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PROFESSOR: Let's look
now at storyboard 25,

00:00:24.810 --> 00:00:27.660
panels A and B.
Most of the pathways

00:00:27.660 --> 00:00:30.810
we have studied so far in
5.07 have been catabolic--

00:00:30.810 --> 00:00:33.270
that is, the breakdown
of molecules, usually

00:00:33.270 --> 00:00:35.790
with the objective of producing
energy rich molecules,

00:00:35.790 --> 00:00:38.550
such as ATP, or
reducing equivalents

00:00:38.550 --> 00:00:41.850
that ultimately could be used
for reductive biosynthesis.

00:00:41.850 --> 00:00:45.510
Now, we're going to turn to
anabolism, or biosynthesis.

00:00:45.510 --> 00:00:48.990
Biosynthetic pathways are
going to require energy input,

00:00:48.990 --> 00:00:50.820
as well as reducing equivalents.

00:00:50.820 --> 00:00:54.900
Overall, biosynthesis is
an endergonic process.

00:00:54.900 --> 00:00:57.210
Looking at panel B,
you can see that we're

00:00:57.210 --> 00:01:01.140
going to start with fatty
acid and lipid biosynthesis.

00:01:01.140 --> 00:01:03.600
Fatty acids are made
in the cytoplasm.

00:01:03.600 --> 00:01:06.450
The mitochondrion is going
to play an important role,

00:01:06.450 --> 00:01:07.290
however.

00:01:07.290 --> 00:01:10.330
We'll see just how in
a couple of minutes.

00:01:10.330 --> 00:01:13.240
Let me say a couple of things
about the mitochondrion.

00:01:13.240 --> 00:01:20.620
First, acetyl CoA, oxaloacetate
and the NADP plus NADPH pair,

00:01:20.620 --> 00:01:22.810
as we have described
above, cannot pass through

00:01:22.810 --> 00:01:24.890
the mitochondrial membrane.

00:01:24.890 --> 00:01:27.610
However, malate and citrate can.

00:01:27.610 --> 00:01:29.900
In fact, they can go
in either direction.

00:01:29.900 --> 00:01:33.400
Keep these points in
mind as we move ahead.

00:01:33.400 --> 00:01:35.380
Now let's look at panel c.

00:01:35.380 --> 00:01:37.870
We're going to break down
fatty-acid biosynthesis

00:01:37.870 --> 00:01:39.800
into five steps.

00:01:39.800 --> 00:01:42.640
The first step involves
getting acetyl coenzyme A

00:01:42.640 --> 00:01:44.290
into the cytoplasm.

00:01:44.290 --> 00:01:47.620
As I just said, acetyl CoA
cannot directly go past

00:01:47.620 --> 00:01:49.450
the mitochondrial membrane.

00:01:49.450 --> 00:01:51.490
To overcome this problem,
nature, quote unquote,

00:01:51.490 --> 00:01:55.980
packages acetyl CoA as citrate,
which can easily traverse

00:01:55.980 --> 00:01:57.670
the mitochondrial membrane.

00:01:57.670 --> 00:02:02.040
Hence, we put acetyl CoA
into a molecule of citrate,

00:02:02.040 --> 00:02:04.640
bring citrate out
into the cytoplasm,

00:02:04.640 --> 00:02:08.840
and then split off the
acetyl CoA once again.

00:02:08.840 --> 00:02:10.680
The second step
involves maintaining

00:02:10.680 --> 00:02:12.740
oxaloacetate balance.

00:02:12.740 --> 00:02:15.170
We just transported
a molecule of citrate

00:02:15.170 --> 00:02:17.240
out of the mitochondrial
matrix, thus

00:02:17.240 --> 00:02:21.380
depleting the TCA cycle of one
of its important intermediates.

00:02:21.380 --> 00:02:24.320
The levels of all TCA
cycle intermediates

00:02:24.320 --> 00:02:27.830
are going to drop,
including oxaloacetate.

00:02:27.830 --> 00:02:29.750
Now think about what's
happened to the citrate

00:02:29.750 --> 00:02:31.990
in the cytoplasm.

00:02:31.990 --> 00:02:36.100
It was split into
oxaloacetate and acetyl CoA.

00:02:36.100 --> 00:02:38.560
If we could send
that on oxaloacetate

00:02:38.560 --> 00:02:41.350
back into the
mitochondrion, the TCA cycle

00:02:41.350 --> 00:02:43.720
would be replenished,
once again.

00:02:43.720 --> 00:02:47.800
Step two will show how
oxaloacetate finds its way back

00:02:47.800 --> 00:02:50.410
into the mitochondrial matrix.

00:02:50.410 --> 00:02:52.720
The third step in
fatty-acid biosynthesis

00:02:52.720 --> 00:02:56.410
involves the synthesis
of malonyl-coenzyme A.

00:02:56.410 --> 00:02:59.290
This molecule is made by
putting a carbon-dioxide

00:02:59.290 --> 00:03:03.280
molecule onto the methyl
carbon of acetyl CoA

00:03:03.280 --> 00:03:05.650
using acetyl coenzyme
A carboxylase--

00:03:05.650 --> 00:03:08.770
an enzyme we looked at earlier
in the carboxylase module

00:03:08.770 --> 00:03:10.300
of 5.07.

00:03:10.300 --> 00:03:13.330
malonyl-coenzyme A is the
actual precursor to all,

00:03:13.330 --> 00:03:17.260
but two of the carbons
in the fatty acid chain.

00:03:17.260 --> 00:03:19.720
The fourth step in
fatty-acid biosynthesis

00:03:19.720 --> 00:03:22.600
involves putting the
malonyl-coenzyme A

00:03:22.600 --> 00:03:27.580
onto a very large protein on
the fatty acid synthase complex.

00:03:27.580 --> 00:03:32.170
This large protein is called
ACP, or acyl carrier protein.

00:03:32.170 --> 00:03:34.750
The fifth step involves
the actual reactions

00:03:34.750 --> 00:03:38.020
of the fatty acid synthase,
sometimes called FAS.

00:03:38.020 --> 00:03:41.180
This is a large
multi-protein complex.

00:03:41.180 --> 00:03:42.880
These are the enzymes
by which we're

00:03:42.880 --> 00:03:46.510
going to see the stepwise
addition of two carbon units,

00:03:46.510 --> 00:03:49.630
up until we get to the
formation of a 16 carbon

00:03:49.630 --> 00:03:52.720
long fatty acid
called palmitic acid.

00:03:52.720 --> 00:03:56.350
Palmitic acid,
abbreviated 16 colon 0,

00:03:56.350 --> 00:03:59.800
is the default fatty acid length
made by the fatty acid synthase

00:03:59.800 --> 00:04:02.180
complex.

00:04:02.180 --> 00:04:05.110
Let's look at panel
D. After the five step

00:04:05.110 --> 00:04:08.330
synthesis of palmitic
acid the C16 fatty

00:04:08.330 --> 00:04:10.250
acid can be elongated.

00:04:10.250 --> 00:04:12.020
It can have double
bonds put into it,

00:04:12.020 --> 00:04:14.100
and it can be branched.

00:04:14.100 --> 00:04:17.160
We're not going to be
looking into those reactions,

00:04:17.160 --> 00:04:21.329
but we shall be looking into
the way that either two or three

00:04:21.329 --> 00:04:25.650
fatty acids will be placed onto
a glycerol backbone in order

00:04:25.650 --> 00:04:28.050
to make either, a
membrane lipid that

00:04:28.050 --> 00:04:32.670
is a diacylglyceride phosphate,
or triacylglycerides.

00:04:32.670 --> 00:04:36.150
Triacylglycerides are our
primary storage form of energy.

00:04:36.150 --> 00:04:38.580
We're not going to be
looking into those reactions,

00:04:38.580 --> 00:04:40.710
but we shall be
looking at the way

00:04:40.710 --> 00:04:42.300
that either two or
three fatty acids

00:04:42.300 --> 00:04:44.700
can be placed onto a
glycerol backbone in order

00:04:44.700 --> 00:04:46.530
to make either a
membrane lipid that

00:04:46.530 --> 00:04:50.350
is a diacylglyceride phosphate,
or triacylglycerides.

00:04:50.350 --> 00:04:53.370
Triacylglycerides are
our primary storage forms

00:04:53.370 --> 00:04:55.300
of energy.

00:04:55.300 --> 00:04:57.930
I'm not going to be
going over another type

00:04:57.930 --> 00:05:01.879
of fatty-acid biosynthesis,
called polyketide biosynthesis,

00:05:01.879 --> 00:05:03.420
but I'll encourage
you to take a look

00:05:03.420 --> 00:05:04.640
at this pathway in the book.

00:05:04.640 --> 00:05:07.950
Polyketide biosynthesis
is a very important source

00:05:07.950 --> 00:05:10.740
of a host of biologically
active lipids.

00:05:10.740 --> 00:05:13.350
Let's look at panel
E. Before we look

00:05:13.350 --> 00:05:15.510
at a schematic of
lipid biosynthesis,

00:05:15.510 --> 00:05:18.110
I'd like to introduce NADPH.

00:05:18.110 --> 00:05:21.390
NADPH is going to be the source
of electrons that are used

00:05:21.390 --> 00:05:23.820
for reductive biosynthesis.

00:05:23.820 --> 00:05:28.230
NADPH is identical to NADH with
the exception of the phosphate

00:05:28.230 --> 00:05:29.940
at the lower right
of the molecule,

00:05:29.940 --> 00:05:32.850
as drawn in panel
E. This phosphate is

00:05:32.850 --> 00:05:35.310
a molecular decoration
that's recognized

00:05:35.310 --> 00:05:36.900
by biosynthetic enzymes.

00:05:36.900 --> 00:05:38.490
That is, enzymes
that are looking

00:05:38.490 --> 00:05:41.880
for a source of
reducing equivalents.

00:05:41.880 --> 00:05:44.460
NADPH is the co-factor
that provides

00:05:44.460 --> 00:05:46.550
those reducing equivalents.

00:05:46.550 --> 00:05:48.810
The hydrides shown
in the blue ellipse,

00:05:48.810 --> 00:05:50.910
at the top of the
molecule, represents

00:05:50.910 --> 00:05:52.650
the reducing equivalents
that were going

00:05:52.650 --> 00:05:54.960
to be used in biosynthesis.

00:05:54.960 --> 00:05:57.060
The two main
pathways that result

00:05:57.060 --> 00:06:00.030
in the production of
NADPH, for biosynthesis,

00:06:00.030 --> 00:06:02.160
are the pentose
phosphate pathway,

00:06:02.160 --> 00:06:04.590
which I'm going to deal
with in a separate lecture,

00:06:04.590 --> 00:06:06.240
and the malic enzyme,
which I'm going

00:06:06.240 --> 00:06:08.320
to introduce in a few minutes.

00:06:08.320 --> 00:06:11.430
Now turn to panel
F. This schematic

00:06:11.430 --> 00:06:14.790
shows a high-level view of
fatty-acid biosynthesis.

00:06:14.790 --> 00:06:17.550
I think it's useful to put
fatty-acid biosynthesis

00:06:17.550 --> 00:06:20.880
in the context of one of
our physiological scenarios.

00:06:20.880 --> 00:06:24.540
The scenario is
eat sugar, get fat.

00:06:24.540 --> 00:06:26.700
Put more scientifically,
this scenario

00:06:26.700 --> 00:06:29.130
is going to show
how the sugar we eat

00:06:29.130 --> 00:06:32.820
can end up being converted,
very efficiently, into fat.

00:06:32.820 --> 00:06:36.060
I think that, probably, most of
us want to avoid getting fat.

00:06:36.060 --> 00:06:38.730
But keep in mind that
converting sugar to fat

00:06:38.730 --> 00:06:42.300
is one way of converting
energy into a very compact

00:06:42.300 --> 00:06:44.430
and mobile form.

00:06:44.430 --> 00:06:46.340
Let's start with
the glucose molecule

00:06:46.340 --> 00:06:48.170
over on the left of the panel.

00:06:48.170 --> 00:06:51.170
Starting with step one, follow
the horizontal hatched line

00:06:51.170 --> 00:06:52.060
to the right.

00:06:52.060 --> 00:06:54.830
The glucose passes
through glycolysis,

00:06:54.830 --> 00:06:58.730
produces pyruvate, pyruvate
goes into the mitochondrion

00:06:58.730 --> 00:07:01.310
and is converted to acetyl CoA.

00:07:01.310 --> 00:07:04.220
Once again, following
the hatched line,

00:07:04.220 --> 00:07:07.070
acetyl CoA condenses
with oxaloacetate.

00:07:07.070 --> 00:07:10.400
This is the citrate synthase
reaction to form citrate.

00:07:10.400 --> 00:07:13.550
Rather than progressing
further into the TCA cycle

00:07:13.550 --> 00:07:16.650
where it would lose its
carbons as carbon dioxide,

00:07:16.650 --> 00:07:19.820
the citrate at step two is
exported by a transporter

00:07:19.820 --> 00:07:22.190
out into the cytoplasm.

00:07:22.190 --> 00:07:24.140
Step three is
catalyzed by an enzyme

00:07:24.140 --> 00:07:27.140
called ATP citrate lyase.

00:07:27.140 --> 00:07:31.100
This enzyme splits the citrate
into acetyl CoA, which is what

00:07:31.100 --> 00:07:33.370
we want, and leave a residue--

00:07:33.370 --> 00:07:35.660
a molecule of oxaloacetate.

00:07:35.660 --> 00:07:38.120
I'm not going to go through
the mechanism of this enzyme,

00:07:38.120 --> 00:07:41.690
but I think it's worthwhile to
look back at storyboard seven,

00:07:41.690 --> 00:07:43.550
look at the citrate
synthase reaction

00:07:43.550 --> 00:07:46.430
and then imagine it
running backwards.

00:07:46.430 --> 00:07:50.120
Now, it's not going to be
exactly the reverse reaction as

00:07:50.120 --> 00:07:52.550
drawn, but at this
point in 5.07 you

00:07:52.550 --> 00:07:56.180
should be able to look at the
cosubstrates for the reaction,

00:07:56.180 --> 00:08:00.440
and be able to figure out how
ATP citrate lyase liberates

00:08:00.440 --> 00:08:02.960
the oxaloacetate residue.

00:08:02.960 --> 00:08:07.100
I'm going to come back to the
oxaloacetate, which is labeled

00:08:07.100 --> 00:08:09.410
by a star, in a few minutes.

00:08:09.410 --> 00:08:11.510
Keep in mind that we
have to find a way

00:08:11.510 --> 00:08:15.590
to return oxaloacetate to the
mitochondrial matrix or else

00:08:15.590 --> 00:08:17.900
we won't have enough
oxaloacetate to enable

00:08:17.900 --> 00:08:20.030
the next molecule
of citrate to be

00:08:20.030 --> 00:08:23.416
formed from an incoming
acetyl coenzyme A.

00:08:23.416 --> 00:08:26.520
At step five we have
acetyl coenzyme A

00:08:26.520 --> 00:08:30.270
in the cytoplasm, where we want
it for fatty-acid biosynthesis.

00:08:30.270 --> 00:08:33.636
But we can also do other
things with this molecule.

00:08:33.636 --> 00:08:35.010
In the last lecture,
for example,

00:08:35.010 --> 00:08:37.260
I talked about ketone
body formation.

00:08:37.260 --> 00:08:40.919
So look at the vertical arrow--
that is the one going up.

00:08:40.919 --> 00:08:44.039
You can see we can
produce HMG CoA, hydroxy

00:08:44.039 --> 00:08:46.440
methyl glutaryl
coenzyme A. Which,

00:08:46.440 --> 00:08:49.770
as I said in the last lecture,
is a biosynthetic precursor

00:08:49.770 --> 00:08:54.240
to, for example, cholesterol,
as well as ketone bodies.

00:08:54.240 --> 00:08:56.490
Then, follow the
arrows through step 5A

00:08:56.490 --> 00:08:59.940
to the deposition of cholesterol
into the plasma membrane.

00:08:59.940 --> 00:09:03.030
Keep in mind, cholesterol is
a plasticizer of the membrane.

00:09:03.030 --> 00:09:05.880
Therefore, it's an
essential molecule.

00:09:05.880 --> 00:09:08.720
Now let's go back and
pick up the acetyl CoA

00:09:08.720 --> 00:09:11.270
as it progresses through
the fatty acid synthase,

00:09:11.270 --> 00:09:14.805
or FAS Reactions of 5B.

00:09:14.805 --> 00:09:17.810
Acetyl CoA is built
up in eight steps

00:09:17.810 --> 00:09:21.230
to form the 16 carbon
fatty-acid palmytate.

00:09:21.230 --> 00:09:23.210
Following the upward
arrow, palmytate

00:09:23.210 --> 00:09:25.760
can go on to form a
phospholipid, which

00:09:25.760 --> 00:09:28.400
will become an integral
part of the membrane,

00:09:28.400 --> 00:09:31.340
or it can go
horizontally, to the left,

00:09:31.340 --> 00:09:33.290
to form a
triacylglyceride, which

00:09:33.290 --> 00:09:35.300
will become embedded
in a lipid globule

00:09:35.300 --> 00:09:38.690
where it can serve as a
storage depot for energy.

00:09:38.690 --> 00:09:40.760
So at this point,
through the series

00:09:40.760 --> 00:09:44.540
of reactions numbered
5A and 5B, we've

00:09:44.540 --> 00:09:48.270
made the membrane plasticizer,
cholesterol, phospholipids,

00:09:48.270 --> 00:09:50.030
which are the main
structural elements

00:09:50.030 --> 00:09:53.150
of a biological membrane,
and a triacylglycerides,

00:09:53.150 --> 00:09:56.870
which is our principal
energy storage molecule.

00:09:56.870 --> 00:10:00.500
Now let's go back to the
asterisked oxaloacetate,

00:10:00.500 --> 00:10:02.000
at step 3.

00:10:02.000 --> 00:10:04.790
This orphaned molecule
of oxaloacetate

00:10:04.790 --> 00:10:07.580
has to be able to get back
into the mitochondrion.

00:10:07.580 --> 00:10:09.860
There are two pathways by
which this can happen--

00:10:09.860 --> 00:10:11.690
4A and 4B.

00:10:11.690 --> 00:10:14.000
Oxaloacetate is a ketone.

00:10:14.000 --> 00:10:18.590
It can be reduced by NADH
to form the alcohol, malate.

00:10:18.590 --> 00:10:20.600
This is the reverse
of the reaction

00:10:20.600 --> 00:10:23.240
that we're used to
seeing in the TCA cycle.

00:10:23.240 --> 00:10:26.510
Nevertheless, this is indeed
the thermodynamically favorable

00:10:26.510 --> 00:10:28.490
direction for this reaction.

00:10:28.490 --> 00:10:30.920
The malate dehydrogenase
enzyme used,

00:10:30.920 --> 00:10:33.680
here, is closely related
to the one that's

00:10:33.680 --> 00:10:35.510
in the mitochondrial matrix.

00:10:35.510 --> 00:10:38.360
Keep in mind, however, that
this is the cytoplasmic version

00:10:38.360 --> 00:10:39.620
of the enzyme.

00:10:39.620 --> 00:10:41.420
I mentioned earlier
that malate can

00:10:41.420 --> 00:10:44.870
go in either direction across
the mitochondrial membrane.

00:10:44.870 --> 00:10:49.250
In this case, path 4A, malate
goes into the mitochondrian,

00:10:49.250 --> 00:10:52.280
becomes part of the
mitochondrial malate pool,

00:10:52.280 --> 00:10:54.920
and then the mitochondrial
malate dehydrogenase

00:10:54.920 --> 00:10:57.350
will convert it to oxaloacetate.

00:10:57.350 --> 00:11:00.470
And thus, we have restored
the molecule of oxaloacetate

00:11:00.470 --> 00:11:04.340
that we borrowed from
the mitochondrial matrix.

00:11:04.340 --> 00:11:06.710
Path 4B represents
an alternative way

00:11:06.710 --> 00:11:08.810
to return the
oxaloacetate molecule

00:11:08.810 --> 00:11:10.910
to the mitochondrial matrix.

00:11:10.910 --> 00:11:13.850
4B involves the
malic enzyme, ME,

00:11:13.850 --> 00:11:19.460
which uses NADP plus to oxidize
malate back to oxaloacetate.

00:11:19.460 --> 00:11:23.300
In the process, NADP
plus is reduced to NADPH,

00:11:23.300 --> 00:11:25.370
the biosynthetic precursor.

00:11:25.370 --> 00:11:27.860
This is one of
the two major ways

00:11:27.860 --> 00:11:31.880
that a cell produces the NADPH
that it needs for biosynthesis.

00:11:31.880 --> 00:11:36.350
The other way to make NADPH is
the pentose phosphate pathway.

00:11:36.350 --> 00:11:40.490
It may seem kind of useless
to have taken an oxaloacetate,

00:11:40.490 --> 00:11:43.790
and then, by using malate
dehydrogenase convert

00:11:43.790 --> 00:11:48.230
that oxaloacetate to malate, and
then, convert the malate back

00:11:48.230 --> 00:11:49.670
to oxaloacetate.

00:11:49.670 --> 00:11:51.740
But by doing this
series of reactions,

00:11:51.740 --> 00:11:57.470
you are effectively
converting NADH to an NADPH.

00:11:57.470 --> 00:12:01.530
And NADPH is desperately needed
for biosynthesis, as we'll see.

00:12:01.530 --> 00:12:05.480
Lipid biosynthesis is
very NADPH intensive.

00:12:05.480 --> 00:12:07.470
Let's look at a couple
of more details.

00:12:07.470 --> 00:12:10.320
Decyl acetate shown
in brackets in step 4B

00:12:10.320 --> 00:12:12.920
exists on the malic enzyme.

00:12:12.920 --> 00:12:15.620
The enzyme facilitates
the decarboxylation

00:12:15.620 --> 00:12:16.430
of that molecule.

00:12:16.430 --> 00:12:20.000
Remember, oxaloacetate
is a beta keto acid.

00:12:20.000 --> 00:12:24.740
Decarboxylation by malic enzyme,
at step 4B, produces pyruvate.

00:12:24.740 --> 00:12:29.390
Now we can follow the pyruvate,
by the continuation of step 4B,

00:12:29.390 --> 00:12:30.890
into the mitochondrion.

00:12:30.890 --> 00:12:34.280
The pyruvate becomes a substrate
for pyruvate carboxylase,

00:12:34.280 --> 00:12:35.790
which we've looked at before.

00:12:35.790 --> 00:12:39.860
Pyruvate carboxylase will
put a CO2 back into pyruvate

00:12:39.860 --> 00:12:43.410
and convert it
into oxaloacetate.

00:12:43.410 --> 00:12:46.760
Pyruvate carboxylase is
an ATP requiring enzyme.

00:12:46.760 --> 00:12:50.840
Hence, path 4B has a
finite energy requirement.

00:12:50.840 --> 00:12:53.360
It would seem as though
this might be a problem.

00:12:53.360 --> 00:12:56.030
Nevertheless, when a cell
is doing biosynthesis,

00:12:56.030 --> 00:13:00.120
it usually has a lot of energy
around in the form of ATP.

00:13:00.120 --> 00:13:02.330
So this small investment
is a good one,

00:13:02.330 --> 00:13:05.690
in order to, first of all,
restore the oxaloacetate

00:13:05.690 --> 00:13:09.110
balance of the mitochondria,
and secondly, keep in mind,

00:13:09.110 --> 00:13:12.440
that 4B also gives you
an NADPH, which you

00:13:12.440 --> 00:13:14.870
need for biosynthesis, anyway.

00:13:14.870 --> 00:13:16.430
Let's summarize this figure.

00:13:16.430 --> 00:13:18.910
At the upper left, we
start with glucose,

00:13:18.910 --> 00:13:21.680
and by the hatched lines,
that glucose is converted

00:13:21.680 --> 00:13:23.780
to citrate in the cytoplasm.

00:13:23.780 --> 00:13:26.330
Citrate then yields
an acetyl CoA molecule

00:13:26.330 --> 00:13:29.810
that is the precursor to
fatty acids and cholesterol.

00:13:29.810 --> 00:13:33.350
Step two dealt with the problem
of getting oxaloacetate back

00:13:33.350 --> 00:13:35.150
into the mitochondrion.

00:13:35.150 --> 00:13:36.950
Remember, fatty-acid
biosynthesis

00:13:36.950 --> 00:13:38.930
happens in the cytoplasm.

00:13:38.930 --> 00:13:42.290
We borrowed an oxaloacetate
from the mitochondrion, hence,

00:13:42.290 --> 00:13:44.480
we have to get it back
to where it belongs

00:13:44.480 --> 00:13:46.400
in the mitochondrial matrix.

00:13:46.400 --> 00:13:49.970
Paths 4A and 4B represent
alternative pathways

00:13:49.970 --> 00:13:53.210
for getting the oxaloacetate
back where it belongs.

00:13:53.210 --> 00:13:56.270
Once the oxaloacetate is
back in the mitochondrion,

00:13:56.270 --> 00:13:58.580
then the next
molecule of acetyl CoA

00:13:58.580 --> 00:14:01.130
can come in, get
converted to citrate,

00:14:01.130 --> 00:14:04.340
and follow the hatched line
path, ultimately resulting

00:14:04.340 --> 00:14:06.560
in synthesis of lipids.

00:14:06.560 --> 00:14:11.450
Let's turn now to story board
26, panel A. At this point,

00:14:11.450 --> 00:14:15.240
we have a pool of acetyl
coenzyme A in the cytoplasm.

00:14:15.240 --> 00:14:18.350
It's now ready to start
making a fatty acid.

00:14:18.350 --> 00:14:20.180
In the carboxylase
module, I talked

00:14:20.180 --> 00:14:23.450
about how acetyl
CoA carboxylase is

00:14:23.450 --> 00:14:26.700
one of the carboxylases
took an active form of CO2

00:14:26.700 --> 00:14:28.850
and placed it onto
the methyl carbon

00:14:28.850 --> 00:14:31.950
at the end of the acetyl
coenzyme A molecule.

00:14:31.950 --> 00:14:33.920
The product of the
carboxylase reaction

00:14:33.920 --> 00:14:37.760
is malonyl coenzyme
A. This carboxylase

00:14:37.760 --> 00:14:41.720
is a biotin containing
enzyme, and requires ATP.

00:14:41.720 --> 00:14:43.880
As I've said
earlier, malonyl CoA

00:14:43.880 --> 00:14:46.250
is going to be the source of
all, but two of the carbons

00:14:46.250 --> 00:14:48.090
that make up the fatty acid.

00:14:48.090 --> 00:14:50.180
The initial fatty acid
that we're going to make

00:14:50.180 --> 00:14:52.940
is the C16 fatty
acid, palmitate.

00:14:52.940 --> 00:14:55.130
Palmitate will be
synthesized entirely

00:14:55.130 --> 00:14:57.640
on the fatty acid synthase.

00:14:57.640 --> 00:15:00.130
Let's look at panel B.
Fatty-acid synthesis

00:15:00.130 --> 00:15:02.470
in bacteria is carried
out by a series

00:15:02.470 --> 00:15:04.900
of enzymes that work as
discrete, or independent,

00:15:04.900 --> 00:15:05.670
units.

00:15:05.670 --> 00:15:07.990
Collectively, this
ensemble of enzymes

00:15:07.990 --> 00:15:11.320
is called the fatty
acid synthase complex.

00:15:11.320 --> 00:15:14.560
In a million cells, all of
the enzymatic activities--

00:15:14.560 --> 00:15:17.830
the same enzymatic activities--
are present in a single, very

00:15:17.830 --> 00:15:19.124
long peptide.

00:15:19.124 --> 00:15:21.040
At this point, I want
to go over the chemistry

00:15:21.040 --> 00:15:24.520
behind each of the activities
in the fatty acid synthase

00:15:24.520 --> 00:15:26.550
complex.

00:15:26.550 --> 00:15:29.300
One of the peptides that I
introduced, briefly, above

00:15:29.300 --> 00:15:33.080
is called ACP, or
acyl carrier protein.

00:15:33.080 --> 00:15:35.390
It contains assisting
sulfhydryl residue that

00:15:35.390 --> 00:15:38.420
will attack the carbonyl
group of malonyl CoA,

00:15:38.420 --> 00:15:42.470
forming a malonyl ACP adduct.

00:15:42.470 --> 00:15:44.340
Keep in mind that
the product is still

00:15:44.340 --> 00:15:46.951
a thioester with all of
the chemical properties

00:15:46.951 --> 00:15:48.367
you would find in
acetyl coenzyme.

00:15:48.367 --> 00:15:51.830
A The enzymatic subunit
that does this chemistry

00:15:51.830 --> 00:15:57.110
is called MAT, or mat, for
malonyl acetyl transferase.

00:15:57.110 --> 00:16:00.080
Another domain, shown at
the bottom of the fatty acid

00:16:00.080 --> 00:16:02.120
synthase, has a
cysteine residue that

00:16:02.120 --> 00:16:04.940
attacks the carbonyl
group of acetyl CoA.

00:16:04.940 --> 00:16:07.670
This reaction is also
catalyzed by MAT.

00:16:07.670 --> 00:16:09.920
Hence, in this case,
the MAT subunit

00:16:09.920 --> 00:16:14.750
forms an acetyl thioester
with the fatty acid synthase.

00:16:14.750 --> 00:16:18.290
At this point, we have a malonyl
group at the-- let's call it,

00:16:18.290 --> 00:16:20.150
northern domain of the
fatty acid synthase,

00:16:20.150 --> 00:16:21.490
as I've drawn it--

00:16:21.490 --> 00:16:25.220
and an acetyl group at what
I'll call, the southern domain.

00:16:25.220 --> 00:16:27.230
Let's look at panel
C. At this point,

00:16:27.230 --> 00:16:29.510
the fatty acid synthase
is fully primed.

00:16:29.510 --> 00:16:33.290
That is, it is loaded with a
malonyl group and acetyl group.

00:16:33.290 --> 00:16:34.850
Now we can start
doing some chemistry

00:16:34.850 --> 00:16:36.920
to join these two
groups together.

00:16:36.920 --> 00:16:40.310
The malonyl group has a beta
keto acid functionality so

00:16:40.310 --> 00:16:43.910
can easily lose CO2, and
generate a nucleophile that

00:16:43.910 --> 00:16:46.490
will then go down and
attack the carbonyl group

00:16:46.490 --> 00:16:49.280
of the acetyl residue that
is on the southern domain

00:16:49.280 --> 00:16:50.180
of the enzyme.

00:16:50.180 --> 00:16:52.940
This reaction results
in the acetyl group,

00:16:52.940 --> 00:16:56.510
from the southern domain,
replacing the CO2 moiety

00:16:56.510 --> 00:16:58.070
on the northern domain.

00:16:58.070 --> 00:17:01.100
That is, the carbonyl group
from the southern domain

00:17:01.100 --> 00:17:04.609
ends up covalently
attached to the CH2 group,

00:17:04.609 --> 00:17:07.640
with the box over it,
in the northern domain.

00:17:07.640 --> 00:17:12.220
The ketoacyl synthase, that
is KS, domain of the protein,

00:17:12.220 --> 00:17:13.819
does this reaction.

00:17:13.819 --> 00:17:17.450
The product of the reaction
shows a beta keto acyl group

00:17:17.450 --> 00:17:19.700
attached to the northern domain.

00:17:19.700 --> 00:17:21.829
Keep in mind, that
the beta keto acyl

00:17:21.829 --> 00:17:25.980
group is attached, specifically,
to the ACL carrier protein.

00:17:25.980 --> 00:17:28.820
The next step is catalyzed
by the ketoacyl ACP

00:17:28.820 --> 00:17:30.590
reductase, or KR.

00:17:30.590 --> 00:17:34.490
In this case, NADPH will
reduce the keto group

00:17:34.490 --> 00:17:36.410
that is next to the
terminal carbon,

00:17:36.410 --> 00:17:40.120
forming an alcohol,
specifically, beta hydroxy

00:17:40.120 --> 00:17:42.330
acyl carrier protein, or ACP.

00:17:42.330 --> 00:17:47.120
This beta hydroxyl four
carbon compound is now primed

00:17:47.120 --> 00:17:48.920
for dehydration
by a dehydratase,

00:17:48.920 --> 00:17:55.130
which removes water to form
the trans-delta-2-enoyl ACP.

00:17:55.130 --> 00:17:58.190
This molecule is an alkyne
with the double bond

00:17:58.190 --> 00:18:02.680
between the second and third
carbons from the thioester.

00:18:02.680 --> 00:18:05.560
Let's turn now to storyboard 27.

00:18:05.560 --> 00:18:09.520
The reaction continues
with enoyl reductase, ER,

00:18:09.520 --> 00:18:12.790
which uses a second
molecule of NADPH

00:18:12.790 --> 00:18:17.050
in order to saturate the
double bond of the enoyl ACP.

00:18:17.050 --> 00:18:21.110
The product is a butyryl
ACP at this point,

00:18:21.110 --> 00:18:23.500
it should be clear as
to what we've done.

00:18:23.500 --> 00:18:26.230
We've taken two
acetyl CoA molecules

00:18:26.230 --> 00:18:28.090
and fused them together.

00:18:28.090 --> 00:18:30.580
In the process we've done
a number of reductions,

00:18:30.580 --> 00:18:32.680
and the product is
a pure hydrocarbon.

00:18:32.680 --> 00:18:36.400
In this case, the four
carbon butyryl coenzyme A.

00:18:36.400 --> 00:18:38.470
In order for the cycle
to repeat itself,

00:18:38.470 --> 00:18:40.840
we have to vacate the
ACP, or northern site

00:18:40.840 --> 00:18:43.390
on fatty acid synthase,
in order to make

00:18:43.390 --> 00:18:45.910
it available for the next
molecule of malonyl CoA

00:18:45.910 --> 00:18:46.990
to come in.

00:18:46.990 --> 00:18:50.710
That's done by the enzyme, or
subunit, called translocase,

00:18:50.710 --> 00:18:53.890
which simply moves the butyryl
group from the northern site

00:18:53.890 --> 00:18:55.420
down to the southern site.

00:18:55.420 --> 00:18:59.200
The northern sulfhydryl on ACP,
of the fatty acid synthase,

00:18:59.200 --> 00:19:02.770
is now available to connect
with the next incoming molecule

00:19:02.770 --> 00:19:06.250
of malonyl coenzyme A. The
biosynthetic cycle will now

00:19:06.250 --> 00:19:10.090
repeat itself six times in
order to form the 16 carbon long

00:19:10.090 --> 00:19:11.980
hydrocarbon, palmitate.

00:19:11.980 --> 00:19:14.680
In the figure, its shown
attached to the northern site

00:19:14.680 --> 00:19:16.840
of the fatty acid synthase.

00:19:16.840 --> 00:19:19.690
Let's look back at the
overall synthesis scheme.

00:19:19.690 --> 00:19:22.000
What you'll see is that the
terminal two carbons, that

00:19:22.000 --> 00:19:24.850
is the two carbons furthest
to the right in the molecule,

00:19:24.850 --> 00:19:26.680
came from acetyl CoA.

00:19:26.680 --> 00:19:28.720
But all the other
carbons originally

00:19:28.720 --> 00:19:32.380
came from alanyl
coenzyme A. The last step

00:19:32.380 --> 00:19:35.590
in the overall reaction
scheme involves thioesterase--

00:19:35.590 --> 00:19:39.460
transferring the thioester bound
palmitate to a water molecule,

00:19:39.460 --> 00:19:42.860
forming palmitic acid,
which is released.

00:19:42.860 --> 00:19:45.110
Now let's turn to storyboard 28.

00:19:45.110 --> 00:19:47.900
As I mentioned earlier, the
default length of a fatty acid

00:19:47.900 --> 00:19:49.160
is 16 carbons.

00:19:49.160 --> 00:19:50.259
That is palmitic acid.

00:19:50.259 --> 00:19:51.800
You can look in the
book for pathways

00:19:51.800 --> 00:19:53.930
leading to elongation,
desaturation,

00:19:53.930 --> 00:19:56.372
and branching of the
parent fatty acid chain.

00:19:56.372 --> 00:19:58.580
I'd like to give you, however,
a little bit of detail

00:19:58.580 --> 00:20:00.620
on how membrane
lipids are formed,

00:20:00.620 --> 00:20:02.900
as well as triacylglycerides.

00:20:02.900 --> 00:20:05.420
Let's look at panel A. To
make these higher order

00:20:05.420 --> 00:20:08.900
lipids, fatty acids will become
connected to a three carbon

00:20:08.900 --> 00:20:10.220
glycerol backbone.

00:20:10.220 --> 00:20:11.900
The glycerol
backbone is made from

00:20:11.900 --> 00:20:13.880
dihydroxyacetone
phosphate, which

00:20:13.880 --> 00:20:16.910
is one of the intermediates
in the glycolytic pathway.

00:20:16.910 --> 00:20:20.240
To make the lipid backbone,
dihydroxyacetone phosphate

00:20:20.240 --> 00:20:23.570
is reduced by glycerol 3
phosphate dehydrogenase

00:20:23.570 --> 00:20:26.180
using NADH as the co-factor.

00:20:26.180 --> 00:20:29.900
The product of this reaction
is glycerol 3 phosphate.

00:20:29.900 --> 00:20:34.010
An acyltransferase will then
take a fatty acyl coenzyme A

00:20:34.010 --> 00:20:36.770
and place it on one
of the hydroxyl groups

00:20:36.770 --> 00:20:41.030
of the glycerol backbone to form
a monoacylglyceride phosphate.

00:20:41.030 --> 00:20:44.840
Then, as shown in panel B,
a second fatty acid group

00:20:44.840 --> 00:20:47.780
will be placed on the
only available hydroxyl

00:20:47.780 --> 00:20:51.260
to form a diacylglyceride
phosphate.

00:20:51.260 --> 00:20:55.640
This is otherwise known as
a membrane phospholipid.

00:20:55.640 --> 00:20:59.660
If the biosynthetic objective
is to make a triacylglyceride,

00:20:59.660 --> 00:21:02.660
which is of course our major
storage form of energy,

00:21:02.660 --> 00:21:05.360
then a phosphatase will
come in and hydrolyze off

00:21:05.360 --> 00:21:07.040
the phosphate from
the three carbon

00:21:07.040 --> 00:21:09.320
of the diacylglyceride
phosphate to form

00:21:09.320 --> 00:21:12.350
what's called a diacylglycerol.

00:21:12.350 --> 00:21:14.390
Then an acyltransferase,
as above,

00:21:14.390 --> 00:21:17.900
will place a third fatty acid
onto the glycerol backbone

00:21:17.900 --> 00:21:21.320
to form a
triacylglyceride, or TAG.

00:21:21.320 --> 00:21:23.930
As I said above, this is
our principal storage form

00:21:23.930 --> 00:21:25.600
of energy.