19,860 --> 00:20:22,950
a way to excrete nitrogen.
It also, obviously, involves

326
00:20:22,950 --> 00:20:27,000
ways to incorporate it and
is really the pathway cells

327
00:20:27,000 --> 00:20:32,930
use to both make and break
down these compounds.

328
00:20:32,930 --> 00:20:34,160
All right.

329
00:20:34,160 --> 00:20:37,970
Now, as humans,
we, of course, eat

330
00:20:37,970 --> 00:20:40,580
a diet that's high in protein--

331
00:20:40,580 --> 00:20:41,870
higher in protein.

332
00:20:41,870 --> 00:20:45,260
Now the current fad is to
eat high-protein diets.

333
00:20:45,260 --> 00:20:48,050
That's the latest
trend that's healthy.

334
00:20:48,050 --> 00:20:52,880
And this involves lots
of nitrogen excess,

335
00:20:52,880 --> 00:20:55,700
and so we have to,
as humans, urinate

336
00:20:55,700 --> 00:20:57,320
several times a
day because we need

337
00:20:57,320 --> 00:20:59,970
to excrete lots of nitrogen.

338
00:20:59,970 --> 00:21:07,610
Now, that's really our
amount of urea out.

339
00:21:07,610 --> 00:21:10,610
So that urea out,
really, in a healthy

340
00:21:10,610 --> 00:21:21,920
person is balanced with the
nitrogen in from the diet?

341
00:21:21,920 --> 00:21:23,510
Makes sense?

342
00:21:23,510 --> 00:21:29,230
If we are not growing and we
eat a certain amount of protein,

343
00:21:29,230 --> 00:21:35,310
if we eat more
protein, that means

344
00:21:35,310 --> 00:21:41,570
we need to excrete
more nitrogen.

345
00:21:41,570 --> 00:21:53,680
If we eat less protein, then we
need to excrete less nitrogen.

346
00:21:53,680 --> 00:21:58,530
And so the nitrogen in and
out should be balanced.

347
00:21:58,530 --> 00:22:02,250
Now the reality is that
not all of our nitrogen

348
00:22:02,250 --> 00:22:05,010
is excreted as urea.

349
00:22:05,010 --> 00:22:09,330
Some of it ends up in the
stool as other compounds.

350
00:22:09,330 --> 00:22:13,860
And so for a healthy
person, nitrogen in

351
00:22:13,860 --> 00:22:23,050
is greater than nitrogen
out in the urine, the urea.

352
00:22:23,050 --> 00:22:25,990
Nitrogen is really
greater than nitrogen out.

353
00:22:25,990 --> 00:22:31,265
And this is what is referred to
medically as positive nitrogen

354
00:22:31,265 --> 00:22:31,765
balance.

355
00:22:36,410 --> 00:22:39,340
So if I collect all of
your urine for 24 hours

356
00:22:39,340 --> 00:22:42,640
and keep track of all the food
that you eat, and I measure

357
00:22:42,640 --> 00:22:45,790
all the nitrogen in the unit
and all the nitrogen in the food

358
00:22:45,790 --> 00:22:47,830
you eat, there's
going to be more taken

359
00:22:47,830 --> 00:22:50,320
in than I measure
in the urine, that's

360
00:22:50,320 --> 00:22:51,790
positive nitrogen balance.

361
00:22:51,790 --> 00:22:58,870
And that is what is standard,
healthy for most humans.

362
00:22:58,870 --> 00:23:02,620
Now, if there's positive
nitrogen balance,

363
00:23:02,620 --> 00:23:12,050
it means you can also have
negative nitrogen balance,

364
00:23:12,050 --> 00:23:16,760
which really means that
the amount of nitrogen

365
00:23:16,760 --> 00:23:21,260
in is less than the
amount of nitrogen out.

366
00:23:21,260 --> 00:23:25,130
That is you are losing
more nitrogen in the urine

367
00:23:25,130 --> 00:23:27,260
than you're taking in.

368
00:23:27,260 --> 00:23:31,040
Medically, this is
a very bad thing.

369
00:23:31,040 --> 00:23:32,220
When does this occur?

370
00:23:32,220 --> 00:23:36,170
Well, it occurs during
periods of starvation.

371
00:23:36,170 --> 00:23:37,250
Make sense?

372
00:23:37,250 --> 00:23:39,410
If you're not eating
any food, you still

373
00:23:39,410 --> 00:23:42,110
need to get energy
from somewhere.

374
00:23:42,110 --> 00:23:45,200
You'll catabolize your glycogen,
you'll catabolize your fat,

375
00:23:45,200 --> 00:23:49,010
but you'll also catabolize the
amino acids in your muscle.

376
00:23:49,010 --> 00:23:52,790
Those amino acids will generate
excess nitrogen, more nitrogen

377
00:23:52,790 --> 00:23:57,230
out than you've
taken in the diet.

378
00:23:57,230 --> 00:24:02,420
This also occurs
during some illnesses--

379
00:24:02,420 --> 00:24:10,610
famously, cancer, AIDS, sepsis,
all of these really horrible

380
00:24:10,610 --> 00:24:13,970
conditions that some people--

381
00:24:13,970 --> 00:24:15,830
can be very life threatening.

382
00:24:15,830 --> 00:24:21,800
Many of these lead to a
phenomenon known as cachexia.

383
00:24:21,800 --> 00:24:23,750
So what's cachexia?

384
00:24:23,750 --> 00:24:27,710
You waste away, lose all
of your lean body mass.

385
00:24:27,710 --> 00:24:33,080
This is also basically a state
of negative nitrogen balance.

386
00:24:33,080 --> 00:24:39,230
And why this is
really, a current topic

387
00:24:39,230 --> 00:24:41,840
that lots of people are
trying to figure out.

388
00:24:41,840 --> 00:24:44,690
But it's really a case
where you are breaking down

389
00:24:44,690 --> 00:24:49,370
more of your muscle protein,
more amino acids, and losing

390
00:24:49,370 --> 00:24:52,580
that nitrogen than
you're taking in.

391
00:24:52,580 --> 00:24:55,950
And treatment of the
underlying condition

392
00:24:55,950 --> 00:24:59,330
is really required for
this to get better.

393
00:24:59,330 --> 00:25:00,320
All right.

394
00:25:00,320 --> 00:25:03,470
Now, it turns out I
can experimentally

395
00:25:03,470 --> 00:25:07,730
cause negative nitrogen
balance in an animal

396
00:25:07,730 --> 00:25:13,820
if I remove certain amino
acids from your diet.

397
00:25:13,820 --> 00:25:14,910
Why is that?

398
00:25:14,910 --> 00:25:28,460
Well, it's because
prokaryotes, plants, yeast,

399
00:25:28,460 --> 00:25:35,510
lots of organisms, many
organisms, can make all 20

400
00:25:35,510 --> 00:25:42,200
of the amino acids that are
required to build protein.

401
00:25:42,200 --> 00:25:50,110
But it turns out that
animals, including humans,

402
00:25:50,110 --> 00:25:59,395
cannot make all 20 amino acids.

403
00:26:04,290 --> 00:26:08,010
It's a quirk of our physiology.

404
00:26:08,010 --> 00:26:11,610
But it turns out some of the
20 amino acids that are needed

405
00:26:11,610 --> 00:26:13,800
to build protein are essential.

406
00:26:13,800 --> 00:26:17,688
That is they have to come
from external sources.

407
00:26:17,688 --> 00:26:19,230
So remember, there's
lots of vitamins

408
00:26:19,230 --> 00:26:20,790
out there that we can't make.

409
00:26:20,790 --> 00:26:22,290
We have to get
those from our diet.

410
00:26:22,290 --> 00:26:23,670
There's essential lipids.

411
00:26:23,670 --> 00:26:26,730
We talked about that
last time again--

412
00:26:26,730 --> 00:26:28,500
have to get those from our diet.

413
00:26:28,500 --> 00:26:32,250
And some of the amino acids
that we need to live also

414
00:26:32,250 --> 00:26:34,410
have to come from
an external source.

415
00:26:34,410 --> 00:26:38,490
And in the 1950s, experiments
actually done on humans,

416
00:26:38,490 --> 00:26:40,770
on medical students,
actually found

417
00:26:40,770 --> 00:26:44,760
that if you take these essential
amino acids out of the diet,

418
00:26:44,760 --> 00:26:48,840
what happens is that you can
shift those medical students

419
00:26:48,840 --> 00:26:51,450
into a state of negative
nitrogen balance.

420
00:26:51,450 --> 00:26:55,050
And that's because they're
unable to synthesize protein.

421
00:26:55,050 --> 00:26:58,680
You're always synthesizing
and breaking down protein.

422
00:26:58,680 --> 00:27:01,680
If you don't have the essential
amino acids to resynthesize

423
00:27:01,680 --> 00:27:04,200
your protein, you now
break down more protein

424
00:27:04,200 --> 00:27:05,970
than you synthesize,
and you end up

425
00:27:05,970 --> 00:27:10,200
with negative nitrogen balance.

426
00:27:10,200 --> 00:27:14,370
So what are these
essential amino acids?

427
00:27:14,370 --> 00:27:18,170
Well, I post them
here on this picture.

428
00:27:18,170 --> 00:27:21,890
And so here's the
essential amino acid.

429
00:27:21,890 --> 00:27:23,900
They have to be
obtained from nutrition.

430
00:27:23,900 --> 00:27:26,450
Here's the non-essential
amino acid.

431
00:27:26,450 --> 00:27:29,600
These can be obtained
from the outside,

432
00:27:29,600 --> 00:27:34,280
but we also have the pathways
in order to make them.

433
00:27:39,392 --> 00:27:40,850
If you count these
up, you will see

434
00:27:40,850 --> 00:27:44,510
that there are nine
essential amino acids and 11

435
00:27:44,510 --> 00:27:47,040
non-essential amino acids.

436
00:27:47,040 --> 00:27:51,110
It turns out that two of the
non-essential amino acids

437
00:27:51,110 --> 00:27:53,448
require essential
amino acids to make,

438
00:27:53,448 --> 00:27:55,240
so they're sort of
conditionally essential.

439
00:27:58,840 --> 00:28:02,200
From a trivia
standpoint, I guess,

440
00:28:02,200 --> 00:28:08,440
the answer is that there's
nine essential amino acids,

441
00:28:08,440 --> 00:28:09,395
and they are--

442
00:28:09,395 --> 00:28:11,840
I'll use the one-letter
abbreviation.

443
00:28:11,840 --> 00:28:21,250
So there's lysine, histidine,
isoleucine, leucine, valine,

444
00:28:21,250 --> 00:28:29,380
threonine, tryptophan,
phenylalanine, and methionine.

445
00:28:29,380 --> 00:28:31,420
Now, there's two other ones.

446
00:28:31,420 --> 00:28:33,850
If you have enough
phenylalanine,

447
00:28:33,850 --> 00:28:36,040
we can synthesize tyrosine.

448
00:28:36,040 --> 00:28:37,990
And if you have
enough methionine,

449
00:28:37,990 --> 00:28:40,450
we can synthesize cystine.

450
00:28:40,450 --> 00:28:44,050
But it turns out that
tyrosine and cystine

451
00:28:44,050 --> 00:28:46,630
because they require
phenylalanine and methionine,

452
00:28:46,630 --> 00:28:48,980
for all intents and
purposes, are also essential.

453
00:28:48,980 --> 00:29:04,680
So really, 11 of the 20 amino
acids come from our diet,

454
00:29:04,680 --> 00:29:08,280
either directly or
indirectly in the case

455
00:29:08,280 --> 00:29:10,980
of tyrosine and cystine.

456
00:29:10,980 --> 00:29:14,070
Now, of course, our diets
involve lots of proteins.

457
00:29:14,070 --> 00:29:16,410
We eat other animals,
and so rarely is it

458
00:29:16,410 --> 00:29:19,020
an issue that we need these,
which is probably how evolution

459
00:29:19,020 --> 00:29:21,420
was able to lose the
ability to make them,

460
00:29:21,420 --> 00:29:26,910
because we take in lots of
amino acids all the time anyway.

461
00:29:26,910 --> 00:29:31,470
And these are amino acids that
are primarily important only

462
00:29:31,470 --> 00:29:35,850
for making proteins.

463
00:29:35,850 --> 00:29:39,840
But this also really
illustrates that proteins

464
00:29:39,840 --> 00:29:43,440
are a large reservoir of
the nitrogen we take in,

465
00:29:43,440 --> 00:29:50,760
and we just take it in premade
already as these amino acids,

466
00:29:50,760 --> 00:29:55,750
even though we can synthesize
the remaining nine.

467
00:29:55,750 --> 00:29:59,708
All right, now of the
remaining nine amino acids--

468
00:29:59,708 --> 00:30:01,750
these are the ones that
we can synthesize-- these

469
00:30:01,750 --> 00:30:06,940
are really largely ones about
how we move nitrogen around

470
00:30:06,940 --> 00:30:08,333
through biological systems.

471
00:30:08,333 --> 00:30:10,000
We'll see that as we
go through the rest

472
00:30:10,000 --> 00:30:13,990
of the lectures on
the pathways involving

473
00:30:13,990 --> 00:30:15,880
amino-acid metabolism.

474
00:30:15,880 --> 00:30:19,060
And where I want
to start with this

475
00:30:19,060 --> 00:30:24,280
is how we're going to deal
with ammonia, which is really,

476
00:30:24,280 --> 00:30:37,160
exchanged in and out of
the biological molecules,

477
00:30:37,160 --> 00:30:44,330
mostly the two of the
remaining amino acids.

478
00:30:44,330 --> 00:30:48,620
And those two are
glutamate and glutamine.

479
00:30:48,620 --> 00:30:52,510
So I'll just remind you
because this is important--

480
00:30:52,510 --> 00:30:55,550
E is the one-letter
abbreviation for glutamate.

481
00:30:55,550 --> 00:30:59,630
Q is the one letter
abbreviation for glutamine.

482
00:30:59,630 --> 00:31:02,330
And glutamate and
glutamine are really

483
00:31:02,330 --> 00:31:07,790
central to exchanging ammonia
in and out of molecules.

484
00:31:12,730 --> 00:31:16,030
Just to remind you
what these look like--

485
00:31:27,030 --> 00:31:33,330
so this is the alpha-keto acid,
alpha-ketoglutarate, ketone,

486
00:31:33,330 --> 00:31:36,720
alpha to the carboxylic
acid, alpha-keto acid,

487
00:31:36,720 --> 00:31:39,120
alpha-ketoglutarate.

488
00:31:39,120 --> 00:31:55,200
So remember, it differs from
the amino acid glutamate.

489
00:31:55,200 --> 00:31:58,860
So exchange that alpha-keto
group for the amino group.

490
00:31:58,860 --> 00:32:00,330
Now I have glutamate.

491
00:32:00,330 --> 00:32:10,110
And glutamine is, rather
than carboxylic acid

492
00:32:10,110 --> 00:32:22,500
on the side chain, you add
this second nitrogen group.

493
00:32:22,500 --> 00:32:26,000
And so what you will see as
we go through the lectures,

494
00:32:26,000 --> 00:32:28,010
that glutamate is really--

495
00:32:28,010 --> 00:32:30,320
the major glutamate,
alpha-ketoglutarate

496
00:32:30,320 --> 00:32:33,620
is really the major
nitrogen donor and acceptor

497
00:32:33,620 --> 00:32:37,280
to get nitrogen in and
out of other amino acids.

498
00:32:37,280 --> 00:32:39,860
In the last lecture,
you'll see that glutamine

499
00:32:39,860 --> 00:32:42,450
is an important nitrogen donor.

500
00:32:42,450 --> 00:32:47,810
This nitrogen on the
side chain ends up

501
00:32:47,810 --> 00:32:55,550
being important for getting
nitrogen into nucleic acids.

502
00:32:55,550 --> 00:33:01,040
Now, a really key enzyme
to getting these nitrogen

503
00:33:01,040 --> 00:33:05,800
in and out is the one that
catalyzes this step right here.

504
00:33:05,800 --> 00:33:14,746
This is catalyzed by an enzyme
called glutamate dehydrogenase.

505
00:33:18,900 --> 00:33:22,170
And glutamate
dehydrogenase really

506
00:33:22,170 --> 00:33:24,315
carries out the
following reaction.

507
00:33:35,142 --> 00:33:37,355
There's alpha-ketoglutarate.

508
00:33:45,810 --> 00:33:50,380
So this can pick up
an ammonia molecule.

509
00:34:39,280 --> 00:34:40,610
OK.

510
00:34:40,610 --> 00:34:44,469
So with this intermediate.

511
00:34:44,469 --> 00:35:04,740
And now we have two electrons
here as a hydride ion,

512
00:35:04,740 --> 00:35:08,040
which, of course, we're
donating two electrons.

513
00:35:08,040 --> 00:35:09,490
That's why it's a dehydrogenase.

514
00:35:09,490 --> 00:35:13,890
It Involves
nicotinamide cofactor.

515
00:35:13,890 --> 00:35:20,850
So these can come from
either NADPH or NADH,

516
00:35:20,850 --> 00:35:26,920
generating NAD+ or NADP+.

517
00:35:26,920 --> 00:35:30,490
And so reduce this molecule--

518
00:35:30,490 --> 00:35:37,090
oxidized NADH, or
NADPH, NAD+, or NADP--

519
00:35:37,090 --> 00:35:53,800
and you get the
amino acid glutamate.

520
00:35:53,800 --> 00:35:56,770
And so here's a
reaction catalyzed

521
00:35:56,770 --> 00:36:00,670
by glutamate dehydrogenase
that can convert

522
00:36:00,670 --> 00:36:03,250
the alpha-keto acid,
alpha-ketoglutarate,

523
00:36:03,250 --> 00:36:07,630
into the amino acid glutamate.

524
00:36:07,630 --> 00:36:16,270
This reaction, which
effectively is ammonia plus

525
00:36:16,270 --> 00:36:27,970
alpha-ketoglutarate plus NADH
or NADPH can reversibility

526
00:36:27,970 --> 00:36:38,750
generate glutamate plus
NAD+ or NADP+ plus water.

527
00:36:41,930 --> 00:36:53,770
So this reaction, again,
glutamate dehydrogenase--

528
00:36:59,220 --> 00:37:06,000
very reversible reaction--
can use either NADH, or NADP,

529
00:37:06,000 --> 00:37:09,180
or NAD+, or NADP+.

530
00:37:09,180 --> 00:37:12,510
Either factor works,
and it's a key reaction

531
00:37:12,510 --> 00:37:16,320
to convert the alpha-keto
acid, alpha-ketoglutarate,

532
00:37:16,320 --> 00:37:21,540
with the amino acid glutamate
and get ammonia in and out

533
00:37:21,540 --> 00:37:25,500
of the amino-acid pool.

534
00:37:25,500 --> 00:37:31,640
All right, so I
mentioned glutamine

535
00:37:31,640 --> 00:37:36,170
is the other major
nitrogen carrier.

536
00:37:36,170 --> 00:37:38,720
And so how is this made?

537
00:37:38,720 --> 00:37:40,380
Well, you can make it.

538
00:37:40,380 --> 00:37:46,480
So if you start with glutamate,
and you phosphorylate

539
00:37:46,480 --> 00:37:49,630
that glutamate with ATP--

540
00:38:07,190 --> 00:38:13,250
so that's now a phospho acid
on the side chain of glutamate.

541
00:38:25,640 --> 00:38:28,100
Lose the inorganic
phosphate, and now we

542
00:38:28,100 --> 00:38:29,990
end up with glutamine--

543
00:38:32,720 --> 00:38:35,130
glutamine.

544
00:38:35,130 --> 00:38:38,400
All right, this is
carried out by an enzyme

545
00:38:38,400 --> 00:38:47,450
called glutamine synthase.

546
00:38:47,450 --> 00:38:52,430
And it's basically another
way to pick up ammonia.

547
00:38:52,430 --> 00:38:55,170
This process can be reversed.

548
00:38:55,170 --> 00:38:57,950
So we can take glutamine.

549
00:38:57,950 --> 00:38:59,915
So I'm going to draw
out the whole molecule.

550
00:38:59,915 --> 00:39:03,390
I'll just draw here
the side chain.

551
00:39:06,200 --> 00:39:08,390
So this here would be the
side chain of glutamine.

552
00:39:14,180 --> 00:39:23,280
Can just use water to remove
the ammonia and regenerate

553
00:39:23,280 --> 00:39:28,480
glutamate carried out by an
enzyme called glutaminase--

554
00:39:32,990 --> 00:39:37,280
so glutamate to glutamine,
glutamine to glutamate--

555
00:39:37,280 --> 00:39:38,610
glutamine synthase--

556
00:39:38,610 --> 00:39:40,600
glutaminase.

557
00:39:40,600 --> 00:39:44,290
Notice, just like any
other reaction or pathway

558
00:39:44,290 --> 00:39:47,410
we talked about, one
direction is favorable.

559
00:39:47,410 --> 00:39:49,120
The other direction is not.

560
00:39:49,120 --> 00:39:52,840
So this direction requires
energy input for it to work.

561
00:39:52,840 --> 00:39:55,960
ATP is used in
glutamine synthase.

562
00:39:55,960 --> 00:39:59,710
Glutaminase doesn't require
ATP because it's favorable--

563
00:39:59,710 --> 00:40:03,170
makes sense with all the
other stuff we talked about.

564
00:40:03,170 --> 00:40:08,050
And so as animals, the way
our tissues deal with nitrogen

565
00:40:08,050 --> 00:40:12,700
is that if we catabolize an
amino acid in our tissue,

566
00:40:12,700 --> 00:40:14,680
we end up generating ammonia.

567
00:40:14,680 --> 00:40:19,930
That ammonia ends up
either in glutamate

568
00:40:19,930 --> 00:40:22,690
via the glutamate
hydrogenation reaction,

569
00:40:22,690 --> 00:40:26,200
or it involves in glutamine.

570
00:40:26,200 --> 00:40:32,920
And then we carry that nitrogen
as glutamate or glutamine

571
00:40:32,920 --> 00:40:33,940
in our blood.

572
00:40:33,940 --> 00:40:37,270
It gets transported to
our liver and kidneys,

573
00:40:37,270 --> 00:40:41,320
and our liver and kidney
handle that excess nitrogen by,

574
00:40:41,320 --> 00:40:43,420
in our case, turning
it into urea.

575
00:40:43,420 --> 00:40:46,810
Other organisms might turn
it into something else

576
00:40:46,810 --> 00:40:49,000
in specialized tissues,
where it can then

577
00:40:49,000 --> 00:40:52,280
be excreted, all right?

578
00:40:52,280 --> 00:40:55,180
And again, it also
ends up being that

579
00:40:55,180 --> 00:40:57,580
glutamate-- if we're going
to make amino acids-- ends up

580
00:40:57,580 --> 00:41:00,490
being a good source of nitrogen.
And glutamine, if we're going

581
00:41:00,490 --> 00:41:03,010
to make nucleic
acids, end up being

582
00:41:03,010 --> 00:41:07,570
a good source of nitrogen. And
so for catabolizing amino acids

583
00:41:07,570 --> 00:41:10,900
for energy, we can
release ammonia

584
00:41:10,900 --> 00:41:13,510
via the glutamate
dehydrogenase reaction moving

585
00:41:13,510 --> 00:41:17,380
in this direction, or we
can incorporate ammonia

586
00:41:17,380 --> 00:41:22,210
by running in the
opposite direction.

587
00:41:22,210 --> 00:41:25,090
And so what you will see
then is that the synthesis

588
00:41:25,090 --> 00:41:27,790
and breakdown, now
focusing on amino acids,

589
00:41:27,790 --> 00:41:31,630
is really going to
involve exchanging

590
00:41:31,630 --> 00:41:36,220
the nitrogen between alpha-keto
acids and amino acids

591
00:41:36,220 --> 00:41:39,460
and glutamine and
glutamate as we

592
00:41:39,460 --> 00:41:43,690
described for the
malate-aspartate shuttle, OK?

593
00:41:43,690 --> 00:41:45,560
So let's just show
an example of that.

594
00:41:45,560 --> 00:41:50,810
So this is what was involved in
the malate-aspartate shuttle.

595
00:41:50,810 --> 00:41:56,440
OK, so here's oxalacetate.

596
00:41:56,440 --> 00:42:01,900
So alpha ketone to the
acid-- so alpha-keto acid.

597
00:42:04,720 --> 00:42:16,080
This here is the
amino acid aspartate.

598
00:42:16,080 --> 00:42:22,220
Remember, in the
malate-aspartate shuttle,

599
00:42:22,220 --> 00:42:27,980
we basically exchanged the
amino group and aspartate to be

600
00:42:27,980 --> 00:42:32,690
glutamate and the alpha-keto
acid, oxalacetate,

601
00:42:32,690 --> 00:42:35,540
for alpha-keto glutarate--

602
00:42:35,540 --> 00:42:39,450
so alpha-keto acid, amino
acid, alpha-keto acid,

603
00:42:39,450 --> 00:42:42,260
amino acid balanced
on both sides.

604
00:42:42,260 --> 00:42:47,780
And nature can basically use
this system to now get nitrogen

605
00:42:47,780 --> 00:42:50,120
in and out of systems, OK?

606
00:42:50,120 --> 00:42:53,630
So take an amino acid alanine.

607
00:42:53,630 --> 00:42:57,840
It's related to the
alpha-keto acid pyruvate.

608
00:42:57,840 --> 00:43:03,170
You can exchange
that amino group

609
00:43:03,170 --> 00:43:05,900
onto alpha-keto glutarate, OK?

610
00:43:05,900 --> 00:43:09,324
That will generate glutamate.

611
00:43:09,324 --> 00:43:16,690
You can use that glutamate.

612
00:43:16,690 --> 00:43:21,010
Transfer the amino
group on to oxalacetate.

613
00:43:21,010 --> 00:43:23,800
That can generate aspartate.

614
00:43:23,800 --> 00:43:28,240
And so really, here, you have
a system where you can use

615
00:43:28,240 --> 00:43:32,980
transfer of these amino acids
between alpha-keto glutarate

616
00:43:32,980 --> 00:43:37,390
and glutamate and
really, any alpha--

617
00:43:37,390 --> 00:43:49,900
or sorry-- any amino acid
and any alpha-keto acid, OK?

618
00:43:49,900 --> 00:43:55,870
And so if we're going to
catabolize an amino acid, say,

619
00:43:55,870 --> 00:43:59,380
alanine, give the nitrogen
to alpha-ketoglutarate,

620
00:43:59,380 --> 00:44:01,780
now it ends up on
glutamate-- that glutamate

621
00:44:01,780 --> 00:44:02,990
can go off in the blood.

622
00:44:02,990 --> 00:44:07,040
It can be used elsewhere
to synthesize aspartate

623
00:44:07,040 --> 00:44:12,100
from oxaloacetate or
some other amino acid

624
00:44:12,100 --> 00:44:15,370
from some other alpha-keto acid.

625
00:44:15,370 --> 00:44:18,520
Or you can just
stay on glutamate,

626
00:44:18,520 --> 00:44:20,320
go here to the
glutamate-dehydrogenase

627
00:44:20,320 --> 00:44:24,170
reaction and generate ammonia.

628
00:44:24,170 --> 00:44:26,860
And so for catabolism
of amino acids,

629
00:44:26,860 --> 00:44:30,430
the first step involves
transferring the nitrogen

630
00:44:30,430 --> 00:44:35,830
to alpha-ketoglutarate, and that
generates glutamate, as well as

631
00:44:35,830 --> 00:44:39,820
the alpha-keto acid, which
can then be metabolized.

632
00:44:39,820 --> 00:44:42,220
If we're going to
make an amino acid,

633
00:44:42,220 --> 00:44:44,770
we can start with
the alpha-keto acid

634
00:44:44,770 --> 00:44:47,980
and transfer the
nitrogen from glutamate.

635
00:44:47,980 --> 00:44:51,680
And that's a way, then,
to make the amino acid.

636
00:44:51,680 --> 00:44:54,910
And so glutamate dehydrogenase
lets you get ammonia in and out

637
00:44:54,910 --> 00:44:59,620
of the system, and then
this basically transferring

638
00:44:59,620 --> 00:45:04,870
of nitrogen between
alpha-keto acids

639
00:45:04,870 --> 00:45:08,740
and amino acids
involving glutamate and

640
00:45:08,740 --> 00:45:16,110
alpha-ketoglutarate now
allows you to move nitrogen

641
00:45:16,110 --> 00:45:17,860
around the system.

642
00:45:17,860 --> 00:45:21,030
And so that's basically,
in a nutshell,

643
00:45:21,030 --> 00:45:25,140
how you synthesize and
break down many amino acids,

644
00:45:25,140 --> 00:45:28,020
although, of course,
there are some exceptions.

645
00:45:28,020 --> 00:45:32,400
And so glutamine
we just described.

646
00:45:32,400 --> 00:45:38,310
It's the glutamine synthase
and the glutaminase reaction.

647
00:45:38,310 --> 00:45:41,220
Obviously, that's how you
synthesize and break down

648
00:45:41,220 --> 00:45:43,140
glutamine relative to glutamate.

649
00:45:46,710 --> 00:45:53,380
Asparagine is like glutamine.

650
00:45:53,380 --> 00:45:55,740
Remember, asparagine--
also, it's

651
00:45:55,740 --> 00:46:00,750
aspartate with an amino
group on the side chain,

652
00:46:00,750 --> 00:46:01,890
just like glutamine.

653
00:46:01,890 --> 00:46:08,670
And so its metabolism
is similar to glutamine.

654
00:46:08,670 --> 00:46:13,080
Arginine-- arginine is
a special amino acid

655
00:46:13,080 --> 00:46:15,120
with three nitrogen molecules.

656
00:46:15,120 --> 00:46:18,720
We'll discuss that at
length in a little bit.

657
00:46:18,720 --> 00:46:22,110
Prolene-- if you look up
the structure of prolene,

658
00:46:22,110 --> 00:46:26,670
what you'll find is that it's
glutamate that's been cyclized,

659
00:46:26,670 --> 00:46:27,840
then reduced.

660
00:46:27,840 --> 00:46:30,690
And so prolene is also
a little bit different.

661
00:46:30,690 --> 00:46:33,150
You make glutamate,
cyclize it, and reduce it.

662
00:46:33,150 --> 00:46:34,800
That's how you get prolene.

663
00:46:34,800 --> 00:46:38,700
And then glycine, it turns
out, is generated from serine.

664
00:46:38,700 --> 00:46:42,310
And we'll discuss that at
length in the next lecture.

665
00:46:42,310 --> 00:46:43,800
But for the other--

666
00:46:43,800 --> 00:46:46,980
most of the other amino
acids, reactions really

667
00:46:46,980 --> 00:46:51,090
involve interconversion
between the alpha-keto acid

668
00:46:51,090 --> 00:46:54,240
and the amino acid and
glutamine-glutamate

669
00:46:54,240 --> 00:46:58,090
via series of reactions
referred to as transamination.

670
00:47:02,190 --> 00:47:06,660
OK and so
transamination is really

671
00:47:06,660 --> 00:47:10,770
these reactions that allow
this swapping of nitrogen

672
00:47:10,770 --> 00:47:13,380
between amino acids
and alpha-keto acids

673
00:47:13,380 --> 00:47:16,710
and alpha-ketoglutarate
and glutamate.

674
00:47:16,710 --> 00:47:20,298
Some of these are very famous.

675
00:47:20,298 --> 00:47:22,740
Many of you want to
go to medical school.

676
00:47:22,740 --> 00:47:25,770
You're going to do
liver-function tests, where you

677
00:47:25,770 --> 00:47:29,370
measure levels of ALT and AST.

678
00:47:29,370 --> 00:47:34,980
These are the liver-function
enzymes, or the transaminases,

679
00:47:34,980 --> 00:47:35,850
all right?

680
00:47:35,850 --> 00:47:38,640
They're basically
the transaminases

681
00:47:38,640 --> 00:47:42,240
that catalyze the exchange
in the case of ALT

682
00:47:42,240 --> 00:47:53,070
between alanine pyruvate and
glutamate alpha-ketoglutarate.

683
00:47:53,070 --> 00:47:59,940
AST is the transaminase that
catalyzes the exchange between

684
00:47:59,940 --> 00:48:06,980
aspartate oxalacetate and
glutamine alpha-ketoglutarate--

685
00:48:06,980 --> 00:48:10,130
very active, highly
reversible enzymes,

686
00:48:10,130 --> 00:48:12,200
very abundant in the liver.

687
00:48:12,200 --> 00:48:14,450
The liver is important
to get nitrogen

688
00:48:14,450 --> 00:48:16,910
in and out of across
different amino acids--

689
00:48:16,910 --> 00:48:19,730
high levels in the liver cells--
so when the liver is damaged,

690
00:48:19,730 --> 00:48:21,840
they leak out, and they
measure in the blood,

691
00:48:21,840 --> 00:48:24,890
and that's a sign
of liver damage.

692
00:48:24,890 --> 00:48:30,140
And so again, just to
drive this point home,

693
00:48:30,140 --> 00:48:32,990
so you have alpha-ketoglutarate.

694
00:48:32,990 --> 00:48:38,330
Alpha-ketoglutarate can
be converted to glutamate.

695
00:48:38,330 --> 00:48:40,340
This is the
glutamate-dehydrogenase

696
00:48:40,340 --> 00:48:46,352
reaction, so it involves a
redox step with NADH or NADPH.

697
00:48:51,100 --> 00:49:01,190
That allows you to
interconvert the ammonia,

698
00:49:01,190 --> 00:49:05,210
basically, take nitrogen in and
out of the system as ammonia.

699
00:49:05,210 --> 00:49:08,470
And then once it's
in the system,

700
00:49:08,470 --> 00:49:12,250
now you can do
transamination to move

701
00:49:12,250 --> 00:49:15,790
the nitrogen between
glutamate alpha-ketoglutarate

702
00:49:15,790 --> 00:49:26,430
with any other alpha-keto acid
and amino acid, all right?

703
00:49:26,430 --> 00:49:30,180
And so if you're going
to catalyze amino acids,

704
00:49:30,180 --> 00:49:31,560
you go in that direction.

705
00:49:37,540 --> 00:49:39,370
And you're a fish--

706
00:49:39,370 --> 00:49:44,550
you go this direction,
release it as ammonia.

707
00:49:44,550 --> 00:49:46,800
Or you can come
in this direction,

708
00:49:46,800 --> 00:49:48,800
and that is anabolism.

709
00:49:48,800 --> 00:49:55,780
That allows you to build
amino acids, all right?

710
00:49:55,780 --> 00:49:56,280
OK.

711
00:49:56,280 --> 00:49:58,980
So these transamination
reactions then

712
00:49:58,980 --> 00:50:03,250
become very important, and we
need to discuss how they work.

713
00:50:03,250 --> 00:50:07,110
So Transamination itself
requires a cofactor.

714
00:50:07,110 --> 00:50:15,330
This cofactor is called
pyridoxal phosphate, OK?

715
00:50:15,330 --> 00:50:19,920
It's derived from a vitamin,
like many cofactors.

716
00:50:19,920 --> 00:50:25,230
In this case, it's
vitamin B6, also referred

717
00:50:25,230 --> 00:50:31,750
to as pyridoxine, vitamin B6.

718
00:50:31,750 --> 00:50:34,135
Vitamin B6 looks like this.

719
00:50:51,290 --> 00:50:52,940
OK, so that's pyridoxine.

720
00:50:52,940 --> 00:50:54,800
That's vitamin B6.

721
00:50:54,800 --> 00:50:57,800
Turns out we eat vitamin B6.

722
00:50:57,800 --> 00:51:21,780
And the active form in
the cell is this molecule,

723
00:51:21,780 --> 00:51:30,160
where this alcohol has been
oxidized to the aldehyde.

724
00:51:30,160 --> 00:51:42,620
And this Coke factor is referred
to as pyridoxal phosphate

725
00:51:42,620 --> 00:51:50,400
and abbreviated PLP,
for Pyridoxal Phosphate.

726
00:51:50,400 --> 00:51:51,000
All right.

727
00:53:12,120 --> 00:53:21,440
All right, now let's go over
now pyridoxal phosphate allows

728
00:53:21,440 --> 00:53:26,640
these transamination
reactions to occur.

729
00:53:26,640 --> 00:53:37,490
So let's start off here with
just some generic amino acid,

730
00:53:37,490 --> 00:53:38,450
OK?

731
00:53:38,450 --> 00:53:45,490
And so the transaminase has--

732
00:53:56,370 --> 00:53:59,400
so here's pyridoxal phosphate.

733
00:53:59,400 --> 00:54:02,820
I'm not going to draw the whole
molecule-- just a skeleton

734
00:54:02,820 --> 00:54:06,180
of the pyridoxal phosphate.

735
00:54:06,180 --> 00:54:08,280
So it reacts with
the amino acid.

736
00:54:30,500 --> 00:54:31,610
OK?

737
00:54:31,610 --> 00:54:35,270
So that's that middle
part here of the molecule.

738
00:55:12,380 --> 00:55:13,160
OK?

739
00:55:13,160 --> 00:55:14,405
Generate the shift base.

740
00:56:33,430 --> 00:56:35,920
Just draw the top part here.

741
00:57:36,780 --> 00:57:40,380
OK, so here now we generate
the alpha-keto acid.

742
00:58:19,190 --> 00:58:27,800
And this alternative version
of pyridoxal phosphate called

743
00:58:27,800 --> 00:58:38,385
pyridoxamine, which is
also abbreviated PMP--

744
00:58:42,720 --> 00:58:51,750
and so basically, I've
exchanged the amino acid,

745
00:58:51,750 --> 00:58:57,960
transferring the nitrogen
onto PLP to make PMP

746
00:58:57,960 --> 00:59:02,040
and generating the
alpha-keto acid.

747
00:59:08,790 --> 00:59:19,810
That PMP can now react with
some other alpha-keto acid.

748
00:59:31,210 --> 00:59:36,400
So here's our PMP.

749
00:59:36,400 --> 00:59:40,960
And basically, if I just reverse
all of those exact same steps

750
00:59:40,960 --> 00:59:41,890
that I just did--

751
01:00:17,790 --> 01:00:21,840
so I'll do, for the
sake of brevity here,

752
01:00:21,840 --> 01:00:23,640
I'll do two steps in one.

753
01:00:31,950 --> 01:00:35,490
OK, so I broke this up here
into two different steps before,

754
01:00:35,490 --> 01:00:40,200
but to save me some drawing,
I'm just going to do one here.

755
01:02:36,467 --> 01:02:41,020
And all I did is just
run the same mechanism

756
01:02:41,020 --> 01:02:42,520
in the opposite direction.

757
01:02:45,910 --> 01:02:49,180
And now I've taken
my alpha-keto acid,

758
01:02:49,180 --> 01:02:54,700
got the nitrogen from PMP,
regenerated now a new amino

759
01:02:54,700 --> 01:02:56,830
acid, as well as--

760
01:03:01,040 --> 01:03:02,570
I'm not going to
draw it all out,

761
01:03:02,570 --> 01:03:07,410
but regenerated
pyridoxal phosphate.

762
01:03:07,410 --> 01:03:11,300
And so pyridoxal
phosphate ends up

763
01:03:11,300 --> 01:03:15,080
being this really
useful cofactor that

764
01:03:15,080 --> 01:03:18,440
enables this transamination
reaction that

765
01:03:18,440 --> 01:03:22,910
is this interconversion
between an amino acid

766
01:03:22,910 --> 01:03:30,330
and alpha-keto acid and another
amino acid or alpha-keto acid.

767
01:03:30,330 --> 01:03:35,540
And so pyridoxal phosphate is
a co-factor that also useful

768
01:03:35,540 --> 01:03:39,410
for other amino-acid-containing
reactions--

769
01:03:39,410 --> 01:03:42,890
basically enables
this sort of chemistry

770
01:03:42,890 --> 01:03:47,610
that enables nitrogen to--

771
01:03:47,610 --> 01:03:51,200
this sort of chemistry
to facilitate reactions

772
01:03:51,200 --> 01:03:52,640
with amino acids.

773
01:03:52,640 --> 01:03:55,640
We will see in the
next lecture that we

774
01:03:55,640 --> 01:03:58,400
will use pyridoxal phosphate
in a slightly different way

775
01:03:58,400 --> 01:04:02,120
to convert serine to glycine
plus a one-carbon unit

776
01:04:02,120 --> 01:04:04,460
that we can use
for something else.

777
01:04:04,460 --> 01:04:07,790
I don't have time
to go into them

778
01:04:07,790 --> 01:04:11,900
in this introductory course,
but pyridoxal phosphate

779
01:04:11,900 --> 01:04:14,910
is also involved in other
amino-acid reactions.

780
01:04:14,910 --> 01:04:17,910
So last time, we
discussed ethanolamine.

781
01:04:17,910 --> 01:04:20,000
And remember,
ethanolamine is an alcohol

782
01:04:20,000 --> 01:04:23,450
that's found in phospholipids,
phosphatidylethanolamine.

783
01:04:23,450 --> 01:04:26,720
Serine is used to
generate ethanolamine.

784
01:04:26,720 --> 01:04:29,000
And pyridoxal
phosphate basically,

785
01:04:29,000 --> 01:04:31,430
enables decarboxylation
of serine

786
01:04:31,430 --> 01:04:33,950
to generate ethanolamine.

787
01:04:33,950 --> 01:04:36,750
Serine can also be turned
into pyruvate and ammonia.

788
01:04:36,750 --> 01:04:39,830
It's an alternative way
to generate ammonia.

789
01:04:39,830 --> 01:04:42,205
That also involves
pyridoxal phosphate.

790
01:04:42,205 --> 01:04:43,580
And if you're
interested, you can

791
01:04:43,580 --> 01:04:46,580
look up these reactions
and their mechanisms

792
01:04:46,580 --> 01:04:49,640
and see how pyridoxal
phosphate is repurposed

793
01:04:49,640 --> 01:04:53,360
for those reactions, all right?

794
01:04:53,360 --> 01:04:56,900
But for today's purposes, this
is what pyridoxal phosphate is.

795
01:04:56,900 --> 01:04:59,420
This is how it allows
transamination.

796
01:04:59,420 --> 01:05:01,520
And so now it
should be very clear

797
01:05:01,520 --> 01:05:04,220
how it is that fish
can eat a bunch

798
01:05:04,220 --> 01:05:07,730
of protein-containing
food and excrete nitrogen

799
01:05:07,730 --> 01:05:10,270
into the ocean, all right?

800
01:05:10,270 --> 01:05:16,660
And that is they can take
whatever, some amino acid.

801
01:05:16,660 --> 01:05:21,130
They can do a
pyridoxal-phosphate-containing

802
01:05:21,130 --> 01:05:26,920
transamination reaction and
generate the alpha-keto acid

803
01:05:26,920 --> 01:05:32,110
with the nitrogen going
on to alpha-ketoglutarate

804
01:05:32,110 --> 01:05:35,230
to generate glutamate.

805
01:05:35,230 --> 01:05:41,120
This alpha-keto acid
can now be oxidized.

806
01:05:41,120 --> 01:05:43,230
That's favorable.

807
01:05:43,230 --> 01:05:47,730
That can be used to generate
ATP and allow the fish to get

808
01:05:47,730 --> 01:05:51,420
the energy it needs to swim.

809
01:05:51,420 --> 01:05:56,290
That glutamate can now
basically carry out

810
01:05:56,290 --> 01:06:01,360
the glutamate-dehydrogenase
reaction, generating ammonia,

811
01:06:01,360 --> 01:06:05,770
regenerating alpha-ketoglutarate
to accept the next nitrogen

812
01:06:05,770 --> 01:06:10,420
to allow oxidation of the carbon
backbone in the amino acid.

813
01:06:10,420 --> 01:06:15,310
And the ammonia can
float off into the ocean

814
01:06:15,310 --> 01:06:17,920
where it doesn't
hurt anybody and be

815
01:06:17,920 --> 01:06:21,220
fertilizer for some algae.

816
01:06:21,220 --> 01:06:21,790
All right?

817
01:06:21,790 --> 01:06:26,380
And so there is a way to take
catabolism of amino acids,

818
01:06:26,380 --> 01:06:31,090
get energy, and excrete
the nitrogen as ammonia,

819
01:06:31,090 --> 01:06:42,390
basically involving
transamination,

820
01:06:42,390 --> 01:06:50,100
which requires pyridoxal
phosphate and glutamate

821
01:06:50,100 --> 01:06:51,270
dehydrogenase.

822
01:06:56,290 --> 01:06:58,560
All right.

823
01:06:58,560 --> 01:07:03,180
Now, it turns out
tadpoles do the same thing

824
01:07:03,180 --> 01:07:04,860
because they live in water.

825
01:07:04,860 --> 01:07:09,410
But when a tadpole becomes a
frog or if you're a mammal,

826
01:07:09,410 --> 01:07:13,550
you don't do this anymore
because now you don't have

827
01:07:13,550 --> 01:07:15,980
infinite water to excrete into.

828
01:07:15,980 --> 01:07:19,400
And so instead, as I
said earlier, we're

829
01:07:19,400 --> 01:07:24,350
going to generate urea
as a way to excrete

830
01:07:24,350 --> 01:07:26,850
nitrogen. And of course,
I mentioned earlier--

831
01:07:26,850 --> 01:07:29,570
I'm not going to draw it
again, but birds and reptiles--

832
01:07:29,570 --> 01:07:33,890
because you'll see making
urea requires a lot of water--

833
01:07:33,890 --> 01:07:36,980
you can save even
that water by making

834
01:07:36,980 --> 01:07:39,260
uric acid appearing instead.

835
01:07:39,260 --> 01:07:40,670
Now, how you make
purines will be

836
01:07:40,670 --> 01:07:42,180
discussed in the last lecture.

837
01:07:42,180 --> 01:07:44,120
You'll see how you
make uric acid there.

838
01:07:44,120 --> 01:07:52,200
But for now, I want to discuss
how we generate urea as a way--

839
01:07:52,200 --> 01:07:56,990
an alternative way to deal
with amino-acid metabolism

840
01:07:56,990 --> 01:08:00,620
and excrete nitrogen.

841
01:08:00,620 --> 01:08:07,700
Turns out that as we discuss
how you synthesize urea also

842
01:08:07,700 --> 01:08:10,490
discusses how we're going
to make and break down

843
01:08:10,490 --> 01:08:14,990
arginine, one of those
exception amino acids.

844
01:08:14,990 --> 01:08:20,270
And you'll see why arginine
and urea are really

845
01:08:20,270 --> 01:08:24,899
tied to metabolism
in the way they are.

846
01:08:24,899 --> 01:08:31,020
And so let me just remind
you what arginine looks like.

847
01:09:05,080 --> 01:09:06,250
OK?

848
01:09:06,250 --> 01:09:12,580
So this here is the amino
acid arginine, all right?

849
01:09:12,580 --> 01:09:16,390
I'm going to redraw
part of this side chain

850
01:09:16,390 --> 01:09:17,590
here in a different color.

851
01:09:28,510 --> 01:09:29,159
OK?

852
01:09:29,159 --> 01:09:31,660
And I did it that way
because I want to show you

853
01:09:31,660 --> 01:09:36,850
how this is related to urea?

854
01:09:36,850 --> 01:09:37,795
So if I take water--

855
01:10:28,180 --> 01:10:32,110
taking off of that end
of the arginine molecule

856
01:10:32,110 --> 01:10:36,960
generates urea, all right?

857
01:10:36,960 --> 01:10:43,730
And so what we're
left with after we

858
01:10:43,730 --> 01:11:01,310
do that is this
other amino acid,

859
01:11:01,310 --> 01:11:06,250
which is not found in protein
but is found in metabolism.

860
01:11:06,250 --> 01:11:12,710
And it is an amino
acid called ornithine.

861
01:11:12,710 --> 01:11:20,030
And so basically, if you break
down arginine into ornithine,

862
01:11:20,030 --> 01:11:22,340
you generate urea.

863
01:11:22,340 --> 01:11:25,100
And so it should be
clear that if I can now

864
01:11:25,100 --> 01:11:27,980
create a cycle where
I take ornithine,

865
01:11:27,980 --> 01:11:31,550
pick up a couple of nitrogen
and regenerate arginine,

866
01:11:31,550 --> 01:11:37,800
now I have a way to basically,
produce urea because then I

867
01:11:37,800 --> 01:11:42,120
can cycle arginine to ornithine,
make urea, pick up two more

868
01:11:42,120 --> 01:11:45,420
nitrogen, regenerate
arginine, and that's

869
01:11:45,420 --> 01:11:49,080
a way to generate urea.

870
01:11:49,080 --> 01:11:56,340
Now, this series of
reactions occurs via cycle

871
01:11:56,340 --> 01:11:59,880
called the urea cycle.

872
01:11:59,880 --> 01:12:02,430
And the urea cycle
was also described

873
01:12:02,430 --> 01:12:04,900
by our old friend, Hans Krebs.

874
01:12:04,900 --> 01:12:08,640
So remember, Hans Krebs
described the TCA cycle.

875
01:12:08,640 --> 01:12:12,120
Turns out the urea cycle
was described by Hans Krebs

876
01:12:12,120 --> 01:12:16,950
before the TCA cycle and
is really the first cycle

877
01:12:16,950 --> 01:12:18,190
described.

878
01:12:18,190 --> 01:12:21,390
And so part of the
inspiration for the TCA cycle

879
01:12:21,390 --> 01:12:24,210
came because Hans Krebs
already had cycles on the brain

880
01:12:24,210 --> 01:12:27,120
from describing
their urea cycle.

881
01:12:27,120 --> 01:12:28,860
Now, it is a common
misconception

882
01:12:28,860 --> 01:12:30,690
that all cells do
the urea cycle.

883
01:12:30,690 --> 01:12:32,610
That is absolutely not true.

884
01:12:32,610 --> 01:12:34,590
Lots of cells
carry out reactions

885
01:12:34,590 --> 01:12:36,900
that involve the urea
cycle because it's also

886
01:12:36,900 --> 01:12:39,390
involved in arginine
metabolism, all right?

887
01:12:39,390 --> 01:12:43,740
But the urea cycle, as
we're going to describe it,

888
01:12:43,740 --> 01:12:47,820
is really, at least in us,
selective for our livers

889
01:12:47,820 --> 01:12:51,810
and our kidneys because this is
really the series of reactions

890
01:12:51,810 --> 01:12:55,170
that we run to
net-generate urea, which

891
01:12:55,170 --> 01:12:57,790
ends up in our urine.

892
01:12:57,790 --> 01:12:58,770
OK?

893
01:12:58,770 --> 01:13:01,680
And so remember, it would
be nitrogen transported

894
01:13:01,680 --> 01:13:06,120
to the kidneys or the liver via
glutamate or glutamine and then

895
01:13:06,120 --> 01:13:10,950
and end up ultimately,
entering this urea cycle

896
01:13:10,950 --> 01:13:13,170
to generate urea.

897
01:13:13,170 --> 01:13:15,630
Now, this involves
reactions that

898
01:13:15,630 --> 01:13:19,030
will occur in multiple
compartments in the cell.

899
01:13:19,030 --> 01:13:21,330
And so most of it
happens on the cytosol,

900
01:13:21,330 --> 01:13:24,630
but there's one reaction that
occurs in the mitochondria.

901
01:13:24,630 --> 01:13:28,620
And so a general overview
is as follows, OK?

902
01:13:28,620 --> 01:13:33,600
So I just showed you how you
can start from the amino acid

903
01:13:33,600 --> 01:13:40,140
arginine and generate the
non-proteinogenic amino acid

904
01:13:40,140 --> 01:13:41,730
ornithine, all right?

905
01:13:46,530 --> 01:13:53,230
And this will involve
production of urea, all right?

906
01:13:53,230 --> 01:13:56,530
That reaction occurs
in the cytosol.

907
01:13:56,530 --> 01:14:04,930
But the next reaction, which
is ornithine picking up a CO2,

908
01:14:04,930 --> 01:14:10,300
as well as or anything picking
up an ammonia to generate

909
01:14:10,300 --> 01:14:16,510
another non-proteinogenic
amino acid called citrulline--

910
01:14:16,510 --> 01:14:19,720
this reaction occurs
in the mitochondria.

911
01:14:24,020 --> 01:14:31,730
And then that citrulline will
pick up another nitrogen--

912
01:14:31,730 --> 01:14:33,950
show you how it does
this in a minute--

913
01:14:33,950 --> 01:14:38,360
to make a molecule
called argininosuccinate.

914
01:14:41,600 --> 01:14:43,790
And then that
argininosuccinate will

915
01:14:43,790 --> 01:14:47,600
be turned back into arginine.

916
01:14:47,600 --> 01:14:50,360
And so a four-step cycle--

917
01:14:50,360 --> 01:14:52,640
this reaction--
ornithine to citrulline

918
01:14:52,640 --> 01:14:56,120
happening in the mitochondria
and then regenerating

919
01:14:56,120 --> 01:15:00,080
the arginine from citrulline
in two steps happening

920
01:15:00,080 --> 01:15:02,960
in the cytosol and then the
arginine-backed ornithine

921
01:15:02,960 --> 01:15:06,830
in the cytosol to generate urea.

922
01:15:06,830 --> 01:15:08,390
All right.

923
01:15:08,390 --> 01:15:13,100
Now let's go through the
details of how this works.

924
01:15:13,100 --> 01:15:16,640
And we're going to start
here and the mitochondria

925
01:15:16,640 --> 01:15:21,240
at this ornithine to
citrulline reaction.

926
01:15:21,240 --> 01:15:23,810
So it turns out the
first part of this

927
01:15:23,810 --> 01:15:27,290
is generating a
molecule referred

928
01:15:27,290 --> 01:15:30,170
to as carbamoyl phosphate.

929
01:15:30,170 --> 01:15:32,520
I'll write that out in a second.

930
01:15:32,520 --> 01:15:37,190
And so this comes
from CO2, which,

931
01:15:37,190 --> 01:15:43,190
of course, is in equilibrium
and cells with bicarbonate, OK?

932
01:15:47,950 --> 01:15:49,720
So there's bicarbonate.

933
01:15:53,290 --> 01:15:58,600
Just like we saw when we did
carboxylation reactions, if I

934
01:15:58,600 --> 01:16:07,060
phosphrylate the
bicarbonate with ATP,

935
01:16:07,060 --> 01:16:10,720
this molecule here then
would react with biotin,

936
01:16:10,720 --> 01:16:14,370
and that was a way I did
carboxylation reactions.

937
01:16:14,370 --> 01:16:20,760
Well, it turns out
that rather than react

938
01:16:20,760 --> 01:16:25,335
with the nitrogen on biotin,
I can also react with ammonia.

939
01:16:28,110 --> 01:16:36,230
That generates this
carbamic-acid intermediate,

940
01:16:36,230 --> 01:16:43,810
which can then be
phosphorylated with ATP, again,

941
01:16:43,810 --> 01:16:59,920
to generate this molecule, which
is called carbamoyl phosphate.

942
01:16:59,920 --> 01:17:03,050
OK, so carbamoyl phosphate.

943
01:17:03,050 --> 01:17:05,620
It turns out this whole step--

944
01:17:05,620 --> 01:17:09,220
two ATPs taking CO2 and
ammonia and generating

945
01:17:09,220 --> 01:17:14,140
carbamoyl phosphate is carried
out by a single enzyme.

946
01:17:14,140 --> 01:17:17,620
This carbamoyl
phosphate will then

947
01:17:17,620 --> 01:17:20,320
react with an
ornithine molecule.

948
01:17:40,840 --> 01:17:42,910
So here's ornithine.

949
01:17:53,740 --> 01:17:54,240
OK.

950
01:18:02,670 --> 01:18:37,020
And that will generate
this molecule,

951
01:18:37,020 --> 01:18:46,570
which is the amino acid
citrulline, ornithine and

952
01:18:46,570 --> 01:18:50,680
citrulline being the two
major amino acids not found

953
01:18:50,680 --> 01:18:53,010
in protein.

954
01:18:53,010 --> 01:18:59,160
All right, now this citrulline,
then, in the cytosol

955
01:18:59,160 --> 01:19:03,600
can undergo a reaction with ATP.

956
01:19:03,600 --> 01:19:06,300
And it's easier to
see if I rearrange

957
01:19:06,300 --> 01:19:10,740
this part here of the
side chain of citrulline.

958
01:19:10,740 --> 01:19:13,620
So I'm just here going
to draw the side chain.

959
01:19:25,140 --> 01:19:33,190
So it turns out this
will react with ATP.

960
01:19:33,190 --> 01:19:41,510
So I'm just going to draw ATP
here in this stylized way.

961
01:19:47,970 --> 01:19:59,270
That's going to release
pyrophosphate, which can be,

962
01:19:59,270 --> 01:20:02,510
of course, pulled
forward by cleaving

963
01:20:02,510 --> 01:20:10,220
the pyrophosphate into two
inorganic-phosphate molecules.

964
01:20:51,140 --> 01:21:01,500
And so now you have
citrulline with an AMP on it.

965
01:21:01,500 --> 01:21:08,960
And that can react
with an aspartate,

966
01:21:08,960 --> 01:21:10,850
the amino acid aspartate--

967
01:21:19,040 --> 01:21:21,440
so this is the amino
acid aspartate--

968
01:21:26,520 --> 01:21:38,467
losing the AMP to generate
this intermediate--

969
01:22:25,690 --> 01:22:29,650
this intermediate, which is
called argininosuccinate.

970
01:22:34,970 --> 01:22:38,420
So argininosuccinate--
why is it called that?

971
01:22:38,420 --> 01:22:42,350
Because this is a
succinate molecule here.

972
01:22:42,350 --> 01:22:46,880
And this part here is
an arginine molecule.

973
01:22:46,880 --> 01:22:50,030
And now what can happen next?

974
01:22:50,030 --> 01:22:54,920
Oh, by the way, this
reaction here from basically,

975
01:22:54,920 --> 01:22:58,850
adding citrulline to
argininosuccinate-- it's

976
01:22:58,850 --> 01:23:02,070
carried out by one of my
favorite enzyme names.

977
01:23:02,070 --> 01:23:06,800
It's called Argininosuccinate
Synthase, abbreviated ASS.

978
01:23:06,800 --> 01:23:12,160
So the enzyme is
the ASS enzyme--

979
01:23:12,160 --> 01:23:15,220
A-S-S-- Argininosuccinate
Synthase--

980
01:23:15,220 --> 01:23:18,350
to turn citrulline
into argininosuccinate.

981
01:23:18,350 --> 01:23:21,400
Once you have
argininosuccinate, you

982
01:23:21,400 --> 01:23:36,190
can now carry out that reaction,
which will generate two things.

983
01:23:36,190 --> 01:23:39,640
From the purple half
here, you will now

984
01:23:39,640 --> 01:23:59,230
end up with this molecule, which
is fumarate from the TCA cycle,

985
01:23:59,230 --> 01:24:07,720
as well as I've regenerated one
of the most tedious amino acids

986
01:24:07,720 --> 01:24:10,570
to draw, which is arginine.

987
01:24:37,110 --> 01:24:42,550
So here, we have our
amino acid arginine.

988
01:24:42,550 --> 01:24:43,570
OK?

989
01:24:43,570 --> 01:24:48,020
And so that fumarate
now needs to be,

990
01:24:48,020 --> 01:24:51,970
of course, turned
back into aspartate.

991
01:24:51,970 --> 01:24:53,200
You know how to do that.

992
01:24:53,200 --> 01:24:56,660
I can take fumarate,
turn it into malate.

993
01:24:56,660 --> 01:24:57,890
That's adding water.

994
01:24:57,890 --> 01:25:01,760
That's the TCA cycle Then
malate to oxalacetate--

995
01:25:01,760 --> 01:25:04,190
that's NAD to NADH.

996
01:25:04,190 --> 01:25:08,660
And then aspartate can be
transaminated into aspartate.

997
01:25:08,660 --> 01:25:13,400
And you can basically see how
I can run that you urea cycle

998
01:25:13,400 --> 01:25:19,370
as a way to excrete
nitrogen one,

999
01:25:19,370 --> 01:25:23,240
from carbamoyl phosphate,
one from aspartate

1000
01:25:23,240 --> 01:25:26,480
to generate urea.

1001
01:25:26,480 --> 01:25:32,300
And so this complete
cycle is actually

1002
01:25:32,300 --> 01:25:36,650
a few cycles put together,
sometimes referred to as Krebs

1003
01:25:36,650 --> 01:25:39,920
bicycle, or Krebs tricycle.

1004
01:25:39,920 --> 01:25:42,930
And I'll just draw it out here.

1005
01:25:42,930 --> 01:25:50,630
So if you start with
argininosuccinate here

1006
01:25:50,630 --> 01:25:55,740
in the middle, that
argininosuccinate

1007
01:25:55,740 --> 01:26:06,900
will generate fumarate
and arginine, OK?

1008
01:26:06,900 --> 01:26:22,280
Arginine into ornithine will
allow generation of urea.

1009
01:26:22,280 --> 01:26:30,910
Ornithine can be
turned into citrulline,

1010
01:26:30,910 --> 01:26:42,790
picking up a carbamoyl
phosphate, which, of course, is

1011
01:26:42,790 --> 01:26:47,130
from CO2 plus ammonia.

1012
01:26:47,130 --> 01:27:02,570
That citrulline and
aspartate come together

1013
01:27:02,570 --> 01:27:04,710
to give you argininosuccinate.

1014
01:27:04,710 --> 01:27:12,380
And then that fumarate
can run via the TCA cycle

1015
01:27:12,380 --> 01:27:17,240
to generate malate.

1016
01:27:17,240 --> 01:27:20,210
So that's water.

1017
01:27:20,210 --> 01:27:27,050
Malate can be used to
generate oxalacetate.

1018
01:27:27,050 --> 01:27:28,555
So that is, of course--

1019
01:27:33,780 --> 01:27:34,670
OK.

1020
01:27:34,670 --> 01:27:40,230
That oxalacetate can be
used to generate aspartate.

1021
01:27:40,230 --> 01:27:43,860
That's a transamination
reaction.

1022
01:27:43,860 --> 01:27:46,920
So that transamination
reaction [INAUDIBLE]

1023
01:27:46,920 --> 01:27:51,540
glutamate to alpha-ketoglutarate
and then glutamate

1024
01:27:51,540 --> 01:27:53,310
to alpha-ketoglutarate.

1025
01:28:23,240 --> 01:28:28,190
Glutamate-dehydrogenase reaction
can bring the nitrogen in.

1026
01:28:28,190 --> 01:28:32,030
And so this here
complete cycle--

1027
01:28:32,030 --> 01:28:34,700
what you'll see is actually
balanced with respect

1028
01:28:34,700 --> 01:28:38,480
to both electrons because of
the glutamate-dehydrogenase

1029
01:28:38,480 --> 01:28:41,360
reaction and the
malate-oxalacetate direction,

1030
01:28:41,360 --> 01:28:43,730
as well as all the
nitrogen, really

1031
01:28:43,730 --> 01:28:47,960
allows you to take
nitrogen as ammonia, put it

1032
01:28:47,960 --> 01:28:52,290
in the system as aspartate, use
that to combine with citrulline

1033
01:28:52,290 --> 01:28:53,520
to make argininosuccinate.

1034
01:28:53,520 --> 01:28:57,240
Go to arginine, make urea
with the other nitrogens

1035
01:28:57,240 --> 01:29:03,000
coming from carbamoyl phosphate
to generate the citrulline.

1036
01:29:03,000 --> 01:29:05,910
And so the ammonia
that's brought

1037
01:29:05,910 --> 01:29:10,830
to the liver and the kidneys
via glutamate or glutamine

1038
01:29:10,830 --> 01:29:14,340
can then be incorporated
into aspartame and carbamoyl

1039
01:29:14,340 --> 01:29:18,840
phosphate and ultimately,
excreted as ammonia.

1040
01:29:18,840 --> 01:29:22,450
You can see that there's lots
of ATP that's required here.

1041
01:29:22,450 --> 01:29:27,630
You need two ATP here to
generate carbamoyl phosphate.

1042
01:29:27,630 --> 01:29:33,150
And you need
another two ATP here

1043
01:29:33,150 --> 01:29:37,110
at the argininosuccinate
synthase step

1044
01:29:37,110 --> 01:29:40,140
to generate argininosuccinate.

1045
01:29:40,140 --> 01:29:43,230
And you also need CO2,
but the net effect

1046
01:29:43,230 --> 01:29:47,060
is that you generate urea.

1047
01:29:47,060 --> 01:29:50,640
OK, I know that the urea
cycle is complicated.

1048
01:29:50,640 --> 01:29:52,880
I will start out
next time running

1049
01:29:52,880 --> 01:29:55,100
through another
review of it just

1050
01:29:55,100 --> 01:29:56,730
to make sure you have it clear.

1051
01:29:56,730 --> 01:29:58,450
Thank you.