BC10M: Introductory Biochemistry, 2005 Semester 2'

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BC10M Introductory Biochemistry, 2005 Semester 2.

• Thursday 17 Mar.
• Lecture 25
• Gluconeogenesis
• PPP
• Calvin cycle
• Andrew Pearson

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Glycogen synthesis proceeds by the sequential
addition of glucose units to the C-4 end of a
chain. The glucose-carrier is UDP-glucose. UDP-gl
ucose is formed from uridine triphosphate and
glucose 1-phosphate, with the release of PP which
is hydrolysed to drive the reaction. Glucose 1-P
is formed from G 6-P by phosphoglucose isomerase.
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Glycogen degradation is stimulated in
Glycogen synthesis is stimulated in
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Gluconeogenesis the synthesis of glucose in
liver (kidney)   Gluconeogenesis (Gneog) is the
new formation of glucose from smaller
carbon-containing biochemicals, and is partly
responsible for converting atmospheric CO2 into
starch in plants, which we eat. It is necessary
in animals to supply glucose to the blood when
the nutritional status is one of fasting dietary
glucose has all been consumed and the liver
stores of glucose in the form of glycogen have
been depleted. The brain and red blood cells are
more or less dependent upon blood glucose for
ATP.   The pathway is activated in response the
signal which indicates that blood glucose is
low glucagon. We have already come across the
control point hepatic glucagon receptors
activate adenylate cyclase to form cAMP which in
turn activates cAMP-dependent protein kinase
which amongst other things, phosphorylates and
inactivates pyruvate kinase in gluconeogenic
tissues.
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The reactions of the Gneog pathway occurring in
liver and kidney can be regarded as being
essentially a reversal of glycolysis, since the
two pathways use the same Michaelis-Menten
enzymes. The 3 glycolysis enzymes that are not
M-M are by-passed.
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Gneog showing the M-M enzymes common carbon
inputs
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Gneog really starts in the mito matrix.
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Mitochondrial oxaloacetate is an intermediate in
the Krebs cycle. Under normal circumstances, the
amount of oxaloacetate in the mitochondrion is
not changed two carbons enter the TCA cycle as
acetyl CoA and two leave as 2 CO2. Putting more
acetyl CoA into the cycle cannot increase the
amount of oxaloacetate present, it can only
increase the rate of its formation and usage
together the cycle cannot grow wider but it
can go faster, up to a Vmax. If oxaloacetate
is removed from the mitochondrion to enter
gluconeogenesis, then it must be replaced from
another source, or the TCA cycle will have too
few intermediates to operate efficiently.
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If there were a pathway which could convert
acetyl CoA directly to oxaloacetate, that would
be ideal for the obese, struggling with an excess
of fat. The Glyoxylate pathway exists in yeasts
those plants that need to convert the fats
stored in their seeds during germination to
glucose for metabolism before photosynthesis can
take over. Animals have no Glyoxylate pathway
and cannot convert fatty acids to glucose.
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Apart from the glucogenic amino acids, other
sources of carbon for gluconeogenesis can be
considered lactate/pyruvate and glycerol.
Lactate released by muscle into the blood can be
returned to pyruvate in the liver and kidney by
their isoenzymes of lactate dehydrogenase.
Pyruvate produced in this way can be converted
by pyruvate carboxylase to oxaloacetate and on up
through gluconeogenesis to glucose. Pyruvate
carboxylase pyruvate ATP HCO3- Þ
oxaloacetate ADP Pi
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Conversion of Mitochondrial Oxaloacetate to
Cytosolic Phosphoenolpyruvate Oxaloacetate can
only cross the inner membrane of the
mitochondrion very slowly, there are two
alternate routes to accomplish this objective.
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Conversion of Phosphoenolpyruvate to Fructose
1,6-bisphosphate   The same enzymes are used for
these reactions as are used in glycolysis Enol
ase Phosphoglycerate Mutase Phosphoglycerate
Kinase Glyceraldehyde 3-P dehydrogenase Triose
phosphate isomerase Aldolase
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Gneog showing the M-M enzymes common carbon
inputs
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Fructose 1,6-bisphosphatase. It is strongly
inhibited by AMP, a situation which would arise
if the liver were not able to maintain its own
ATP levels. Fructose 1,6-bisphosphatase is also
strongly inhibited by the powerful PFK1
stimulant fructose 2,6-bisphosphate, which as
well as inhibiting in its own right also
increases the inhibition by AMP.
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Phosphoglucose isomerase   This rapidly
interconverts   Glucose 6-P Û Fructose 6-P
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Glucose 6-Phosphatase   This is found only in
liver and kidney, the only two tissues that can
release glucose into the blood. Recall that
glucose transporters will assist transport down
the concentration gradient into the low glucose
blood.  
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Pentose Phosphate Pathway (PPP), alias Hexose
Monophosphate Shunt, alias Phosphogluconate
Pathway. This is a set of enzymes in the cytosol
of most cells that can carry out intermediary
metabolism of some monosaccharides and simple
derivatives. In some cells these enzymes perform
a catabolic role like glycolysis some bacteria
without a complete glycolytic pathway rely on
these enzymes for the fermentative generation of
ATP. Muscle cells have very low levels of PPP
activity, whereas normal erythrocytes have high
levels.
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Pentose Phosphate Pathway
The two major outputs from this in animal cells
are ribose 5-phosphate for nucleic acid
synthesis and NADPH for biosynthetic reduction
or plasma membrane reduction and maintenance.
Sadly, most textbooks describe all of the
possible reactions of this matrix, and the
first-time student is often hard put to see the
wood for the trees.
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Pentose Phosphate Pathway
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Pentose Phosphate Pathway
At the heart of this area of intermediary
carbohydrate metabolism are two types of
reaction transketolases and transaldolases.
Transketolase Reactions. These transfer 2-carbon
ketol groups
from a variety of ketose sugars, such as ribulose
ribulose 5-phosphate, xylulose xylulose
5-phosphate, fructose fructose 6-phosphate,
sedoheptulose sedoheptulose 7-phosphate. The
group is transiently held on a thiamin
pyrophosphate cofactor, before being transferred
onto the 1-carbon of an aldose sugar, which is
always an aldehyde, such as glyceraldehyde
3-phosphate.
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A typical transketolase reaction in the PPP
is Step 1. Xylulose 5-phosphate
? glyceraldehyde 3-phosphate (5
carbon) (3 carbon) TPP-Enz.
Ketol-TPP-Enz. (2 carbon) Step
2. Ketol-TPP-Enz. sedoheptulose
7-phosphate. (2 carbon) (7 carbon) ribose
5-phosphate ? TPP-Enz (5 carbon)
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Transaldolase Reactions. These transfer 3-carbon
aldol groups
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A typical transaldolase reaction in the PPP
is Step 1. sedoheptulose 7-phosphate
? erythrose 4-phosphate (7 carbon) (4
carbon) lys-Enz.
Aldol-lys-Enz. (3 carbon) Step
2. Aldol-lys-Enz. fructose 6-phosphate. (3
carbon) ? (6 carbon) glyceraldehyde
3-phosphate lys-Enz (3 carbon)
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The oxidative branch of the PPP 1. Glucose
6-phosphate dehydrogenase. This enzyme is
deficient in some individuals this deficiency
offers some protection against malaria caused by
Plasmodium falciparum. Glucose 6-phosphate is
present, to some extent, in the cytosol of all
cells, being formed by one of the hexokinase
isoenzymes. The enzyme transfers reducing
equivalents and a proton from the 1-carbon of
glucose to NADP, forming 6-phosphoglucono-d-lacto
ne.
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1. Glucose 6-phosphate dehydrogenase. 2. Lactonase
hydrolysis of 6- phosphoglucono-d-lactone. 3
. 6-phosphogluconate dehydrogenase. 4. Phosphopent
ose isomerase Summary of route 1 Glucose
6-phosphate ribose 5-phosphate CO2
? 2NADP 2NADPH 2H
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