Biochemistry 2/e - Garrett & Grisham
Chapter 23
Gluconeogenesis. Glycogen
Metabolism, and the Pentose
Phosphate Pathway
to accompany
Biochemistry, 2/e
by
Reginald Garrett and Charles Grisham
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Biochemistry 2/e - Garrett & Grisham
Outline
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23.1 Gluconeogenesis
23.2 Regulation of Gluconeogenesis
23.3 Glycogen Catabolism
23.4 Glycogen Synthesis
23.5 Control of Glycogen Metabolism
23.6 The Pentose Phosphate Pathway
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Gluconeogenesis
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Synthesis of "new glucose" from common
metabolites
Humans consume 160 g of glucose per day
75% of that is in the brain
Body fluids contain only 20 g of glucose
Glycogen stores yield 180-200 g of glucose
So the body must be able to make its own
glucose
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Biochemistry 2/e - Garrett & Grisham
Substrates for
Gluconeogenesis
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Pyruvate, lactate, glycerol, amino acids
and all TCA intermediates can be utilized
Fatty acids cannot!
Why?
Most fatty acids yield only acetyl-CoA
Acetyl-CoA (through TCA cycle) cannot
provide for net synthesis of sugars
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Gluconeogenesis I
• Occurs mainly in liver and kidneys
• Not the mere reversal of glycolysis for 2
reasons:
– Energetics must change to make
gluconeogenesis favorable (delta G of
glycolysis = -74 kJ/mol
– Reciprocal regulation must turn one on and
the other off - this requires something new!
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Gluconeogenesis II
Something Borrowed, Something New
• Seven steps of glycolysis are retained:
– Steps 2 and 4-9
• Three steps are replaced:
– Steps 1, 3, and 10 (the regulated steps!)
• The new reactions provide for a
spontaneous pathway (G negative in the
direction of sugar synthesis), and they
provide new mechanisms of regulation
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Pyruvate Carboxylase
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Pyruvate is converted to oxaloacetate
The reaction requires ATP and bicarbonate as
substrates
That should make you think of biotin!
Biotin is covalently linked to an active site lysine
Acetyl-CoA is an allosteric activator
The mechanism (Figure 23.4) is typical of biotin!
Regulation: when ATP or acetyl-CoA are high,
pyruvate enters gluconeogenesis
Note the "conversion problem" in mitochondria
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PEP Carboxykinase
Conversion of oxaloacetate to PEP
• Lots of energy needed to drive this
reaction!
• Energy is provided in 2 ways:
– Decarboxylation is a favorable reaction
– GTP is hydrolyzed
• GTP used here is equivalent to an ATP
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Fructose-1,6-bisphosphatase
Hydrolysis of F-1,6-P to F-6-P
• Thermodynamically favorable - G in
liver is -8.6 kJ/mol
• Allosteric regulation:
– citrate stimulates
– fructose-2,6--bisphosphate inhibits
– AMP inhibits
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Biochemistry 2/e - Garrett & Grisham
Glucose-6-Phosphatase
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Conversion of Glucose-6-P to Glucose
Presence of G-6-Pase in ER of liver and
kidney cells makes gluconeogenesis possible
Muscle and brain do not do gluconeogenesis
G-6-P is hydrolyzed as it passes into the ER
ER vesicles filled with glucose diffuse to the
plasma membrane, fuse with it and open,
releasing glucose into the bloodstream.
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Lactate Recycling
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How your liver helps you during exercise....
Recall that vigorous exercise can lead to a
buildup of lactate and NADH, due to oxygen
shortage and the need for more glycolysis
NADH can be reoxidized during the reduction
of pyruvate to lactate
Lactate is then returned to the liver, where it
can be reoxidized to pyruvate by liver LDH
Liver provides glucose to muscle for exercise
and then reprocesses lactate into new glucose
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Regulation of Gluconeogenesis
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Reciprocal control with glycolysis
When glycolysis is turned on, gluconeogenesis
should be turned off
When energy status of cell is high, glycolysis
should be off and pyruvate, etc., should be used
for synthesis and storage of glucose
When energy status is low, glucose should be
rapidly degraded to provide energy
The regulated steps of glycolysis are the very
steps that are regulated in the reverse direction!
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Biochemistry 2/e - Garrett & Grisham
Gluconeogenesis Regulation II
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Allosteric and Substrate-Level Control
See Figure 23.11
Glucose-6-phosphatase is under substratelevel control, not allosteric control
The fate of pyruvate depends on acetyl-CoA
F-1,6-bisPase is inhibited by AMP, activated by
citrate - the reverse of glycolysis
Fructose-2,6-bisP is an allosteric inhibitor of
F-1,6-bisPase
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23.3 Glycogen Catabolism
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Getting glucose from storage (or diet)
-Amylase is an endoglycosidase
It cleaves amylopectin or glycogen to maltose,
maltotriose and other small oligosaccharides
It is active on either side of a branch point, but
activity is reduced near the branch points
Debranching enzyme cleaves "limit dextrins"
Note the 2 activities of the debranching
enzyme
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Metabolism of Tissue Glycogen
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Digestive breakdown is unregulated - 100%!
But tissue glycogen is an important energy
reservoir - its breakdown is carefully controlled
Glycogen consists of "granules" of high MW
Glycogen phosphorylase cleaves glucose from
the nonreducing ends of glycogen molecules
This is a phosphorolysis, not a hydrolysis
Metabolic advantage: product is a sugar-P - a
"sort-of" glycolysis substrate
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Glycogen Phosphorylase
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A beautiful protein structure!
A dimer of identical subunits (842 res. each)
Each subunit contains a PLP, which
participates in phosphorolysis, but not in the
usual way!
Note that NaBH4 reduction does not affect
activity
See pages 473-479 to review glycogen
phosphorylase
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Biochemistry 2/e - Garrett & Grisham
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Biochemistry 2/e - Garrett & Grisham
23.4 Glycogen Synthesis - I
Glucose units are activated for transfer by
formation of sugar nucleotides
• What are other examples of "activation"?
– acetyl-CoA, biotin, THF,
• Leloir showed in the 1950s that glycogen
synthesis depends on sugar nucleotides
• UDP-glucose pyrophosphorylase - Fig. 23.18
– a phosphoanhydride exchange
– driven by pyrophosphate hydrolysis
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Glycogen Synthase
Forms -(1 4) glycosidic bonds in glycogen
• Glycogenin (a protein!) forms the core of a
glycogen particle
• First glucose is linked to a tyrosine -OH
• Glycogen synthase transfers glucosyl units
from UDP-glucose to C-4 hydroxyl at a
nonreducing end of a glycogen strand.
• Note another oxonium ion intermediate (Fig.
23.19)
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23.5 Control of Glycogen
Metabolism
A highly regulated process, involving
reciprocal control of glycogen
phosphorylase and glycogen synthase
• GP allosterically activated by AMP and
inhibited by ATP, glucose-6-P and caffeine
• GS is stimulated by glucose-6-P
• Both enzymes are regulated by covalent
modification - phosphorylation
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Phosphorylation of GP and GS
Covalent control
• Edwin Krebs and Edmond Fisher showed in
1956 that a "converting enzyme" converted
phosphorylase b to phosphorylase a(P)
• Phosphorylation causes the amino terminus
of the protein (res 10-22) to swing through
120 degrees, moving into the subunit
interface and moving Ser-14 by more than 3.6
nm
• Nine Ser residues on GS are phosphorylated!
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Enzyme Cascades and
GP/GS
Hormonal regulation
• Hormones (glucagon, epinephrine)
activate adenylyl cyclase
• cAMP activates kinases and phosphatases
that control the phosphorylation of GP and
GS
• GTP-binding proteins (G proteins) mediate
the communication between hormone
receptor and adenylyl cyclase
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Hormonal Regulation
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of Glycogen Synthesis and Degradation
Insulin is secreted from the pancreas (to
liver) in response to an increase in blood
glucose
Note that the portal vein is the only vein in
the body that feeds an organ!
Insulin stimulates glycogen synthesis and
inhibits glycogen breakdown
Note other effects of insulin (Figure 23.22)
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Hormonal Regulation II
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Glucagon and epinephrine
Glucagon and epinephrine stimulate glycogen
breakdown - opposite effect of insulin!
Glucagon (29 res) is also secreted by pancreas
Glucagon acts in liver and adipose tissue only!
Epinephrine (adrenaline) is released from
adrenal glands
Epinephrine acts on liver and muscles
The phosphorylase cascade amplifies the
signal!
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Biochemistry 2/e - Garrett & Grisham
Epinephrine and Glucagon
The difference....
• Both are glycogenolytic but for different
reasons!
• Epinephrine is the fight or flight hormone
– rapidly mobilizes large amounts of energy
• Glucagon is for long-term maintenance of
steady-state levels of glucose in the blood
– activates glycogen breakdown
– activates liver gluconeogenesis
• Relate these points to sites of action!
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Pentose Phosphate Pathway
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aka hexose monophosphate shunt
Provides NADPH for biosynthesis
Produces ribose-5-P
Two oxidative processes followed by five
non-oxidative steps
Operates mostly in cytoplasm of liver and
adipose cells
NADPH is used in cytosol for fatty acid
synthesis
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Oxidative Steps
of the Pentose Phosphate Pathway
• Glucose-6-P Dehydrogenase
– Irreversible 1st step - highly regulated!
• Gluconolactonase
– Uncatalyzed reaction happens too
• 6-Phosphogluconate Dehydrogenase
– An oxidative decarboxylation (in that
order!)
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The Nonoxidative Steps
Five steps, only 4 types of reaction...
• Phosphopentose isomerase
– converts ketose to aldose
• Phosphopentose Epimerase
– epimerizes at C-3
• Transketolase (TPP-dependent)
– transfer of two-carbon units
• Transaldolase (Schiff base mechanism)
– transfers a three-carbon unit
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Variations on the Pentose
Phosphate Pathway
• 1) Both ribose-5-P and NADPH are
needed
• 2) More ribose-5-P than NADPH is
needed
• 3) More NADPH than ribose-5-P is
needed
• 4) NADPH and ATP are needed, but
ribose-5-P is not
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