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JOHN ESSIGMANN: The second
carbohydrate biosynthetic

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pathway we're going to look
at is called gluconeogenesis.

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00:00:26,700 --> 00:00:29,130
Gluconeogenesis is the
synthesis of glucose

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00:00:29,130 --> 00:00:31,500
from noncarbohydrate precursors.

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00:00:31,500 --> 00:00:35,730
Let's look at Panel A of this
storyboard, Storyboard 31.

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There are some organs in the
body that require glucose

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00:00:38,540 --> 00:00:40,560
as their metabolic
fuel, yet they

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00:00:40,560 --> 00:00:44,220
don't have the capacity to
make it, or to make much of it.

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00:00:44,220 --> 00:00:48,030
Examples are the brain, renal
medulla, red blood cells,

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00:00:48,030 --> 00:00:49,960
and the testes.

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00:00:49,960 --> 00:00:52,810
To give an idea of the scale
of the problem the body

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00:00:52,810 --> 00:00:57,130
faces, we only have about 200
grams of glucose or glycogen

20
00:00:57,130 --> 00:01:01,420
stored away in our body, and the
brain alone uses more than half

21
00:01:01,420 --> 00:01:03,550
of that each day,
so we don't have

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00:01:03,550 --> 00:01:06,370
much in the way of a
carbohydrate reserve.

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00:01:06,370 --> 00:01:09,640
The body compensates by having
certain organs, specifically

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00:01:09,640 --> 00:01:14,260
the liver and renal cortex, act
as specialized manufacturing

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00:01:14,260 --> 00:01:17,320
centers to produce
and export glucose.

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00:01:17,320 --> 00:01:19,660
The chemical raw
materials they use

27
00:01:19,660 --> 00:01:23,440
include organic acids such
as lactate, some amino acids,

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00:01:23,440 --> 00:01:26,380
and the three-carbon residues
from odd-chain fatty acids

29
00:01:26,380 --> 00:01:28,600
that enter catabolism.

30
00:01:28,600 --> 00:01:30,730
To get an idea of
how gluconeogenesis

31
00:01:30,730 --> 00:01:32,680
works, let's take
a look at the box

32
00:01:32,680 --> 00:01:34,560
here at the bottom of Panel A.

33
00:01:34,560 --> 00:01:37,300
You see a muscle working very
hard, as evidenced by the fact

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00:01:37,300 --> 00:01:40,340
that it's secreting
lactate out into the blood.

35
00:01:40,340 --> 00:01:42,400
The lactate travels
from the working muscle

36
00:01:42,400 --> 00:01:46,390
by the blood to the liver,
where the lactate is taken in.

37
00:01:46,390 --> 00:01:49,240
The pathway of
gluconeogenesis, with energy

38
00:01:49,240 --> 00:01:52,810
and reducing equivalent
input, rebuilds glucose

39
00:01:52,810 --> 00:01:54,910
from that lactate precursor.

40
00:01:54,910 --> 00:01:58,030
The glucose is then sent back
out into the blood, returns

41
00:01:58,030 --> 00:02:01,750
to the muscle, enabling the
muscle to continue hard work.

42
00:02:01,750 --> 00:02:04,300
As I've described
earlier in 5.07,

43
00:02:04,300 --> 00:02:07,720
this functional relationship
between a muscle and the liver

44
00:02:07,720 --> 00:02:10,240
is called the Cori Cycle.

45
00:02:10,240 --> 00:02:12,950
Let's look at Panel B.
Depending on how you look at it,

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00:02:12,950 --> 00:02:14,350
there are seven
or possibly eight

47
00:02:14,350 --> 00:02:18,130
noncarbohydrate precursors to
glucose in gluconeogenesis.

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00:02:18,130 --> 00:02:19,522
They're listed in this panel.

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00:02:19,522 --> 00:02:20,980
And I've indicated
these precursors

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00:02:20,980 --> 00:02:23,350
by numbers 1 through 8.

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00:02:23,350 --> 00:02:25,490
The first seven are
not carbohydrates,

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00:02:25,490 --> 00:02:29,050
so they qualify as
noncarbohydrate precursors

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00:02:29,050 --> 00:02:33,430
as fits the definition for
a gluconeogenic compound.

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00:02:33,430 --> 00:02:35,290
The eighth is actually
a carbohydrate.

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00:02:35,290 --> 00:02:36,660
It's ribose.

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00:02:36,660 --> 00:02:39,820
I'm semi-illegally
squeezing it in here

57
00:02:39,820 --> 00:02:43,450
because ribose acts as a major
gluconeogenesis precursor

58
00:02:43,450 --> 00:02:46,960
by way of the Pentose
Phosphate Pathway, or PPP.

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00:02:46,960 --> 00:02:50,080
We haven't covered the
Pentose Phosphate Pathway yet,

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00:02:50,080 --> 00:02:51,760
but we'll see, when
we do cover it,

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00:02:51,760 --> 00:02:55,870
exactly how this pathway
works to manufacture glucose.

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00:02:55,870 --> 00:02:58,120
In other words,
ribose from the diet

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00:02:58,120 --> 00:03:01,240
can be converted into glucose
by way of the Pentose Phosphate

64
00:03:01,240 --> 00:03:04,832
Pathway coupled to
gluconeogenesis.

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00:03:04,832 --> 00:03:06,290
A little bit later
in this lecture,

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00:03:06,290 --> 00:03:08,560
we'll see how each of
these precursors-- that is,

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00:03:08,560 --> 00:03:09,910
numbers 1 through 8--

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00:03:09,910 --> 00:03:13,930
are converted to glucose by
the pathway of gluconeogenesis.

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00:03:13,930 --> 00:03:16,600
The best way to start
thinking about gluconeogenesis

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00:03:16,600 --> 00:03:19,960
is to think of it as
glycolysis running in reverse.

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00:03:19,960 --> 00:03:22,690
As shown here in
Panel C, glycolysis

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00:03:22,690 --> 00:03:26,020
is conversion of glucose,
ultimately, to pyruvate.

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00:03:26,020 --> 00:03:29,950
As we know, that pathway goes
with a net generation of ATP.

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00:03:29,950 --> 00:03:32,020
That is, it is exergonic.

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00:03:32,020 --> 00:03:32,990
Accordingly.

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00:03:32,990 --> 00:03:34,930
If we go from pyruvate
back to glucose,

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00:03:34,930 --> 00:03:38,140
we know that in order to make
that reverse pathway exergonic,

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00:03:38,140 --> 00:03:40,870
we're going to need
to have energy input.

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00:03:40,870 --> 00:03:43,540
As we learned earlier, there
are 10 steps in glycolysis

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00:03:43,540 --> 00:03:45,050
going from glucose to pyruvate.

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00:03:45,050 --> 00:03:48,370
Three of those steps are
thermodynamically irreversible.

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00:03:48,370 --> 00:03:50,890
Those are the steps
that commit molecules

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00:03:50,890 --> 00:03:54,490
to flow from left to right,
as drawn in this panel.

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00:03:54,490 --> 00:03:56,710
The thermodynamically
irreversible steps

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00:03:56,710 --> 00:04:00,610
of glycolysis are first,
hexokinase/glucokinase,

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00:04:00,610 --> 00:04:03,520
that is, the conversion of
glucose to glucose 6-phosphate.

87
00:04:03,520 --> 00:04:06,760
The second irreversible step
is phosphofructokinase 1,

88
00:04:06,760 --> 00:04:10,798
the conversion of fructose
6-phosphate to fructose 1,

89
00:04:10,798 --> 00:04:12,310
6-bisphosphate.

90
00:04:12,310 --> 00:04:15,430
The third irreversible
step is pyruvate kinase,

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00:04:15,430 --> 00:04:18,040
the conversion of
phosphoenolpyruvate

92
00:04:18,040 --> 00:04:19,149
to pyruvate.

93
00:04:19,149 --> 00:04:21,579
We have to find a way
in gluconeogenesis

94
00:04:21,579 --> 00:04:25,300
to bypass these three
thermodynamically irreversible

95
00:04:25,300 --> 00:04:26,530
steps.

96
00:04:26,530 --> 00:04:29,260
The enzymes that were invented
to circumvent the three

97
00:04:29,260 --> 00:04:32,590
thermodynamically irreversible
steps of glycolysis

98
00:04:32,590 --> 00:04:34,960
are shown in green
asterisks in this panel.

99
00:04:34,960 --> 00:04:37,240
For example, glucose
6-phosphatase

100
00:04:37,240 --> 00:04:40,930
hydrolyzes the sixth phosphate
from glucose 6-phosphate,

101
00:04:40,930 --> 00:04:42,570
converting it to glucose.

102
00:04:42,570 --> 00:04:46,230
A second asterisked enzyme of
gluconeogenesis is fructose

103
00:04:46,230 --> 00:04:49,690
1,6-bisphosphatase,
which converts fructose

104
00:04:49,690 --> 00:04:54,250
1,6-bisphosphate by hydrolysis
to fructose 6-phosphate.

105
00:04:54,250 --> 00:04:56,770
The third irreversible
step that we need to bypass

106
00:04:56,770 --> 00:04:58,540
is pyruvate kinase.

107
00:04:58,540 --> 00:05:01,600
Two enzymes were invented
to enable this bypass.

108
00:05:01,600 --> 00:05:03,490
Pyruvate carboxylase,
which we have

109
00:05:03,490 --> 00:05:06,640
studied before in
several contexts in 5.07,

110
00:05:06,640 --> 00:05:09,310
and a new enzyme,
phosphoenolpyruvate

111
00:05:09,310 --> 00:05:11,480
carboxykinase, or PEPCK.

112
00:05:11,480 --> 00:05:14,410
Pyruvate, carboxylase,
and PEPCK work

113
00:05:14,410 --> 00:05:17,080
in a partnership with
several other enzymes,

114
00:05:17,080 --> 00:05:20,680
actually in a mini-pathway,
to bypass the pyruvate kinase

115
00:05:20,680 --> 00:05:24,930
step of glycolysis, and
hence, enable gluconeogenesis.

116
00:05:24,930 --> 00:05:27,100
Lastly, at the
right of this panel

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00:05:27,100 --> 00:05:31,450
is a large inverted L. This
is meant to symbolize the 8

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00:05:31,450 --> 00:05:33,760
precursors to gluconeogenesis.

119
00:05:33,760 --> 00:05:36,365
These precursors will enter
the gluconeogenesis pathway,

120
00:05:36,365 --> 00:05:39,160
and flow by the
hatched arrow lines

121
00:05:39,160 --> 00:05:41,860
all the way up to glucose.

122
00:05:41,860 --> 00:05:45,490
This network of reactions allows
the conversion of molecules,

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00:05:45,490 --> 00:05:47,920
such as the amino acid
glutamate, all the way

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00:05:47,920 --> 00:05:49,490
to glucose.

125
00:05:49,490 --> 00:05:52,270
Now we're going to
turn to Storyboard 32.

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00:05:52,270 --> 00:05:54,610
Let's look at Panel
A. And let's start out

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00:05:54,610 --> 00:05:58,130
by looking at the mechanisms
by which the four key enzymes

128
00:05:58,130 --> 00:06:00,190
of gluconeogenesis works.

129
00:06:00,190 --> 00:06:03,980
The first two enzymes, glucose
6-phosphatase and glucose

130
00:06:03,980 --> 00:06:08,170
1,6-bisphosphatase, carry
out simple phosphate-ester

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00:06:08,170 --> 00:06:10,650
hydrolyses as shown.

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00:06:10,650 --> 00:06:13,110
The third enzyme is
pyruvate carboxylase.

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00:06:13,110 --> 00:06:14,970
We looked at this
enzyme mechanistically

134
00:06:14,970 --> 00:06:18,390
back in the fatty acid
catabolism lectures,

135
00:06:18,390 --> 00:06:20,700
when I taught about the
mechanisms by which CO2

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00:06:20,700 --> 00:06:24,030
can be captured and
added to an organic acid.

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00:06:24,030 --> 00:06:25,890
Pyruvate carboxylase
will convert

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00:06:25,890 --> 00:06:27,930
pyruvate to oxaloacetate.

139
00:06:27,930 --> 00:06:30,060
Hence, it's in an
anaplerotic enzyme

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00:06:30,060 --> 00:06:34,170
that helps maintain the
levels of TCA intermediates.

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00:06:34,170 --> 00:06:35,870
In the cases we've
looked at so far,

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00:06:35,870 --> 00:06:40,290
PC helps maintain specifically
oxaloacetate levels.

143
00:06:40,290 --> 00:06:42,370
Refer back to the
lecture on carboxylases

144
00:06:42,370 --> 00:06:46,080
to see how this enzyme works
from a mechanistic perspective.

145
00:06:46,080 --> 00:06:50,340
Now let's look at Panel B.
The last and fourth key enzyme

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00:06:50,340 --> 00:06:52,650
I want to discuss
in gluconeogenesis

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00:06:52,650 --> 00:06:57,150
is phosphoenolpyruvate
carboxykinase, or PEPCK.

148
00:06:57,150 --> 00:06:59,730
We haven't seen an enzyme
that works like this before,

149
00:06:59,730 --> 00:07:02,790
so let me go through its
mechanism in some detail.

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00:07:02,790 --> 00:07:05,490
PEPCK is going to
convert oxaloacetate

151
00:07:05,490 --> 00:07:07,500
to phosphoenolpyruvate.

152
00:07:07,500 --> 00:07:10,260
The enzyme is active in
the liver and renal cortex

153
00:07:10,260 --> 00:07:13,560
under physiological conditions
that mandate gluconeogenesis

154
00:07:13,560 --> 00:07:15,030
be turned on.

155
00:07:15,030 --> 00:07:18,060
The enzyme takes oxaloacetate,
which, as we know,

156
00:07:18,060 --> 00:07:19,560
is a beta keto acid.

157
00:07:19,560 --> 00:07:22,500
Thus, it's prone
to decarboxylation.

158
00:07:22,500 --> 00:07:25,950
When CO2 is removed
from oxaloacetate,

159
00:07:25,950 --> 00:07:30,450
you get a transient pyruvate
enolate, which is an anion.

160
00:07:30,450 --> 00:07:33,360
The oxygen anion of
the pyruvate enolate

161
00:07:33,360 --> 00:07:35,250
will attack the
gamma phosphate, that

162
00:07:35,250 --> 00:07:37,800
is, the terminal
phosphate of GTP,

163
00:07:37,800 --> 00:07:41,280
and thus, the pyruvate enolate
will become phosphorylated.

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00:07:41,280 --> 00:07:46,560
The product of phosphorylation
is phosphoenolpyruvate, or PEP.

165
00:07:46,560 --> 00:07:49,470
Hence, the concerted
action of two enzymes,

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00:07:49,470 --> 00:07:52,980
pyruvate carboxylase
and PEPCK, enables

167
00:07:52,980 --> 00:07:55,980
reversal of the
otherwise irreversible PK

168
00:07:55,980 --> 00:07:58,770
step, the pyruvate kinase
step, and allows the cell

169
00:07:58,770 --> 00:08:01,260
to do gluconeogenesis.

170
00:08:01,260 --> 00:08:03,570
Let's look now at
Panel C. Now that we've

171
00:08:03,570 --> 00:08:05,970
seen the identities of
the eight precursors

172
00:08:05,970 --> 00:08:08,700
to glucose in
gluconeogenesis, and we've

173
00:08:08,700 --> 00:08:11,880
done a little bit of learning
about the mechanisms of some

174
00:08:11,880 --> 00:08:13,920
of the key enzymes
in the pathway,

175
00:08:13,920 --> 00:08:15,630
we can turn our
attention to a network

176
00:08:15,630 --> 00:08:18,540
analysis of the overall
gluconeogenic pathway.

177
00:08:18,540 --> 00:08:21,570
The pathway in this panel
may seem a bit daunting,

178
00:08:21,570 --> 00:08:23,910
because it's pretty
much everything

179
00:08:23,910 --> 00:08:27,040
we've learned about
metabolism so far in 5.07.

180
00:08:27,040 --> 00:08:29,880
So let's go through it
slowly in little pieces.

181
00:08:29,880 --> 00:08:32,760
Let's start over on the left,
where the glucose molecule is

182
00:08:32,760 --> 00:08:36,120
situated in a squiggly box.

183
00:08:36,120 --> 00:08:38,110
Note that the glucose
in the squiggly box

184
00:08:38,110 --> 00:08:40,100
is being sent out of the cell--

185
00:08:40,100 --> 00:08:42,945
that is, it's going in
the opposite direction

186
00:08:42,945 --> 00:08:45,320
from the direction in which
we have dealt with glucose so

187
00:08:45,320 --> 00:08:46,910
far in metabolism.

188
00:08:46,910 --> 00:08:49,820
Usually, we think of glucose
as coming into the cell

189
00:08:49,820 --> 00:08:51,380
and entering metabolism.

190
00:08:51,380 --> 00:08:53,450
This is a liver
cell or renal cortex

191
00:08:53,450 --> 00:08:55,820
cell that's in the
process of manufacturing

192
00:08:55,820 --> 00:08:58,400
glucose from those eight
precursors, the ones I

193
00:08:58,400 --> 00:09:01,310
mentioned above, and sending
the manufactured glucose out

194
00:09:01,310 --> 00:09:02,570
into the blood.

195
00:09:02,570 --> 00:09:05,060
In this metabolic map,
the hatch lines-- that is,

196
00:09:05,060 --> 00:09:07,040
the lines that look
like railroad tracks--

197
00:09:07,040 --> 00:09:09,380
represent that
gluconeogenic pathways.

198
00:09:09,380 --> 00:09:11,930
Now let's start walking
through the metabolic map.

199
00:09:11,930 --> 00:09:15,040
Slightly to the right of
the middle of the diagram,

200
00:09:15,040 --> 00:09:18,170
you see the amino acid alanine
and the three-carbon organic

201
00:09:18,170 --> 00:09:19,610
acid lactate.

202
00:09:19,610 --> 00:09:22,130
Lactate is precursor
number one in my scheme.

203
00:09:22,130 --> 00:09:23,840
The numbering,
again, is in Panel B

204
00:09:23,840 --> 00:09:25,900
of the previous storyboard.

205
00:09:25,900 --> 00:09:28,570
And alanine is
precursor number two.

206
00:09:28,570 --> 00:09:30,110
Let's start with lactate.

207
00:09:30,110 --> 00:09:33,110
Lactate would typically come
into the liver from the blood,

208
00:09:33,110 --> 00:09:34,580
perhaps from a working muscle.

209
00:09:34,580 --> 00:09:37,240
Think about the Cori
Cycle for a moment.

210
00:09:37,240 --> 00:09:39,680
Lactate dehydrogenase
will convert the lactate

211
00:09:39,680 --> 00:09:43,310
into pyruvate in the liver
or in the renal cortex,

212
00:09:43,310 --> 00:09:46,130
in the other
gluconeogenic organ.

213
00:09:46,130 --> 00:09:48,110
Let's look next at alanine.

214
00:09:48,110 --> 00:09:51,500
Alanine, gluconeogenic
precursor number two,

215
00:09:51,500 --> 00:09:53,480
is, of course, an amino acid.

216
00:09:53,480 --> 00:09:57,620
And a PLP-mediated reaction
will convert it to pyruvate.

217
00:09:57,620 --> 00:10:00,350
That reaction involves
specifically the loss

218
00:10:00,350 --> 00:10:02,660
of alanine's amino group.

219
00:10:02,660 --> 00:10:05,090
Now imagine the
resulting pyruvate,

220
00:10:05,090 --> 00:10:07,500
that is, from either
lactate or alanine,

221
00:10:07,500 --> 00:10:10,900
translocating to the right
into the mitochondrion, where

222
00:10:10,900 --> 00:10:15,650
it's carboxylated by pyruvate
carboxylase into oxaloacetate.

223
00:10:15,650 --> 00:10:19,730
This is the reaction mechanism
we looked at a short time ago.

224
00:10:19,730 --> 00:10:22,880
Now remember that malate
dehydrogenase, or MDH,

225
00:10:22,880 --> 00:10:26,060
thermodynamically favors
the formation of malate

226
00:10:26,060 --> 00:10:28,310
from oxaloacetate.

227
00:10:28,310 --> 00:10:31,940
So our two molecules navigating
the gluconeogenic pathway--

228
00:10:31,940 --> 00:10:36,590
that is, precursors one and
two, transit from oxaloacetate

229
00:10:36,590 --> 00:10:38,330
into malate.

230
00:10:38,330 --> 00:10:41,180
Malate is in the
mitochondrion at this point.

231
00:10:41,180 --> 00:10:43,567
In the next step, it
exits the mitochondrion

232
00:10:43,567 --> 00:10:44,650
and goes into the cytosol.

233
00:10:44,650 --> 00:10:48,440
As I've mentioned before,
the transporter for malate

234
00:10:48,440 --> 00:10:50,400
works in both directions.

235
00:10:50,400 --> 00:10:53,150
Malate, now transported
into the cytosol,

236
00:10:53,150 --> 00:10:55,580
can be converted
back to oxaloacetate

237
00:10:55,580 --> 00:11:00,020
by the cytoplasmic version
of malate dehydrogenase.

238
00:11:00,020 --> 00:11:03,170
The oxaloacetate thus
produced in the cytosol

239
00:11:03,170 --> 00:11:05,540
goes upward, following
the vertical arrow.

240
00:11:05,540 --> 00:11:08,060
Lastly, PEPCK,
phosphoenolpyruvate

241
00:11:08,060 --> 00:11:11,600
carboxykinase, converts
the oxaloacetate

242
00:11:11,600 --> 00:11:15,050
into cytoplasmic
phosphoenolpyruvate.

243
00:11:15,050 --> 00:11:17,330
Let's pause here for
a minute and review.

244
00:11:17,330 --> 00:11:19,250
Look at where
phosphoenolpyruvate

245
00:11:19,250 --> 00:11:22,490
is on the metabolic map,
and then look to the right.

246
00:11:22,490 --> 00:11:24,020
On the right,
you'll see pyruvate

247
00:11:24,020 --> 00:11:27,170
on the rightward side
of the one-way arrow.

248
00:11:27,170 --> 00:11:30,410
You will also see lactate and
alanine, molecules one and two,

249
00:11:30,410 --> 00:11:32,600
respectively, having
their carbons flow

250
00:11:32,600 --> 00:11:34,700
into the pool of pyruvate.

251
00:11:34,700 --> 00:11:38,270
The pyruvate pool, which
starts in the cytosol,

252
00:11:38,270 --> 00:11:40,880
gets shuttled into the
mitochondrion, where

253
00:11:40,880 --> 00:11:43,580
pyruvate carboxylase converts
the molecules derived

254
00:11:43,580 --> 00:11:46,370
from pyruvate-- that is,
lactate and alanine--

255
00:11:46,370 --> 00:11:49,730
into mitochondrial oxaloacetate.

256
00:11:49,730 --> 00:11:54,560
Then the mitochondrial pool
is converted from oxaloacetate

257
00:11:54,560 --> 00:11:58,370
into malate, which
exits the mitochondrion.

258
00:11:58,370 --> 00:12:01,194
And the molecules now
represent a malate pool

259
00:12:01,194 --> 00:12:02,110
that's in the cytosol.

260
00:12:02,110 --> 00:12:07,710
That malate pool, which began as
pyruvate, alanine, and lactate,

261
00:12:07,710 --> 00:12:09,960
is converted to oxaloacetate.

262
00:12:09,960 --> 00:12:13,170
And lastly, our
new enzyme, PEPCK,

263
00:12:13,170 --> 00:12:16,120
converts the oxaloacetate
into cytoplasmic

264
00:12:16,120 --> 00:12:18,060
phosphoenolpyruvate.

265
00:12:18,060 --> 00:12:20,430
All that this was done
in order to bypass

266
00:12:20,430 --> 00:12:23,660
the one-way step between
phosphoenolpyruvate

267
00:12:23,660 --> 00:12:25,610
and pyruvate.

268
00:12:25,610 --> 00:12:29,390
In other words, I can go
from phosphoenolpyruvate

269
00:12:29,390 --> 00:12:32,630
to pyruvate quite easily
by pyruvate kinase.

270
00:12:32,630 --> 00:12:35,900
But because this reaction
is so strongly exothermic,

271
00:12:35,900 --> 00:12:38,420
I cannot go the other way.

272
00:12:38,420 --> 00:12:40,540
The reverse step
is irreversible.

273
00:12:40,540 --> 00:12:43,130
The mini-network that
we've just navigated

274
00:12:43,130 --> 00:12:46,550
with all of its steps,
some of which require ATP,

275
00:12:46,550 --> 00:12:50,150
was done in order to bypass the
irreversible pyruvate kinase

276
00:12:50,150 --> 00:12:51,480
reaction.

277
00:12:51,480 --> 00:12:55,280
We've just worked very hard
to move atoms from pyruvate,

278
00:12:55,280 --> 00:12:58,280
alanine, and lactate
into the cytosol, where

279
00:12:58,280 --> 00:13:02,870
those atoms are now packaged
as PEP, or phosphoenolpyruvate.

280
00:13:02,870 --> 00:13:05,820
Now, let's focus on
phosphoenolpyruvate,

281
00:13:05,820 --> 00:13:10,070
and let it flow backward through
the reverse of glycolysis all

282
00:13:10,070 --> 00:13:13,240
the way up to the next
irreversible step at fructose

283
00:13:13,240 --> 00:13:15,530
1,6-bisphosphate.

284
00:13:15,530 --> 00:13:18,770
All of the steps between
phosphoenolpyruvate

285
00:13:18,770 --> 00:13:22,250
and fructose 1,6-bisphosphate
are common to both

286
00:13:22,250 --> 00:13:25,370
gluconeogenesis and glycolysis.

287
00:13:25,370 --> 00:13:29,350
Fructose 1,6-bisphosphate
cannot be converted to fructose

288
00:13:29,350 --> 00:13:34,400
6-phosphate by reversal of the
PFK 1, phosphofructokinase 1,

289
00:13:34,400 --> 00:13:35,720
enzymatic step.

290
00:13:35,720 --> 00:13:38,780
That step is too
thermodynamically irreversible.

291
00:13:38,780 --> 00:13:43,070
Consequently, Nature invented
fructose 1,6-bisphosphatase.

292
00:13:43,070 --> 00:13:47,210
We saw the mechanism of that
enzyme earlier in the lecture.

293
00:13:47,210 --> 00:13:50,690
It allows the conversion of
fructose 1,6-bisphosphate

294
00:13:50,690 --> 00:13:52,400
to fructose 6-phosphate.

295
00:13:52,400 --> 00:13:56,000
Then the molecule continues
upstream to form glucose

296
00:13:56,000 --> 00:13:56,810
6-phosphate.

297
00:13:56,810 --> 00:14:00,740
And once again, a
gluconeogenic-specific enzyme,

298
00:14:00,740 --> 00:14:04,490
glucose 6-phosphatase, will
convert glucose 6-phosphate

299
00:14:04,490 --> 00:14:06,080
into glucose.

300
00:14:06,080 --> 00:14:09,920
Lastly, the manufactured glucose
is exported from the cell

301
00:14:09,920 --> 00:14:12,530
by the glucose transporter.

302
00:14:12,530 --> 00:14:16,490
So while it was a long
passage, the carbon atoms

303
00:14:16,490 --> 00:14:19,310
from alanine and lactate
have traveled all the way

304
00:14:19,310 --> 00:14:20,780
to glucose.

305
00:14:20,780 --> 00:14:23,150
And then the glucose has
been sent out of the cell.

306
00:14:23,150 --> 00:14:26,180
That is, their carbons are now
incorporated into the molecules

307
00:14:26,180 --> 00:14:29,210
of glucose that go off from
the liver and renal cortex

308
00:14:29,210 --> 00:14:32,000
to organs that need glucose
to meet their energy

309
00:14:32,000 --> 00:14:34,910
needs, or their needs
for the glucose skeleton

310
00:14:34,910 --> 00:14:37,130
for other biochemical purposes.

311
00:14:37,130 --> 00:14:40,580
Gluconeogenic precursor
three is glutamate.

312
00:14:40,580 --> 00:14:43,790
This is the amino acid
equivalent of the organic acid

313
00:14:43,790 --> 00:14:45,740
alpha ketoglutarate.

314
00:14:45,740 --> 00:14:49,250
So look at alpha ketoglutarate
at about 3 o'clock

315
00:14:49,250 --> 00:14:51,000
on the TCA cycle.

316
00:14:51,000 --> 00:14:53,450
You'll see glutamate
precursor three

317
00:14:53,450 --> 00:14:57,140
being de-aminated by a
PLP-mediated process.

318
00:14:57,140 --> 00:15:00,740
This converts glutamate
into alpha ketoglutarate.

319
00:15:00,740 --> 00:15:02,990
Most of the carbons of
the alpha ketoglutarate

320
00:15:02,990 --> 00:15:06,710
will then move clockwise
around the TCA cycle,

321
00:15:06,710 --> 00:15:09,590
and end up at about
9 o'clock as malate.

322
00:15:09,590 --> 00:15:12,440
Then, as we saw for
precursors one and two,

323
00:15:12,440 --> 00:15:15,740
the malate will be exported
from the mitochondrion.

324
00:15:15,740 --> 00:15:19,130
And next, most of its atoms will
find their way back to glucose,

325
00:15:19,130 --> 00:15:23,250
just as we saw with alanine
and lactate earlier.

326
00:15:23,250 --> 00:15:25,680
Precursor four is aspartic acid.

327
00:15:25,680 --> 00:15:29,940
Aspartic acid is the amino acid
equivalent of oxaloacetate.

328
00:15:29,940 --> 00:15:32,940
Aspartate is de-aminated
into oxaloacetate,

329
00:15:32,940 --> 00:15:36,360
which enters the gluconeogenic
pathway at about 10 o'clock

330
00:15:36,360 --> 00:15:38,130
on the TCA cycle.

331
00:15:38,130 --> 00:15:41,940
At this point, the oxaloacetate
formed from aspartate

332
00:15:41,940 --> 00:15:45,570
joins the oxaloacetate made from
alanine and lactate, precursors

333
00:15:45,570 --> 00:15:49,280
one and two, and continues
all the way back to glucose.

334
00:15:49,280 --> 00:15:53,130
Gluconeogenic precursor five
comes from odd-chained fatty

335
00:15:53,130 --> 00:15:55,860
acids, which are depicted
way over on the right

336
00:15:55,860 --> 00:15:57,420
of the metabolic map.

337
00:15:57,420 --> 00:15:59,460
As we have seen,
odd-chain fatty acids

338
00:15:59,460 --> 00:16:02,130
are catabolized into
three- carbon compounds,

339
00:16:02,130 --> 00:16:05,970
such as propionyl coenzyme
A. One of the carboxlysases,

340
00:16:05,970 --> 00:16:08,040
propionyl coenzyme
A carboxylase,

341
00:16:08,040 --> 00:16:11,470
will add a carbon to this
three-carbon molecule.

342
00:16:11,470 --> 00:16:14,610
Then methylmalonyl-coenzyme
A mutase and its partner,

343
00:16:14,610 --> 00:16:17,040
epimerase, will
complete conversion

344
00:16:17,040 --> 00:16:19,350
of the three-carbon
propionyl-CoA

345
00:16:19,350 --> 00:16:22,680
to the four-carbon product
succinyl-CoA, which will then

346
00:16:22,680 --> 00:16:25,280
integrate into the TCA cycle.

347
00:16:25,280 --> 00:16:28,650
Succinyl-coenzyme A enters
at about five o'clock

348
00:16:28,650 --> 00:16:30,390
on the TCA cycle.

349
00:16:30,390 --> 00:16:34,440
The carbons then flow
clockwise to malate.

350
00:16:34,440 --> 00:16:36,480
Just as we've seen before,
most of those carbons

351
00:16:36,480 --> 00:16:40,350
will flow all the way back
to be built into glucose.

352
00:16:40,350 --> 00:16:43,620
Gluconeogenic precursors
indicated by number six

353
00:16:43,620 --> 00:16:48,330
are the amino acids, isoleucine,
methionine, and valine.

354
00:16:48,330 --> 00:16:51,060
These are, once again, way
over on the right-hand side

355
00:16:51,060 --> 00:16:52,260
of the figure.

356
00:16:52,260 --> 00:16:55,560
These amino acids are also
converted to propionyl-CoA.

357
00:16:55,560 --> 00:16:58,080
And just as with the
odd-chain fatty acids,

358
00:16:58,080 --> 00:17:01,380
they will be converted
into succinyl-coenzyme A.

359
00:17:01,380 --> 00:17:03,120
Then their atoms
follow the pathway

360
00:17:03,120 --> 00:17:07,660
all the way back to glucose, as
we have seen now several times.

361
00:17:07,660 --> 00:17:09,880
Slightly to the left
of center, you'll

362
00:17:09,880 --> 00:17:12,099
see gluconeogenic
precursor number

363
00:17:12,099 --> 00:17:14,079
seven, which is glycerol.

364
00:17:14,079 --> 00:17:17,800
Glycerol is a tri-alcohol
produced from the metabolism

365
00:17:17,800 --> 00:17:19,510
of complex lipids.

366
00:17:19,510 --> 00:17:21,579
It derives from the
three-carbon backbone

367
00:17:21,579 --> 00:17:25,800
that holds fatty acids and
phosphates by ester linkages.

368
00:17:25,800 --> 00:17:29,640
Glycerol is first phosphorylated
to glycerol 3-phosphate.

369
00:17:29,640 --> 00:17:31,590
And then that
product is oxidized

370
00:17:31,590 --> 00:17:33,750
to dihydroxyacetone phosphate.

371
00:17:33,750 --> 00:17:35,850
We've seen this
chemistry before.

372
00:17:35,850 --> 00:17:39,240
From DHAP, or
dihydroxyacetone phosphate,

373
00:17:39,240 --> 00:17:42,090
the molecules flow leftward
on the chart to fructose

374
00:17:42,090 --> 00:17:45,390
1,6-bisphosphate, and then
they continue on through

375
00:17:45,390 --> 00:17:48,290
gluconeogenesis to glucose.

376
00:17:48,290 --> 00:17:51,140
Lastly, precursor
number eight is ribose.

377
00:17:51,140 --> 00:17:54,239
We mainly obtain
ribose from our diet.

378
00:17:54,239 --> 00:17:56,780
In the next lecture, I'm going
to cover the pentose phosphate

379
00:17:56,780 --> 00:17:57,350
pathway.

380
00:17:57,350 --> 00:17:59,500
In this pathway,
ribose from the diet

381
00:17:59,500 --> 00:18:02,570
will enter what I call
the "carbon scrambling

382
00:18:02,570 --> 00:18:05,300
phase" of the pentose
phosphate pathway.

383
00:18:05,300 --> 00:18:07,460
Ribose goes into the
carbon scrambling

384
00:18:07,460 --> 00:18:10,130
phase of the pathway,
and GAP, glyceraldehyde

385
00:18:10,130 --> 00:18:14,410
3-phosphate and fructose
6-phosphate come out.

386
00:18:14,410 --> 00:18:17,920
Both of these molecules, GAP
and fructose 6-phosphate,

387
00:18:17,920 --> 00:18:20,860
will flow from right to
left all the way to glucose

388
00:18:20,860 --> 00:18:23,830
in the pathway that I've
drawn out in Panel C. Hence,

389
00:18:23,830 --> 00:18:27,310
ribose is a good
gluconeogenic precursor.

390
00:18:27,310 --> 00:18:29,590
Let me summarize the
functional dimensions

391
00:18:29,590 --> 00:18:32,440
of the gluconeogenesis
pathway from a high altitude.

392
00:18:32,440 --> 00:18:34,780
Imagine that overnight,
as you sleep,

393
00:18:34,780 --> 00:18:36,640
you're not consuming any food.

394
00:18:36,640 --> 00:18:38,840
Consequently, at some
point, your blood sugar

395
00:18:38,840 --> 00:18:40,300
is going to drop.

396
00:18:40,300 --> 00:18:43,000
Your gluconeogenic organs,
the liver and renal cortex,

397
00:18:43,000 --> 00:18:45,490
will sense this
drop in blood sugar,

398
00:18:45,490 --> 00:18:48,910
and turn on the hatched-line
pathways we've seen above,

399
00:18:48,910 --> 00:18:52,030
in order to take readily
available noncarbohydrate

400
00:18:52,030 --> 00:18:55,390
precursors, such as lactate,
alanine, ribose, and so on,

401
00:18:55,390 --> 00:18:57,970
and convert those
noncarbohydrate precursors

402
00:18:57,970 --> 00:18:59,570
into glucose.

403
00:18:59,570 --> 00:19:03,280
The liver and renal cortex
will then send that glucose out

404
00:19:03,280 --> 00:19:05,620
into the bloodstream
to client organs,

405
00:19:05,620 --> 00:19:10,330
for example, the brain, red
blood cells, and renal medulla.

406
00:19:10,330 --> 00:19:12,190
Ultimately, these
client organs will

407
00:19:12,190 --> 00:19:15,310
be able to rely on a
constant level of glucose.

408
00:19:15,310 --> 00:19:19,830
That, in a nutshell, is the role
of the gluconeogenic pathway.