Download Reactions of glycolysis and gluconeogenesis

Chapter 13
Carbohydrate Metabolism:
Glycolysis, Gluconeogenesis,
Glycogen Metabolism, and the
Pentose Phosphate Pathway
13 - 1
Copyright © 2013 Pearson Canada Inc.
Biochemistry, 4th Edition
•
•
•
•
•
•
•
•
•
•
•
Chapter 13 Outline:
Glycolysis: An Overview
Reactions of Glycolysis
Metabolic Fates of Pyruvate
Energy and Electron Balance Sheets
Gluconeogenesis
Coordinated Regulation of Glycolysis and
Gluconeogenesis
Entry of Other Sugars into the Glycolytic Pathway
Polysaccharide Metabolism
Glycogen Metabolism in Muscle and Liver
Coordinated Regulation of Glycogen Metabolism
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
Copyright © 2013 Pearson Canada Inc.
13 - 2
Biochemistry, 4th Edition
Glycolysis: An Overview
Catabolic and anabolic processes in anaerobic
carbohydrate metabolism:
•
The gold arrows show the glycolytic pathway and the
breakdown of polysaccharides that supply this pathway.
•
Glycolysis generates ATP anaerobically and provides
fuel for the aerobic energy-generating pathways.
•
The green arrows show the gluconeogenesis pathway,
the synthesis of polysaccharides such as glycogen.
•
The blue arrow shows the pentose phosphate
pathway, an alternative carbohydrate oxidation pathway
needed for nucleotide synthesis.
•
The numbers 1, 2, and 3 identify the three stages of
metabolism
Copyright © 2013 Pearson Canada Inc.
13 - 3
Biochemistry, 4th Edition
Glycolysis: An Overview
The two phases of glycolysis and the products of glycolysis:
Copyright © 2013 Pearson Canada Inc.
13 - 4
Biochemistry, 4th Edition
Glycolysis: An Overview
•
A fermentation is an energy-yielding metabolic pathway with no net
change in the oxidation state of products compared to substrates.
•
Anaerobic glycolysis (like aerobic glycolysis) leads to pyruvate, but the
pyruvate is then reduced, so no net oxidation of glucose occurs.
Copyright © 2013 Pearson Canada Inc.
13 - 5
Biochemistry, 4th Edition
Glycolysis: An Overview
•
Alcoholic fermentation involves cleavage of pyruvate to
acetaldehyde and CO2 with the acetaldehyde then reduced
to ethanol by alcohol dehydrogenase:
Copyright © 2013 Pearson Canada Inc.
13 - 6
Biochemistry, 4th Edition
Glycolysis: An Overview
Fermentations use a
common strategy to
regenerate oxidized
NAD+.
Copyright © 2013 Pearson Canada Inc.
13 - 7
Biochemistry, 4th Edition
Reactions of Glycolysis
The chemical strategy of glycolysis can be condensed into three
processes:
1.Add phosphoryl groups to glucose, yielding compounds with low
phosphate group–transfer potential priming.
2.Chemically convert these low phosphate group–transfer
potential intermediates into compounds with high phosphate
group–transfer potential.
3.Chemically couple the energy–yielding hydrolysis of these high
phosphate group–transfer potential compounds to the synthesis of
ATP by transfer of the phosphate group to ADP.
Copyright © 2013 Pearson Canada Inc.
13 - 8
Biochemistry, 4th Edition
Reactions of Glycolysis
•
ATP is synthesized by three major routes:
o Substrate-level phosphorylation
o Oxidative phosphorylation
o Photophosphorylation
•
Copyright © 2013 Pearson Canada Inc.
Glycolysis produces ATP via substrate-level
phosphorylation
13 - 9
Biochemistry, 4th Edition
Reactions of Glycolysis
An overview of glycolysis:
•
This condensed view of glycolysis
shows the key intermediates and
reactions in each of the two major
phases.
•
In the energy-generating phase, two
ATPs are produced for each ATP
utilized in the energy-investment phase.
Copyright © 2013 Pearson Canada Inc.
13 - 10
Biochemistry, 4th Edition
Reactions of Glycolysis
•
Copyright © 2013 Pearson Canada Inc.
The first five reactions constitute the energyinvestment phase of glycolysis.
13 - 11
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 1: The First ATP Investment
Glycolysis begins with the ATP-dependent phosphorylation
of glucose, catalyzed by hexokinase.
Copyright © 2013 Pearson Canada Inc.
13 - 12
Biochemistry, 4th Edition
Reactions of Glycolysis
•
Mammals possess several molecular forms of hexokinase.
•
Different molecular forms of an enzyme catalyzing the same
reaction are called isoenzymes, isozymes, or isoforms.
•
Most tissues express hexokinase I, II, or III, all low KM
enzymes.
•
Because intracellular glucose levels (2–15 mM) are usually far
higher than the KM value for hexokinase, the enzyme often
functions in vivo at saturating substrate concentrations.
•
A low affinity isozyme of hexokinase IV in liver with a sigmoidal
dependence on glucose concentration permits that organ to
adjust glucose utilization to glucose supply at high blood
glucose levels.
Copyright © 2013 Pearson Canada Inc.
13 - 13
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 2: Isomerization of Glucose-6-phosphate.
•
The next reaction, catalyzed by glucose-6-phosphate
isomerase (also called phosphoglucoisomerase), is the
readily reversible isomerization of the aldose, glucose-6phosphate (G6P), to the corresponding ketose, fructose-6phosphate (F6P).
Copyright © 2013 Pearson Canada Inc.
13 - 14
Biochemistry, 4th Edition
Reactions of Glycolysis
•
This isomerization reaction proceeds via an enediolate
intermediate.
•
B: and B-H represent active-site amino acid residues acting
as bases and acids, respectively.
Copyright © 2013 Pearson Canada Inc.
13 - 15
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 3: The Second Investment of ATP
•
Phosphofructokinase catalyzes a second ATP-dependent phosphorylation, to
give a hexose derivative, fructose-1,6-bisphosphate (FBP), phosphorylated at
both carbons 1 and 6.
•
Transferring the carbonyl oxygen from carbon 1 to carbon 2 has two important
effects:
•
The hydroxyl group generated at carbon 1 can be readily phosphorylated in
the reaction 3.
•
It also sets up the sugar for a symmetric aldol cleavage in reaction 4.
•
The phosphofructokinase reaction is the primary step at which glycolysis is
regulated.
Copyright © 2013 Pearson Canada Inc.
13 - 16
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 4: Cleavage to Two Triose Phosphates
•Fructose-1,6-bisphosphate aldolase, catalyzes the “splitting of sugar”
that is connoted by the term glycolysis.
•The six-carbon sugar fructose-1,6-bisphosphate is cleaved to give 2
three-carbon intermediates, glyceraldehyde-3-phosphate and
dihydroxyacetone phosphate (DHAP).
•Aldolase cleaves fructose-1,6-bisphosphate under intracellular
conditions, even though the equilibrium lies far toward fructose-1,6bisphosphate under standard conditions.
Copyright © 2013 Pearson Canada Inc.
13 - 17
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction mechanism for fructose-1,6-bisphosphate aldolase:
•A protonated Schiff base intermediate (iminium ion) occurs between
the substrate and an active site lysine residue.
•An aspartate residue facilitates the reaction via general acid–base
catalysis.
Copyright © 2013 Pearson Canada Inc.
13 - 18
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 5: Isomerization of Dihydroxyacetone Phosphate
•
The function of reaction 5, catalyzed by triose phosphate
isomerase, is conversion of one of the products from reaction
4, dihydroxyacetone phosphate (DHAP), to the other,
glyceraldehyde-3-phosphate (GAP).
Copyright © 2013 Pearson Canada Inc.
13 - 19
Biochemistry, 4th Edition
Reactions of Glycolysis
Reactions 6–10:
The Energy
Generation Phase
Copyright © 2013 Pearson Canada Inc.
13 - 20
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 6: Generation of the First Energy-Rich Compound
•
This reaction, catalyzed by glyceraldehyde-3-phosphate
dehydrogenase, is among the most important in glycolysis, partly
because it generates the first intermediate with high phosphate
transfer potential and partly because it generates a pair of
reducing equivalents.
Copyright © 2013 Pearson Canada Inc.
13 - 21
Biochemistry, 4th Edition
Reactions of Glycolysis
Copyright © 2013 Pearson Canada Inc.
13 - 22
Biochemistry, 4th Edition
Reactions of Glycolysis
•
This mechanism conserves the energy of oxidation by
coupling an exergonic reaction to an endergonic reaction:
Copyright © 2013 Pearson Canada Inc.
13 - 23
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 7: The First Substrate-Level Phosphorylation
•Because of its high phosphate transfer potential, 1,3bisphosphoglycerate has a strong tendency to transfer its acylphosphate group to ADP, with resultant formation of ATP.
•This substrate-level phosphorylation reaction is catalyzed by
phosphoglycerate kinase.
Copyright © 2013 Pearson Canada Inc.
13 - 24
Biochemistry, 4th Edition
Reactions of Glycolysis
•
Phosphoglycerate kinase catalyzes the first glycolytic
reaction that forms ATP.
•
The glyceraldehyde-3-phosphate dehydrogenase and
phosphoglycerate kinase are thermodynamically coupled:
Copyright © 2013 Pearson Canada Inc.
13 - 25
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 8: Preparing for Synthesis of the Next HighEnergy Compound
•
Activation of 3-phosphoglycerate begins with an
isomerization catalyzed by phosphoglycerate mutase.
•
The enzyme transfers phosphate from position 3 to position
2 of the substrate to yield 2-phosphoglycerate.
Copyright © 2013 Pearson Canada Inc.
13 - 26
Biochemistry, 4th Edition
Reactions of Glycolysis
•
Reaction 8 is slightly endergonic under standard conditions.
•
However, the intracellular level of 3-phosphoglycerate is high relative to
that of 2-phosphoglycerate (2PG), so that in vivo the reaction proceeds to
the right without difficulty.
•
The enzyme contains a phosphohistidine residue in the active site.
•
In the first step of the reaction, the phosphate is transferred from the
enzyme to the substrate to give an intermediate, 2,3 bisphosphoglycerate.
•
Transfer of the phosphate from C-3 to the active site of the enzyme
regenerates the phosphorylated enzyme and forms the product, which is
released.
Copyright © 2013 Pearson Canada Inc.
13 - 27
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 9: Synthesis of the Second High-Energy Compound
•
Reaction 9, catalyzed by enolase, generates
phosphoenolpyruvate (PEP), another compound with very high
phosphate transfer potential.
•
PEP participates in the second substrate-level phosphorylation of
glycolysis.
Copyright © 2013 Pearson Canada Inc.
13 - 28
Biochemistry, 4th Edition
Reactions of Glycolysis
Reaction 10: The Second Substrate-Level Phosphorylation
•
In the last reaction, catalyzed by pyruvate kinase,
phosphoenolpyruvate transfers its phosphoryl group to ADP in
another substrate-level phosphorylation.
•
Note that the enzyme is named as if it were acting leftward in the
reaction shown below, even though it is strongly exergonic in the
rightward direction.
•
Many enzymes were named before the function or direction of
intracellular catalysis had been identified.
Copyright © 2013 Pearson Canada Inc.
13 - 29
Biochemistry, 4th Edition
Reactions of Glycolysis
The reaction mechanism of pyruvate kinase:
Copyright © 2013 Pearson Canada Inc.
13 - 30
Biochemistry, 4th Edition
Reactions of Glycolysis
•
Pyruvate kinase (PK) is another site for metabolic regulation.
•
PK, like many enzymes, exists in animal tissues as multiple
isozymes.
•
Mammals carry two paralogous pyruvate kinase genes, which,
through alternative splicing, produce four different PK isozymes.
•
All four PK isozymes are homotetramers of 60 kDa subunits, and
except for the M1 isozyme, require allosteric activation by
fructose-1,6-bisphosphate for full activity.
•
The L and R isozymes are allosterically inhibited at high ATP
concentrations.
•
The M1 isozyme, on the other hand, is constitutively active.
•
Dietary carbohydrate induces the biosynthesis of pyruvate kinase
and increases the ability of the body to obtain energy from
glycolysis.
Copyright © 2013 Pearson Canada Inc.
13 - 31
Biochemistry, 4th Edition
Reactions of Glycolysis
Copyright © 2013 Pearson Canada Inc.
13 - 32
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
•
Pyruvate represents a central metabolic branch point.
•
Its fate depends crucially on the oxidation state of the cell,
which is related to the reaction catalyzed by glyceraldehyde3-phosphate dehydrogenase (reaction 6).
•
Recall that this reaction converts 1 mole of NAD+ per mole of
triose phosphate to NADH.
•
The cytoplasm has a finite supply of NAD+ so this NADH
must be reoxidized to for glycolysis to continue.
Copyright © 2013 Pearson Canada Inc.
13 - 33
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
•
Pyruvate must be reduced to lactate when tissues are insufficiently
aerobic to oxidize all of the NADH formed in glycolysis.
•
Microorganisms can oxidize NADH via fermentations producing
lactate or ethanol.
Copyright © 2013 Pearson Canada Inc.
13 - 34
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
Yeast converts pyruvate to ethanol in a two-step pathway:
•
This alcoholic fermentation starts with the nonoxidative
decarboxylation of pyruvate to acetaldehyde, catalyzed by
pyruvate decarboxylase.
•
NAD+ is regenerated in the next reaction, the NADHdependent reduction of acetaldehyde to ethanol, catalyzed by
alcohol dehydrogenase.
Copyright © 2013 Pearson Canada Inc.
13 - 35
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
•
The first reaction requires thiamine pyrophosphate as a coenzyme.
•
The vitamin is structurally complex, but its conversion to the coenzyme
form, thiamine pyrophosphate, or TPP, involves simply an ATP-dependent
pyrophosphorylation.
•
Thiamine pyrophosphate is the coenzyme for al decarboxylations of aketo acids.
•
Unlike b-keto acids, a-keto acids cannot stabilize the carbanion transition
state that develops during decarboxylation, and they thus require the aid
of a cofactor (TPP).
•
The thiazole ring of TPP is the functional part of the coenzyme, allowing it
to bind and transfer activated aldehydes.
Copyright © 2013 Pearson Canada Inc.
13 - 36
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
•
Because the thiazole ring is weakly acidic (pKa ~ 18 for the ring
hydrogen), a general base is required in its ionization.
•
As shown in the following diagram, a glutamate carboxyl group in
the enzyme deprotonates N-1 of the pyrimidine, which in turn
increases the basicity of the amino group, facilitating the
deprotonation of C-2 of the thiazole ring to give a dipolar thiazolium
carbanion (or ylid).
Copyright © 2013 Pearson Canada Inc.
13 - 37
Biochemistry, 4th Edition
Metabolic Fates of Pyruvate
Thiamine pyrophosphate in the pyruvate decarboxylase reaction:
•
The key reaction (step 2) is attack by the carbanion of TPP on the carbonyl
carbon of pyruvate and is followed by nonoxidative decarboxylation of the
coenzyme-bound pyruvate to give another carbanion (step 3), which is
resonance-stabilized.
Copyright © 2013 Pearson Canada Inc.
13 - 38
Biochemistry, 4th Edition
Energy and Electron Balance Sheets
•
Glycolysis, which yields 2 ATP per glucose, is fast but
releases only a small fraction of the energy available from
glucose.
•
In alcoholic fermentation as in lactate fermentation, an
exergonic process coupled to an endergonic process.
Copyright © 2013 Pearson Canada Inc.
13 - 39
Biochemistry, 4th Edition
Energy and Electron Balance Sheets
Energy profile of anaerobic glycolysis:
•
Most of the reactions function at or near
equilibrium and are freely reversible in vivo.
•
The three enzymes that catalyze reactions so
highly exergonic as to be virtually
irreversible(arrows) are subject to allosteric
control.
Copyright © 2013 Pearson Canada Inc.
13 - 40
Biochemistry, 4th Edition
Gluconeogenesis
Synthesis and use of glucose in the human body:
•
Liver and kidney cortex are the primary
gluconeogenic tissues.
•
Brain, skeletal muscle, kidney medulla,
erythrocytes, and testes use glucose as their sole
or primary energy source, but they lack the
enzymatic machinery to synthesize it.
•
Synthesis of glucose from noncarbohydrate
precursors is essential for maintenance of blood
glucose levels within acceptable limits.
Copyright © 2013 Pearson Canada Inc.
13 - 41
Biochemistry, 4th Edition
Gluconeogenesis
•
Gluconeogenesis uses specific enzymes to bypass
three irreversible reactions of glycolysis.
Reactions of glycolysis and gluconeogenesis:
•
Irreversible reactions of glycolysis are shown in dark
purple.
•
The opposed reactions in gluconeogenesis, which
bypass these steps, are shown in dark blue.
•
Pale arrows identify reversible reactions used in both
pathways.
Copyright © 2013 Pearson Canada Inc.
13 - 42
Biochemistry, 4th Edition
Gluconeogenesis
Bypass 1: Conversion of Pyruvate to Phosphoenolpyruvate
•
The bypass of pyruvate kinase begins in the mitochondrion and
involves two reactions.
•
Pyruvate carboxylase catalyzes the ATP- and biotin-dependent
conversion of pyruvate to oxaloacetate.
•
The enzyme requires acetyl-CoA as an allosteric activator:
Copyright © 2013 Pearson Canada Inc.
13 - 43
Biochemistry, 4th Edition
Gluconeogenesis
•
To be used for gluconeogenesis, oxaloacetate must move out of
the mitochondrion to the cytosol, where the remainder of the
pathway occurs.
•
However, the mitochondrial membrane does not have an effective
transporter for oxaloacetate.
•
Therefore, oxaloacetate is reduced by mitochondrial malate
dehydrogenase to malate, which is transported into the cytosol by
exchange for orthophosphate and then reoxidized by cytosolic
malate dehydrogenase.
•
Once in the cytosol, oxaloacetate is acted on by
phosphoenolpyruvate carboxykinase (PEPCK) to give
phosphoenolpyruvate:
Copyright © 2013 Pearson Canada Inc.
13 - 44
Biochemistry, 4th Edition
Gluconeogenesis
•
Bypass 2: Conversion of Fructose-1,6-bisphosphate to
Fructose-6-phosphate
o The PFK reaction of glycolysis is essentially irreversible, but
only because it is driven by phosphate transfer from ATP.
o A bypass reaction in gluconeogenesis involves a simple
hydrolytic reaction, catalyzed by fructose-1,6bisphosphatase.
•
Bypass 3: Conversion of Glucose-6-phosphate to Glucose
o Glucose-6-phosphate cannot be converted to glucose by
reverse action of hexokinase or glucokinase because of the
high positive DGo of that reaction; phosphate transfer from
ATP makes that reaction virtually irreversible.
o Another enzyme specific to gluconeogenesis, glucose-6phosphatase, also involves a simple hydrolysis.
Copyright © 2013 Pearson Canada Inc.
13 - 45
Biochemistry, 4th Edition
Gluconeogenesis
PEPCK isozymes provide alternative
routes to PEP and cytoplasmic reducing
equivalents:
Copyright © 2013 Pearson Canada Inc.
13 - 46
Biochemistry, 4th Edition
Gluconeogenesis
Copyright © 2013 Pearson Canada Inc.
13 - 47
Biochemistry, 4th Edition
Gluconeogenesis
Outline of pathways for glucose
synthesis from the major
gluconeogenic precursors:
Copyright © 2013 Pearson Canada Inc.
•
Note that both glucose and lactate are
carried in the blood.
•
The equivalent of 11 high-energy
phosphates are consumed per mole of
glucose synthesized by
gluconeogenesis.
•
The most important gluconeogenic
precursors are lactate, alanine,
glycerol, and propionate.
•
Animals cannot undergo net conversion
of fat to carbohydrate.
13 - 48
Biochemistry, 4th Edition
Gluconeogenesis
The Cori cycle:
•
Lactate produced in glycolysis during muscle exertion is transported
to the liver, for resynthesis of glucose by gluconeogenesis.
•
Transport of glucose back to muscle for synthesis of glycogen, and
its reutilization in glycolysis, completes the cycle.
Copyright © 2013 Pearson Canada Inc.
13 - 49
Biochemistry, 4th Edition
Gluconeogenesis
•
In all organisms a three-carbon acyl-CoA, propionyl-CoA, is
generated either from the breakdown of some amino acids or from
the oxidation of fatty acids with odd numbers of carbon atoms.
•
Propionyl-CoA enters gluconeogenesis via its conversion to
succinyl-CoA and then to oxaloacetate.
•
The process involves a coenzyme derived from vitamin B12.
Copyright © 2013 Pearson Canada Inc.
13 - 50
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
•
Louis Pasteur observed that when anaerobic yeast
cultures metabolizing glucose were exposed to air, the
rate of glucose utilization decreased dramatically.
•
It became clear that this phenomenon, known as the
Pasteur effect, involves the inhibition of glycolysis by
oxygen.
•
This effect makes biological sense because far more
energy is derived from complete oxidation of glucose
than from glycolysis alone.
Copyright © 2013 Pearson Canada Inc.
13 - 51
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Periodic oscillations of the levels of glycolytic
intermediates in yeast cells undergoing
glycolysis:
•The culture was shifted from aerobic to anaerobic
conditions at the point labeled “Air off.”
•The upper tracing depicts continuous monitoring of
fluorescence of the cell suspension, which is related
to the intracellular concentration of NADH.
•The nucleotide levels were determined in a parallel
experiment in which samples of culture were removed
at various times, extracted, and assayed for their
contents of ATP, ADP, and AMP
Copyright © 2013 Pearson Canada Inc.
13 - 52
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Alternative fates of
glycolytic intermediates in
biosynthetic pathways:
Copyright © 2013 Pearson Canada Inc.
13 - 53
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Comparative substrate cycles in glycolysis/gluconeogenesis:
Copyright © 2013 Pearson Canada Inc.
13 - 54
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Major control mechanisms
affecting glycolysis and
gluconeogenesis:
•
Note the strongly
exergonic reactions of
glycolysis and
gluconeogenesis and the
major activators and
inhibitors of these
reactions.
Conditions that promote
glycolysis inhibit
gluconeogenesis, and vice
versa.
Copyright © 2013 Pearson Canada Inc.
13 - 55
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
•
Fructose-2,6-bisphosphate is the most important regulator of
glycolysis and gluconeogenesis.
•
It is synthesized and degraded by different forms of the same
enzyme.
Copyright © 2013 Pearson Canada Inc.
13 - 56
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Allosteric control of liver PFK:
(a) Activation by fructose2,6-bisphosphate.
(b) How ATP increases the
apparent KM for
substrate fructose-6phosphate.
(c) Model of E. coli PFK
homotetramer.
Copyright © 2013 Pearson Canada Inc.
13 - 57
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Bifunctional PFK-2/FBPase-2:
Copyright © 2013 Pearson Canada Inc.
•
The X-ray crystal structure of one subunit of
the homodimeric human liver enzyme
•
The kinase active site is marked by a bound
nonhydrolyzable ATP analog.
13 - 58
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Regulation of the synthesis and degradation of fructose2,6-bisphosphate in liver:
•
The bifunctional PFK-2/FBPase-2 is controlled by reversible
phosphorylation of a specific serine residue near the Nterminus of each subunit of the homodimeric protein.
•
In the unphosphorylated form, the 6-phosphofructo-2-kinase
domain (K) is active, and fructose-2,6-bisphosphate
(F2,6BP) is synthesized.
•
In the phosphorylated form, the fructose-2,6bisphosphatase domain (B) is active, and F2,6BP is
degraded.
•
Glucagon stimulates phosphorylation by activating cAMPdependent protein kinase (PKA). Insulin and glucose (via
xyulose-5-phosphate) stimulate dephosphorylation by
activating a protein phosphatase.
Copyright © 2013 Pearson Canada Inc.
13 - 59
Biochemistry, 4th Edition
Coordinated Regulation of
Glycolysis and Gluconeogenesis
Regulation of liver hexokinase by protein–protein
interactions.
•Activity of the liver hexokinase isozyme IV (HKIV) is regulated by alternative
protein–protein interactions with the glucokinase regulatory protein (GKRP)
in the nucleus and dephosphorylated PFK- 2/FBPase-2 in the cytoplasm.
•Insulin binding to its plasma membrane receptor stimulates
dephosphorylation of PFK-2/FBPase-2. Glut2, plasma membrane glucose
transporter.
Copyright © 2013 Pearson Canada Inc.
13 - 60
Biochemistry, 4th Edition
Entry of Other Sugars into the
Glycolytic Pathway
Routes for utilizing substrates other than glucose in glycolysis:
•
In animals, most of the carbohydrate other than glucose and
glycogen comes from the diet, and most of the glycerol is derived
from lipid catabolism.
Copyright © 2013 Pearson Canada Inc.
13 - 61
Biochemistry, 4th Edition
Entry of Other Sugars into the
Glycolytic Pathway
Leloir pathway for utilizing galactose by
converting it to glucose-6-phosphate:
•
The galactose and glucose rings are colored
blue and orange, respectively, to emphasize
that reaction 3 is a group transfer, not an
epimerization.
•
Recall that glucose and galactose are
epimers at C-4.
Copyright © 2013 Pearson Canada Inc.
13 - 62
Biochemistry, 4th Edition
Entry of Other Sugars into the
Glycolytic Pathway
The three disaccharides most abundant in foods are maltose,
lactose, and sucrose:
•
Maltose is available primarily as an artificial sweetener, derived
from starch, while lactose and sucrose are abundant natural
products.
•
In animal metabolism they are hydrolyzed in cells lining the small
intestine, to give the constituent hexose sugars:
Copyright © 2013 Pearson Canada Inc.
13 - 63
Biochemistry, 4th Edition
Entry of Other Sugars into the
Glycolytic Pathway
•
The digestion of neutral fat (triacylglycerols) and most
phospholipids generates glycerol as one product.
•
In animals, glycerol first enters the glycolytic pathway by the action
in liver of glycerol kinase.
•
The product is then oxidized by glycerol-3-phosphate
dehydrogenase to yield dihydroxyacetone phosphate, which is
catabolized by glycolysis.
Copyright © 2013 Pearson Canada Inc.
13 - 64
Biochemistry, 4th Edition
Polysaccharide Metabolism
•
In animal metabolism, two primary sources of glucose are
derived from polysaccharides:
1. Digestion of dietary polysaccharides, chiefly starch from
plant foodstuffs and glycogen from meat.
2. Mobilization of the animal’s own glycogen reserves.
Copyright © 2013 Pearson Canada Inc.
13 - 65
Biochemistry, 4th Edition
Glycogen Metabolism in Muscle and Liver
•
The principal glycogen stores in vertebrates are in skeletal
muscle and liver.
•
Breakdown of these stores into usable energy, or mobilization of
glycogen, involves sequential phosphorolytic cleavages of
bonds, catalyzed by glycogen phosphorylase.
•
In plants, starch is similarly mobilized by the action of starch
phosphorylase .
•
Both reactions release glucose-1-phosphate from nonreducing
ends of the glucose polymer:
Copyright © 2013 Pearson Canada Inc.
13 - 66
Biochemistry, 4th Edition
Polysaccharide Metabolism
Cleavage of a glycosidic bond by hydrolysis or
Phosphorolysis:
This formal diagram shows how the elements of water
or phosphoric acid, respectively, are added across a
glycosidic bond.
Copyright © 2013 Pearson Canada Inc.
13 - 67
Biochemistry, 4th Edition
Polysaccharide Metabolism
•
Dietary polysaccharides are metabolized by
hydrolysis to monosaccharides.
•
Intracellular carbohydrate stores, as glycogen, are
mobilized as phosphorylated monosaccharides by
phosphorolysis.
•
a-Amylase in saliva cleaves bonds a(14)
between the maltose units of amylopectin (or
glycogen).
•
However, it cannot cleave a(16) glycosidic bonds
in the branched polymer, and a limit dextrin
accumulates unless a(16)-glucosidase
(debranching enzyme) is present.
Copyright © 2013 Pearson Canada Inc.
13 - 68
Biochemistry, 4th Edition
Glycogen Metabolism in Muscle and Liver
The debranching process in glycogen catabolism:
(a) A glycogen chain following activity by phosphorylase,
which cleaves off glucose residues to within four residues
of the branch point.
(b) The glycogen chain following transferase activity by the
debranching enzyme. The three remaining glucose
residues with linkage have been transferred to a nearby
nonreducing end.
(c) The glycogen chain following a(16)-glucosidase activity
by the debranching enzyme, which has removed the last
remaining glucose residue of the branch.
•
Phosphorylase will cleave off all but four glucose units of
the newly elongated branch, beginning the debranching
process again.
Copyright © 2013 Pearson Canada Inc.
13 - 69
Biochemistry, 4th Edition
Glycogen Metabolism in Muscle and Liver
•
UDP-glucose is the
metabolically activated form
of glucose for glycogen
synthesis.
•
Glycogen biosynthesis
requires glycogen synthase
for polymerization and a
transglycosylase to create
branches.
Copyright © 2013 Pearson Canada Inc.
13 - 70
Biochemistry, 4th Edition
Glycogen Metabolism in Muscle and Liver
The glycogen synthase reaction:
Copyright © 2013 Pearson Canada Inc.
13 - 71
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
•
The branching process in glycogen synthesis.
•
Branching is brought about by the action of amylo-(1,41,6)transglycosylase.
Copyright © 2013 Pearson Canada Inc.
13 - 72
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
The regulatory cascade controlling glycogen breakdown:
Copyright © 2013 Pearson Canada Inc.
13 - 73
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
•
Glycogen mobilization is controlled hormonally by a metabolic
cascade that is activated by cAMP formation and involves
successive phosphorylations of enzyme proteins.
•
The rapid mobilization of muscle glycogen triggered by
epinephrine is one of several components of the “fight or flight”
response.
Copyright © 2013 Pearson Canada Inc.
13 - 74
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
Control of glycogen phosphorylase activity:
Copyright © 2013 Pearson Canada Inc.
13 - 75
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
•
Conditions that activate glycogen breakdown inhibit glycogen
synthesis, and vice versa.
•
Glycogen synthase activity is controlled by phosphorylation,
through mechanisms comparable to those controlling glycogen
breakdown by phosphorylase but having reciprocal effects on
enzyme activity.
Copyright © 2013 Pearson Canada Inc.
13 - 76
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
Control of glycogen synthase activity:
Copyright © 2013 Pearson Canada Inc.
13 - 77
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
Regulation of phosphoprotein phosphatase 1 (PP1) in muscle:
Copyright © 2013 Pearson Canada Inc.
13 - 78
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
Regulation of phosphoprotein phosphatase 1 (PP1) in liver:
Copyright © 2013 Pearson Canada Inc.
13 - 79
Biochemistry, 4th Edition
Coordinated Regulation of Glycogen Metabolism
Copyright © 2013 Pearson Canada Inc.
13 - 80
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
The pentose phosphate pathway converts
glucose to various other sugars, which can
be used for energy.
Its most important products, however, are
NADPH and ribose-5-phosphate.
In stage 1, the oxidative phase, glucose-6phosphate is oxidized to ribulose-5-phosphate
and CO2, with production of NADPH.
The remaining stages constitute the
nonoxidative phase of the pathway.
Copyright © 2013 Pearson Canada Inc.
13 - 81
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
Oxidative phase of the pentose phosphate
pathway:
•
The three reactions of the oxidative phase
include two oxidations, which produce
NADPH.
•
The pentose phosphate pathway primarily
generates NADPH for reductive
biosynthesis and ribose-5-phosphate for
nucleotide biosynthesis.
Copyright © 2013 Pearson Canada Inc.
13 - 82
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
In the nonoxidative phase, some of the ribulose-5phosphate produced in the oxidative phase is converted to
ribose-5 phosphate by phosphopentose isomerase.
Copyright © 2013 Pearson Canada Inc.
13 - 83
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
The reaction sequence of the nonoxidative branch converts
3 five-carbon sugar phosphates to 2 six-carbon sugar
phosphates and 1 three-carbon sugar phosphate.
•
The hexose phosphates formed can be catabolized either
by recycling through the pentose phosphate pathway or by
glycolysis.
•
The triose phosphate is glyceraldehyde-3-phosphate, a
glycolytic intermediate.
•
Three enzymes involved are:
o phosphopentose epimerase
o transketolase
o transaldolase
Copyright © 2013 Pearson Canada Inc.
13 - 84
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
Copyright © 2013 Pearson Canada Inc.
The nonoxidative branch begins with both ribulose-5phosphate and ribose-5-phosphate.
13 - 85
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
Transketolase catalyzes a transfer of a two-carbon fragment
from xylulose-5-phosphate to ribose-5-phosphate to give a
triose phosphate, glyceraldehyde-3-phosphate, and a sevencarbon sugar, sedoheptulose-7-phosphate.
•
In the final reaction of pentose phosphate catabolism,
transketolase acts on another molecule of xylulose-5phosphate, transferring a glycolaldehyde fragment to
erythrose-4-phosphate and generating a three-carbon
product and a six-carbon product—glyceraldehyde-3phosphate and fructose-6-phosphate, respectively.
Copyright © 2013 Pearson Canada Inc.
13 - 86
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
Copyright © 2013 Pearson Canada Inc.
Transketolase requires a TPP cofactor to transfer its 2carbon unit as an activated aldehyde.
13 - 87
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
Copyright © 2013 Pearson Canada Inc.
13 - 88
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
Comparison of C-C bond cleavage in
transketolase and transaldolase
reactions:
Copyright © 2013 Pearson Canada Inc.
13 - 89
Biochemistry, 4th Edition
A Biosynthetic Pathway That Oxidizes Glucose:
The Pentose Phosphate Pathway
•
The pentose phosphate pathway
has different modes of operation to
meet varying metabolic needs.
a)
When the primary need is for nucleotide
biosynthesis, the primary product is ribose-5phosphate. Reducing equivalents from
NADPH are used to reduce ribonucleotides to
deoxyribonucleotide.
b)
When the primary need is for reducing power
(NADPH), fructose phosphates are
reconverted to glucose-6-phosphate for
reoxidation in the oxidative phase.
c)
When only moderate quantities of pentose
phosphates and NADPH are needed, the
pathway can also be used to supply energy,
with the reaction products being oxidized
through glycolysis and the citric acid cycle.
Copyright © 2013 Pearson Canada Inc.
13 - 90