Transcript (a) (b)

Chapter 6. Metabolism and its regulation
Section I. Carbohydrate Metabolism
1
1. What Are the Essential Features of Glycolysis?
In the glycolysis pathway, a molecule of glucose is converted in 10
enzyme-catalyzed steps to two molecules of 3-carbon pyruvate.
1930s, Most of the details of this pathway were worked out by Otto
Warburg, Gustav Embden, and Otto Meyerhof (German). This pathway is
often referred to as the Embden–Meyerhof Pathway (EMP).
Why is glycolysis so important to organisms?
For some tissues and cells, glucose is the only source of metabolic energy.
In addition, the product of glycolysis---pyruvate is a versatile metabolite
that can be used in several ways.
2
3
Reaction 1: Glucose Is Phosphorylated
Glucokinase—The First Priming Reaction
by
Hexokinase
or
 G° = -RT ln Keq
Hydrolysis of ATP releases 30.5 kJ/mol, and the phosphorylation of
glucose “costs” 13.8 kJ/mol. Thus, the reaction liberates 16.7 kJ/mol
under standard-state conditions (1mM, 25oC)
4
The incorporation of a phosphate into glucose is important for several
reasons:
First, phosphorylation keeps the substrate in the cell.
Moreover, rapid conversion of glucose to glucose-6-phosphate keeps the
intracellular concentration of glucose low, favoring faciliated diffusion of
glucose into the cell.
In addition, because
regulatory control can
be imposed only on
reactions
not
at
equilibrium,
the
favorable
thermodynamics of this first
reaction makes it an
important
site
for
regulation.
5
Enzyme that phosphorylates glucose is hexokinase, which required Mg2+ as
cofactor for this reaction.
There are four isozymes of hexokinase. Type I (Km=0.03mM) is the
principal form in the brain. Hexokinase in skeletal muscle is a mixture of
types I and II (Km=0.3mM). [normal blood glucose level is about 4 mM ]
The type IV, called glucokinase, is found predominantly in the liver and
pancreas with Km=10 mM.
6
Hexokinase Binds Glucose and ATP with an Induced Fit
In most organisms, hexokinase occurs in a single form: a two-lobed 50-kD
monomer that resembles a clamp, with a large groove in one side.
(c)
Glucose
The (a) open and (b) closed states of
yeast hexokinase. Binding of glucose
(green) induces a conformation change
that closes the active site.
Mammalian hexokinase I contains an Nterminal domain (top) and a C-terminal
domain (bottom) joined by a long -helix.
Each of these domains is similar in sequence
7
and structure to yeast hexokinase
Reaction 2: Phosphoglucoisomerase Catalyzes the Isomerization of
Glucose-6-Phosphate
G = -2.92kJ/mol
Phosphoglucoisomerase, with fructose6-P (blue) bound
The
phosphoglucoisomerase
mechanism
involves opening of the pyranose ring (step 1),
proton abstraction leading to enediol formation
(step 2), and proton addition to the double8bond,
followed by ring closure (step 3).
Reaction 3: ATP Drives a Second Phosphorylation
Phosphofructokinase—The Second Priming Reaction
by
Phosphofructokinase (PFK) is the “valve”
controlling the rate of glycolysis. In addition to
its role as a substrate, ATP is also an allosteric
inhibitor of this enzyme.
At high [ATP], PFK behaves cooperatively and
the plot of enzyme activity versus [fructose-6phosphate] is sigmoid. High [ATP] thus
decreases the enzyme’s affinity for fructose-6phosphate.
9
Reaction 4: Cleavage by Fructose Bisphosphate Aldolase Creates Two 3Carbon Intermediates
ΔGo' = 23.9 kJ/mol
ΔG= -0.23 kJ/mol
Class I aldolases, found in animals and plants, characterized by the
formation of a covalent Schiff base intermediate between an active-site
lysine and the carbonyl group of the substrate.
Class II enzymes, in fungi and bacteria, do not form the Schiff base
intermediate. Instead, a zinc ion is required for its activity.
10
(a)
(b)
(a) The Schiff base formed between
the substrate carbonyl and an activesite lysine acts as an electron sink,
increasing the acidity of the hydroxyl group and facilitating
cleavage as shown. The catalytic
residues in the rabbit muscle enzyme
are Lys229 and Asp33. (b) In Class II
aldolases,
an
active-site
Zn2
stabilizes the enolate intermediate,
leading to polarization of the
substrate carbonyl group.
11
Reaction 5: Interconversion of the Triose Phosphates
Only one of the two triose phosphates formed by aldolase, glyceraldehyde
3-phosphate, can be directly degraded in the subsequent steps of glycolysis.
The other product, dihydroxyacetone phosphate, is rapidly and reversibly
converted to glyceraldehyde 3-phosphate.
12
Reaction 6: Glyceraldehyde-3-Phosphate Dehydrogenase Creates a HighEnergy Intermediate
ΔGo' = +6.3 kJ/mol
ΔG = -1.29 kJ/mol
Glyceraldehyde-3-phosphate is oxidized to 1,3-bisphosphoglycerate by
glyceraldehyde-3-phosphate dehydrogenase. The overall reaction involves
both formation of a carboxylic–phosphoric anhydride (a high-energy
phosphate compound) and the reduction of NAD to NADH.
13
A
mechanism
for
the
glyceraldehyde-3-phosphate
dehydrogenase
reaction.
Reaction of an enzyme
sulfhydryl with the carbonyl
carbon of glyceraldehyde-3-P
forms a thiohemiacetal, which
loses a hydride to NAD to
become
a
thioester.
Phosphorolysis of this thioester
releases
1,314
bisphosphoglycerate.
Reaction 7: Phosphoryl Transfer from 1,3-Bisphosphoglycerate to ADP
substrate-level
phosphorylation
The phosphoglycerate kinase reaction is sufficiently exergonic to pull the
G-3-P dehydrogenase reaction along.
15
Reaction 8: Phosphoglycerate Mutase Catalyzes a Phosphoryl Transfer
ΔGo' = 4.4 kJ/mol
ΔG = 0.83 kJ/mol
Phosphoglycerate mutase is a
phosphoenzyme,
with
a
phosphoryl group covalently
bound to a histidine residue at
the active site. This phosphoryl
group is transferred to the C-2
position of the substrate to
form a transient, enzymebound 2,3-bisphosphoglycerate,
which then decomposes by a
second phosphoryl transfer
from the C-3 position of the
intermediate to the histidine
residue on the enzyme..
The catalytic histidine (His183) at the active site of E. coli
phosphoglycerate mutase. Note that His10 is phosphorylated.
16
Reaction 9: Dehydration by Enolase Creates PEP
ΔGo' = 1.8 kJ/mol
ΔG = 1.1 kJ/mol
What the enolase reaction does is rearrange the substrate into a form from
which more of this potential energy can be released upon hydrolysis.
(a)
(b)
The yeast enolase dimer is asymmetric. The active site of one subunit (a) contains 2phosphoglycerate, the substrate. Also shown are a Mg2+ (blue), a Li + (purple), and His159,
which participates in catalysis. The other subunit (b) binds phosphoenolpyruvate, the 17
product of the enolase reaction.
Reaction 10: Pyruvate Kinase Yields More ATP
Pyruvate kinase is a suitable target site for regulation of glycolysis and
possesses allosteric sites for numerous effectors. It is activated by AMP
and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and
alanine.
18
What are The metabolic fates of NADH and Pyruvate
Partially depends on the availability of oxygen.
Under aerobic conditions, pyruvate can be sent into the citric acid cycle,
whereas NADH is reoxidized to NAD+ in the mitochondrial electrontransport chain.
Anaerobic Metabolism of Pyruvate Leads to Lactate or Ethanol
Pyruvate
decarboxylase
(a) Pyruvate reduction to ethanol in yeast. (b) In oxygen-depleted muscle, NAD is 19
regenerated in the lactate dehydrogenase reaction.
GLYCOLYSIS PATHWAY
20
3. The Tricarboxylic Acid Cycle
For most eukaryotic cells and many bacteria,
which live under aerobic conditions and oxidize
their organic fuels to carbon dioxide and water--cellular respiration.
First stage, organic fuel molecules are oxidized
to yield two-carbon fragments of acetylcoenzyme A (acetyl-CoA).
Second stage, the acetyl groups are oxidized to
CO2 via TCA pathway; the energy released is
conserved in the reduced electron carriers
NADH and FADH2.
Third stage, reduced coenzymes are via the
respiratory chain, energy released is conserved
in the form of ATP.
21
1. Production of Acetyl-CoA
The pyruvate dehydrogenase complex requires five coenzymes:
thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD),
coenzyme A (CoA), nicotinamide adenine dinucleotide (NAD), and
lipoate.
Reactive
thiol group
22
The Pyruvate Dehydrogenase Complex Consists of Three Distinct Enzymes:
pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and
dihydrolipoyl dehydrogenase (E3)—each present in multiple copies.
In the bovine enzyme complex, 60 identical copies of E2 form a
dodecahedron core with a diameter of about 25 nm.
E2 has three domains: the amino-terminal lipoyl domain; the central E1- and
E3-binding domain; and the inner-core acyltransferase domain.
The active site of E1 and E3 has bound TPP and FAD, respectively. The
attachment of lipoate to E2 produces a long, flexible arm that can move
from the active site of E1 to the active sites of E2 and E3.
23
In Substrate Channeling, Intermediates Never Leave the Enzyme Surface
(1) Pyruvate reacts with the
bound
TPP
of
E1,
undergoing decarboxylation
(2) E1 catalyze the transfer
of two electrons and the
acetyl group from TPP to
lipoyllysyl group of E2, to
form the acetyl thioester of
the reduced lipoyl group.
(3) Transesterification in which the -SH group of CoA replaces the -SH group of
E2 to yield acetyl-CoA and the fully reduced form of the lipoyl group. (4) E3
promotes transfer of two hydrogen atoms from the reduced lipoyl groups to the
FAD. (5) The reduced FADH2 transfers a hydride ion to NAD, forming NADH.
2. Citric Acid Cycle
25
Step 1. Formation of Citrate
In this reaction the methyl carbon of the acetyl group is joined to the
carbonyl group (C-2) of oxaloacetate. Citroyl-CoA is a transient
intermediate formed on the active site of the enzyme.
26
CoA-SH
3
H2O
2
27
Step 2. Formation of Isocitrate via cis-Aconitate
∆G0=13.3 kJ/mol
This reaction is pulled to the right because isocitrate is rapidly consumed in
the next step of the cycle, lowering its steady-state concentration.
The active site of aconitase. The
iron–sulfur cluster (pink) is
coordinated
by
cysteines
(orange) and isocitrate (purple)
28
Step 3. Oxidation of Isocitrate to α-Ketoglutarate and CO2
Two different forms of isocitrate dehydrogenase, one requiring NAD as
electron acceptor (occurs in the mitochondrial matrix and serves in the
citric acid cycle) and the other requiring NADP found in both the
mitochondrial matrix and the cytosol (reductive anabolic reactions).
Both the two forms need Mg2+ as cofactor.
29
Step 4. Oxidation of α-Ketoglutarate to Succinyl-CoA and CO2
In this reaction, NAD serves as electron acceptor and CoA as the carrier of
the succinyl group.
This reaction is virtually identical to the pyruvate dehydrogenase reaction,
and the α-ketoglutarate dehydrogenase complex closely resembles the
PDH complex in both structure and function (E1, E2 and E3).
30
Step5. Conversion of Succinyl-CoA to Succinate
In step 1 a phosphoryl group replaces the CoA of
succinyl-CoA, forming a high-energy acyl phosphate. In
step 2 the succinyl phosphate donates its phosphoryl
group to a His residue on the enzyme, forming a highenergy phosphohistidyl enzyme. In step 3 the
phosphoryl group is transferred from the His residue to
GDP (or ADP), forming GTP (or ATP).
31
Step 6. Oxidation of Succinate to Fumarate
Step 7. Hydration of Fumarate to Malate
This enzyme is highly stereospecific; it
catalyzes hydration of the trans double
bond of fumarate but not the cis double
bond of. In the reverse direction (from Lmalate to fumarate), fumarase is equally
stereospecific: D-malate is not a substrate.
32
Step 8. Oxidation of Malate to Oxaloacetate
∆Go = 29.7 kJ/mol
In intact cells, oxaloacetate is continually removed by the highly exergonic
citrate synthase reaction. This keeps the concentration of oxaloacetate in the
cell extremely low (<10-6 M), pulling the malate dehydrogenase reaction
toward the formation of oxaloacetate.
33
The Energy of Oxidations in the Cycle Is Efficiently Conserved
The citric acid cycle directly generates only one ATP, four NADH and
one FADH2.
In oxidative phosphorylation, passage of two electrons from NADH (or
FADH2) to O2 drives the formation of about 2.5 ATP (or about1.5 ATP).
In round numbers, 32×30.5 kJ/mol = 976 kJ/mol, or 34% of the
theoretical maximum of about 2,840 kJ/mol available from the complete
oxidation of glucose (standard free energy changes) were conserved.
When corrected for the actual free energy required to form ATP within
cells, the calculated efficiency of the process is closer to 65%.
34
Citric Acid Cycle Components Are Important Biosynthetic Intermediates
In aerobic organisms, the citric acid cycle is an amphibolic pathway, one
that serves in both catabolic and anabolic processes.
Role of the citric acid
cycle in anabolism.
35
3. Gluconeogenesis
The central role of glucose in metabolism--fuel and building block
In mammals, some tissues depend almost
completely on glucose for their metabolic
energy (like brain and nervous system,
erythrocytes, renal medulla).
Supply of glucose from stores is not always
sufficient, like during longer fasts, or after
vigorous exercise.
Through a pathway called gluconeogenesis,
converts pyruvate and related three- and
four-carbon compounds to glucose.
Precursor
36
Gluconeogenesis and glycolysis are not
identical pathways running in opposite
directions, although they do share several
steps. However, three reactions of
glycolysis are essentially irreversible and
cannot be used in gluconeogenesis:
•conversion
of
pyruvate
to
phosphoenolpyruvate
•conversion of fructose 1,6-bisphosphate
to fructose 6-phosphate
•conversion of glucose 6-phosphate to
glucose
37
Conversion of Pyruvate to Phosphoenolpyruvate Requires Two
Exergonic Reactions
This reaction cannot occur by reversal of the pyruvate kinase reaction of
glycolysis. Instead, the phosphorylation of pyruvate requires enzymes in both
the cytosol and mitochondria.
In mitochondria, pyruvate is converted to
oxaloacetate in a biotin-requiring reaction
catalyzed by pyruvate carboxylase.
Mitochondrial membrane has no transporter
for oxaloacetate
In the cytosol, oxaloacetate is converted
to
phosphoenolpyruvate
by
PEP
carboxykinase.
38
Gluconeogenesis Is Energetically Expensive, but Essential
Citric Acid Cycle Intermediates and Many Amino Acids Are Glucogenic
Citrate, isocitrate, -ketoglutarate, succinyl-CoA, succinate, fumarate, and
malate—all are citric acid cycle intermediates that can undergo oxidation
†
to oxaloacetate.
Some or all of the carbon atoms
of most amino acids are
ultimately
catabolized
to
pyruvate or to intermediates of
the citric acid cycle.
In contrast, no net conversion of
fatty acids to glucose occurs in
mammals.
39
Glucogenic Amino Acids. †site of entry
Chapter 6. Metabolism and its regulation
Section II. Metabolic Regulation: Glucose
and Glycogen
BEIJING, METRO MAP
METABOLIC PATHWAYS
40
1. The Metabolism of Glycogen in Animals
In a wide range of organisms, excess glucose is converted to polymeric
forms for storage—glycogen in vertebrates and many microorganisms,
starch in plants. [may represent up to 10% of the weight of liver and 1% to
2% of the weight of muscle]
The glycogen in muscle is there to
provide a quick source of energy
for either aerobic or anaerobic
metabolism.
Liver glycogen serves as a
reservoir of glucose for other
tissues when dietary glucose is not
available
Glycogen granules in a hepatocyte
41
(1) Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase
Glucose units of glycogen enter
glycolytic pathway through the action of
three enzymes: glycogen phosphorylase,
glycogen debranching enzyme, and
phosphoglucomutase.
Stop at the fourth
glucose
residues
away from an (1 → 6)
branch point
(1→4)
Transfer
branches
42
Glucose 1-phosphate is converted to
glucose
6-phosphate
by
phosphoglucomutase.
The glucose 6-phosphate in skeletal
muscle can enter glycolysis and serve as
an energy source.
In liver, it can be released into the blood
when the blood glucose level drops, which
need glucose 6-phosphatase with the
active site on the lumenal side of the ER.
43
(2) The Sugar Nucleotide UDP-Glucose Donates Glucose for Glycogen
Synthesis
Many reactions in which hexoses are transformed or polymerized involve
sugar nucleotides (active form). What’s the aim of this transformation?
•Their formation is metabolically irreversible, contributing to the irreversible
synthetic pathways in which they are intermediates.
•Nucleotide moiety has many
groups that can undergo
noncovalent interactions with
enzymes.
•Nucleotidyl group is an
excellent
leaving
group,
facilitating nucleophilic attack
by activating the sugar carbon
to which it is attached.
•By “tagging” some hexoses
with nucleotidyl groups, cells
can set them aside in a pool
for glycogen synthesis.
G 0’ = -19.2 kJ/mol
44
Glycogen synthesis takes place prominently in the liver and skeletal muscles.
The starting point for synthesis of glycogen is glucose 6-phosphate.
hexokinase I and II in muscle
and hexokinase IV in liver
phosphoglucomutase
UDP-glucose pyrophosphorylase
UDP-glucose is immediate
donor of glucose residues
in the reaction of glycogen
synthase
45
Glycogen synthase cannot make the (1→6) bonds found at the branch points
of glycogen; these are formed by the glycogen-branching enzyme, also
called amylo (1→4) to (1→6) transglycosylase.
The glycogen-branching enzyme catalyzes transfer of a terminal fragment of
6 or 7 glucose residues from the nonreducing end of a glycogen branch
having at least 11 residues to the C-6 hydroxyl group of a glucose residue at
a more interior position of the same or another glycogen chain, thus creating
a new branch.
46
How is a new glycogen molecule initiated?
A: Glycogenin primes the initial sugar residues in glycogen, and it also
catalyze this biosynthesis reaction.
Muscle
glycogenin
forms dimers. The
substrate,
UDPglucose (shown in red
ball-and-stick),
is
bound to a Rossman
fold near the amino
terminus.
Glycogenin catalyzes two distinct reactions. (1)
Transfer of a glucose residue from UDP-glucose
to the hydroxyl group of Tyr194 of glycogenin,
catalyzed
by
the
protein’s
intrinsic
glucosyltransferase activity; (2) The nascent
chain is extended by the sequential addition of
seven more glucose residues, each derived from
UDP-glucose, catalyzed by the chain-extending
activity of glycogenin.
47
At this point, glycogen synthase takes
over, further extending the glycogen
chain. Glycogenin remains buried
within the particle, covalently
attached to the single reducing end of
the glycogen molecule.
Structure of the glycogen particle. Starting at
a central glycogenin molecule, glycogen
chains (12 to 14 residues) extend in tiers.
Inner chains have two (1→6) branches each.
Chains in the outer tier are unbranched. There
are 12 tiers in a mature glycogen particle
(only 5 are shown here), consisting of about
55,000 glucose residues in a molecule of
about 21 nm diameter and Mr 107.
48
2. Coordinated Regulation of Glycolysis and Gluconeogenesis
Gluconeogenesis employs most of
the enzymes that act in glycolysis,
but it is not simply the reversal of
glycolysis.
Seven of the glycolytic reactions
are freely reversible, other three
reactions of glycolysis are so
exergonic as to be irreversible:
those catalyzed by hexokinase,
PFK, and pyruvate kinase.
49
(1) Hexokinase Isozymes Are Affected Differently by Their Product
There are four hexokinase isozymes (designated I to IV). I~III type in
muscle, working for energy production, whereas IV in liver working for
maintaining blood glucose homeostasis.
At
normal
blood
glucose
concentration, hexokinase I~III acts
at or near its maximal rate, whereas
the activity of type IV is very low;
When
the
blood
glucose
concentration is high, as it is after a
meal rich in carbohydrates, excess
glucose
is
transported
into
hepatocytes, where hexokinase IV
converts it to glucose 6-phosphate.
Km=0.1 mM
glucose in blood 4~5 mM
Km=10 mM
50
Hexokinase IV is subject to inhibition by the reversible binding of a
regulatory protein specific to liver.
Glucose causes dissociation of the regulatory protein from hexokinase IV,
relieving the inhibition (immediately after meal). The binding is much
tighter in the presence of the allosteric effector fructose 6-phosphate (blood
glucose drops below 5 mM), draws this enzyme into nucleus.
51
(2) Phosphofructokinase-1 Is under Complex Allosteric Regulation
In addition to its substrate-binding sites, PFK has several regulatory sites
at which allosteric activators or inhibitors bind (e.g. ATP, citrate, fructose
2,6-bisphosphate).
Substrate
(fructose 1,6bisphosphate)
Allosteric
effector (ADP)
52
(3) Pyruvate Kinase Is Allosterically Inhibited by ATP
Pyruvate kinase is allosterically inhibited by ATP, acetyl-CoA, long-chain
fatty acids (signals for energy supply) and Ala (one-step transformation of
pyruvate), and fructose 1,6-bisphosphate triggers its activation.
In liver, glucagon inactivates pyruvate kinase (L isoenzyem) via
phosphorylation. When the glucagon level drops, pyruvate kinase is
activating via dephosphorylation.
53
(4) Gluconeogenesis Is Regulated at Several Steps
The 1st control point, pyruvate can be
converted to glucose and glycogen via
gluconeogenesis or oxidized to acetyl-CoA
for energy production. This enzyme in
each path is regulated allosterically;
acetyl-CoA, produced either by fatty acid
oxidation
or
by
the
pyruvate
dehydrogenase
complex,
stimulates
pyruvate carboxylase and inhibits pyruvate
dehydrogenase.
54
The second control point in
gluconeogenesis is the reaction
catalyzed by fructose 1,6bisphosphatase-1, which is
strongly inhibited by AMP.
The corresponding glycolytic
enzyme, PFK-1, is stimulated
by AMP and ADP but inhibited
by citrate and ATP.
In general, when sufficient concentrations of acetyl-CoA or citrate are
present, or when a high energy charge, gluconeogenesis is favored. AMP
promotes glycogen degradation and glycolysis by activating glycogen
phosphorylase (via activation of phosphorylase kinase) and stimulating the
activity of PFK-1.
55
3. Coordinated Regulation of Glycogen Synthesis and Breakdown
(1) Regulation of Glycogen Phosphorylase
Two interconvertible forms of glycogen phosphorylase, a (active) and b
(inactive).
Phosphorylase b predominates in resting muscle, but during vigorous
muscular activity the epinephrine triggers phosphorylation of
phosphorylase b, converting it to it’s a form, catalyzed by phosphorylase b
kinase.
Cascade mechanism of epinephrine
and glucagon action.
Glucose binding to an allosteric site of the phosphorylase a isozyme of liver induces a
conformational change that exposes its phosphorylated Ser residues to the action of
phosphorylase a phosphatase 1(PP1). phosphorylase a was converted to phosphorylase b.
56
(2) Glycogen Synthase Is Also Regulated by Phosphorylation and
Dephosphorylation
Two forms of glycogen Synthase: glycogen synthase a, active form, is
unphosphorylated; glycogen synthase b, inactive, is phosphorylated
(several sites).
Glycogen synthase kinase 3 (GSK3)
catalyzed the phosphorylation of
glycogen synthase (three sites) and
inactive it.
57
The action of GSK3 needs other protein kinase casein kinase II (CKII),
has first phosphorylated the glycogen synthase on a nearby residue, an
event called priming.
In liver, conversion of glycogen synthase b to the active form is promoted
by phosphorprotein phosphatase (PP1), which removes the phosphoryl
groups. Glucose 6-phosphate binds to an allosteric site on glycogen
synthase b, making the enzyme a better substrate for dephosphorylation by
PP1 and causing its activation.
58
(3) Glycogen Synthase Kinase 3 Mediates the Actions of Insulin
Insulin binding to its
receptor activates a tyrosine
protein
kinase,
which
phosphorylates
insulin
receptor substrate-1 (IRS-1).
IRS-1 is then bound by phosphatidylinositol 3-kinase (PI-3K), which
converts phosphatidylinositol 4,5bisphosphate (PIP2) in to PIP3.
tyrosine protein kinase
A protein kinase (PDK-1) that is activated
when bound to PIP3 activates a second
protein
kinase
(PKB),
which
phosphorylates glycogen synthase kinase 3
(GSK3). The inactivation of GSK3 allows
phosphoprotein phosphatase 1 (PP1) to
dephosphorylate glycogen synthase (active
form), therefore, stimulates glycogen
synthesis
59
(4) Phosphoprotein Phosphatase 1 Is Central to Glycogen Metabolism
•Phosphoprotein Phosphatase 1 can remove phosphoryl groups from
phosphorylase kinase, glycogen phosphorylase, and glycogen synthase,
which involved in response to glucagon and epinephrine.
•PP1 does not exist free in the cytosol, but is tightly bound to glycogentargeting proteins (GM) that bind glycogen and above mentioned enzymes.
Glycogen
granule
(1)Insulin-stimulated phosphorylation
of GM site 1 activates PP1, which
dephosphorylates
phosphorylase
kinase, glycogen phosphorylase, and
glycogen synthase. (2) Epinephrine
stimulated phosphorylation of GM
site 2 causes dissociation of PP1 from
the glycogen particle, preventing its
access to glycogen phosphorylase and
glycogen synthase. PKA also
phosphorylates a protein (inhibitor 1)
that, when phosphorylated, inhibits
PP1.
60
(5) Allosteric and Hormonal Signals Coordinate Carbohydrate
Metabolism
Carbohydrate metabolism is well regulated by insulin, glucagon, and
epinephrine.
Elevated of blood glucose triggers insulin release. In hepatocyte, insulin has
two immediate effects: it inactivates GSK3 and activates PP1, therefore,
activates glycogen synthesis.
High capacity
PP1 inactivates glycogen
transporter
phosphorylase
a
and
phosphorylase
kinase,
stopping
glycogen
breakdown.
Glucose enters the hepatocyte
through GLUT2, and the
elevated intracellular glucose
leads to hexokinase IV
entering the cytosol and
stimulating glycolysis and
supplying the precursor for
61
glycogen synthesis.
Low blood glucose triggers the release of glucagon, which activates PKA.
This enzyme can
•Phosphorylate phosphorylase kinase,
activating it and leading to the activation
of glycogen phosphorylase.
•Phosphorylate
glycogen
synthase,
inactivating it and blocking glycogen
synthesis.
•Phosphorylate PFK-2/FBPase-2, leading
to a drop in the concentration of the
regulator fructose 2,6-bisphosphate,
which inactivate the glycolytic enzyme
PFK-1 and activate the gluconeogenic
enzyme FBPase-1.
•Phosphorylate and inactivates the
glycolytic enzyme pyruvate kinase.
62
Insulin Changes the Expression of Many Genes Involved in Carbohydrate
and Fat Metabolism
In addition to its effects on the activity of existing enzymes, insulin also
regulates the expression of as many as 150 genes, including some related
to fuel metabolism.
63
FURTHER READING:
Simoni RD, Hill RL, Vaughan M. (2002) Carbohydrate metabolism: glycogen
phosphorylase and the work of Carl F. and Gerty T. Cori. J. Biol. Chem. 277
(www.jbc.org/cgi/content/full/277/29/e18).
Gibbons BJ, Roach PJ, Hurley TD. (2002) Crystal structure of the autocatalytic
initiator of glycogen biosynthesis, glycogenin. J. Mol. Biol. 319, 463–477.
Barford D. (1999) Structural studies of reversible protein phosphorylation and
protein phosphatases. Biochem. Soc. Trans. 27, 751–766.
de la Iglesia N, Mukhtar M, Seoane J, Guinovart JJ, Agius L. (2000) The role of the
regulatory protein of glucokinase in the glucose sensory mechanism of the
hepatocyte. J. Biol. Chem. 275, 10,597–10,603.
Nordlie RC, Foster JD, Lange AJ. (1999) Regulation of glucose production by the
liver. Annu. Rev. Nutr. 19, 379–406.
Harwood AJ. (2001) Regulation of GSK-3: a cellular multiprocessor. Cell 105,
821–824.
Radziuk J, Pye S. (2001) Hepatic glucose uptake, gluconeogenesis and the
regulation of glycogen synthesis. Diabetes/Metab. Res. Rev. 17, 250–272.
64