Transcript glycogen metabolism

GLYCOGEN METABOLISM
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Glycogen Structure
• Most of the glucose residues in glycogen are linked by
a-1,4-glycosidic bonds.
• Branches at about every tenth residue are created by a1,6-glycosidic bonds.
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Glycogen is an important fuel reserve
for several reasons
• Glycogen serves as a buffer to maintain bloodglucose levels
– Especially important because glucose is virtually
the only fuel used by the brain.
– Is good source of energy for sudden, strenuous
activity
• Unlike fatty acids, it can provide energy in the
absence of oxygen
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The major sites of glycogen storage
1. liver
2. The skeletal muscles
• Glycogen is present in the
cytosol in the form of
granules ranging in
diameter from 10 to 40 nm
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Glycogen Metabolism
Glycogenesis
• Addition of α1-4
linkages to nonreducing ends.
• 1 ATP per linkage.
• Branching enzymes.
• Inactivated by cAMP
Glycogenolysis
• Cleave α1-4 linkages at
non-reducing ends.
• Phosphorolysis (Pi in place
of H2O.
• Debranching enzymes.
• Activated by cAMP
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Glycogen degradation
Consists of three steps e
1. release of G1-P from glycogen
2. The remodeling of the glycogen substrate to permit
further degradation
3. The conversion of G1-P into G6-P.
It is the initial substrate for glycolysis
1. it can be processed by the pentose phosphate pathway to
yield NADPH and ribose derivatives
2. it can be converted into free glucose for release into the
bloodstream.
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Glycogen degradation
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Regulation of Glycogen metabolism
• Allosterically:
– By adjustment of enzyme activity to meet the needs of
the cell.
• Hormones stimulate cascades that lead to reversible
phosphorylation of the enzymes, which alters their
kinetic properties.
– Regulation by hormones allows glycogen metabolism to
adjust to the needs of the entire organism.
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Glycogen Breakdown Requires the Interplay
of Several Enzymes
• Four enzyme activities:
– one to degrade glycogen,
– two to remodel glycogen so that it remains a
substrate for degradation
– one to convert the product of glycogen
breakdown into a form suitable for further
metabolism.
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Glycogen Phosphorylase: A key enzyme
• Cleaves its substrate by the addition of orthophosphate
(Pi) to yield G1-P (phosphorolysis)
• Catalyzes the sequential removal of glycosyl residues
from the nonreducing ends of the glycogen molecule
(the ends with a free 4-OH groups)
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• DG°´ for this reaction is small because a
glycosidic bond is replaced by a phosphoryl
ester bond that has a nearly equal transfer
potential.
• Phosphorolysis proceeds far in the direction of
glycogen breakdown in vivo because the
[Pi]/[G6-P] ratio is usually >100, substantially
favoring phosphorolysis.
• The phosphorolytic cleavage of glycogen is
energetically advantageous because the
released sugar is already phosphorylated
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Two Remodeling Enzymes
• Transferase:
– Shifts a block of three glycosyl residues from one
outer branch to the other
• a-1,6-glucosidase (debranching enzyme)
– Hydrolyzes the a-1, 6-glycosidic bond, resulting in
the release of a free glucose molecule.
– Glucose is phosphorylated by hixokinase
(glycolysis)
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• This paves the way for
further cleavage by
phosphorylase.
• In eukaryotes, the
transferase and the a1,6-glucosidase activities
are present in a single
polypeptide chain, in a
bifunctional enzyme
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Phosphoglucomutase
• G1-P formed in the phosphorolytic cleavage of glycogen
must be converted into G6-P to enter the metabolic
mainstream.
• This enzyme is also used in galactose metabolism
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Serine
G1-P
G6-P
G1,6-BP
Serine
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• You may recall that the enzyme glucose-6phosphatase (G6Pase) catalyzes the last step of
gluconeogenesis - conversion of G6P to glucose +
phosphate.
• This enzyme necessary also for release of glucose
into the bloodstream from glycogen metabolism
(glycogen -> G1P -> G6P -> Glucose).
• It is interesting to note that G6Pase is ABSENT FROM
MUSCLE.
• This is because muscle does NOT export glucose. the
liver, on the other hand, DOES export glucose and
thus has abundant supplies of the enzyme.
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Liver Contains G6-Pase; a hydrolytic
enzyme absent from Muscle
 A major function of the liver is to maintain a near constant
level of glucose in the blood.
 The liver G6-Pase, cleaves the phosphoryl group to form free
glucose and orthophosphate.
 This G6-Pase, is located on the lumenal side of the smooth
endoplasmic reticulum membrane
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Glycogen Phosphorylase
Pyridoxal Phosphate
integral group of the
Enzyme
The Pi
substrate
binding site
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• In human, liver phosphorylase and muscle
phosphorylase are approximately 90%
identical in amino acid sequence.
• The differences result in important shifts in
the stability of various forms (isozymes) of the
enzyme.
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Phosphorylase exists in two states
The T state is less active
because the catalytic site is
partly blocked.
The R state, catalytic site is
more accessible and a binding
site for orthophosphate is well
organized.
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Phosphorylase Is Regulated by:
• Allosteric Interactions:
– By several allosteric effectors that signal the energy state of
the cell
• Reversible Phosphorylation:
– responsive to hormones such as:
• Insulin
• Epinephrine
• Glucagon
• The glycogen metabolism regulation differs in muscle
than in liver because:
– The muscle uses glucose to produce energy for itself,
whereas the liver maintains glucose homeostasis of the
organism as a whole
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phosphorylase
B
usually inactive
ATP
Phosphorylase kinase
P
A
usually active
Phosphorylase a differs
from b by a phosphoryl
group on each subunit
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The R and T states of each of the a or b forms are in
equilibrium
The equilibrium for
phosphorylase a, favors the Rstate
The equilibrium for
phosphorylase b, favors the Tstate
Depending on
cellular conditions
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In muscles-Posphorylase b
• High AMP, binds to a nucleotide-binding site and stabilizes the
conformation of phosphorylase b in the R-state.
• ATP acts as a negative allosteric effector by competing with
AMP and so favors the T-state.
• G6-P also favors the T-state of phosphorylase b, an example of
feedback inhibition
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• Under most physiological conditions,
phosphorylase b is inactive because of the
inhibitory effects of ATP and G6-P.
• In contrast, phosphorylase a is fully active,
regardless of the levels of AMP, ATP, and G6-P.
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Liver Phosphorylase Produces Glucose for Use by
Other Tissues
• In contrast with the muscle enzyme, liver
phosphorylase a but not b exhibits the most
responsive T-to-R transition.
• The binding of glucose shifts the allosteric
equilibrium of the a form from the R to the T state,
deactivating the enzyme
• Unlike the enzyme in muscle, the liver phosphorylase
is insensitive to regulation by AMP because the liver
does not undergo the dramatic changes in energy
charge seen in a contracting muscle
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Phosphorylase
kinase
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 Phosphorylase kinase in the skeletal muscle: is
(abgd)4
 g is catalytic
 abd are regultory
 d is calmodulin
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Epinephrine and Glucagon Signal the Need
for Glycogen Breakdown
• Muscular activity or its anticipation leads to
the release of epinephrine (adrenaline),from
the adrenal medulla.
• Epinephrine markedly stimulates glycogen
breakdown in muscle and, to a lesser extent,
in the liver.
• The liver is more responsive to glucagon, a
polypeptide hormone that is secreted by the
a cells of the pancreas when the blood-sugar
level is low.
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• Epinephrine binds to the b-adrenergic receptor in muscle,
whereas glucagon binds to the glucagon receptor in liver.
• These binding events activate the a subunit of the
heteromeric Gs protein.
• A specific external signal is transmitted into the cell
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Glycogen Is Synthesized and Degraded by
Different Pathways
• glycogen is synthesized by a pathway that
utilizes uridine diphosphate glucose (UDPglucose) rather than G1-P as the activated
glucose donor.
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UDP-Glucose Is an Activated Form of
Glucose
• UDP-glucose, the glucose donor
in the biosynthesis of glycogen,
is an activated form of glucose.
• The C-1 carbon atom of the
glucosyl unit of UDP-glucose is
activated because its hydroxyl
group is esterified to the
diphosphate moiety of UDP.
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• UDP-glucose is synthesized from G1-P and (UTP) in a
reaction catalyzed by UDP-glucose
pyrophosphorylase.
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• This reaction is readily reversible.
• Pyrophosphate is rapidly hydrolyzed in vivo to
orthophosphate by an inorganic pyrophosphatase.
• The essentially irreversible hydrolysis of
pyrophosphate drives the synthesis of UDP-glucose.
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Glycogen Synthase Catalyzes the Transfer of
Glucose from UDP-Glucose to a Growing Chain
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• glycogen synthase, is the key regulatory enzyme in
glycogen synthesis.
• It can add glucosyl residues only if the polysaccharide
chain already contains more than four residues.
• Thus, glycogen synthesis requires a primer.
– This priming function is carried out by glycogenin, a protein
composed of two identical 37-kd subunits, each bearing an
oligosaccharide of a-1,4-glucose units.
– C1 of the first unit of this chain, the reducing end, is
covalently attached to the phenolic hydroxyl group of a
specific tyrosine in each glycogenin subunit.
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How is this chain formed?
• Each subunit of glycogenin catalyzes the addition of
eight glucose units to its partner in the glycogenin
dimer.
• UDP-glucose is the donor in this autoglycosylation.
• At this point, glycogen synthase takes over to extend
the glycogen molecule
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A Branching Enzyme Forms a-1,6 Linkages
• Branching occurs after a number of glucosyl residues
are joined in a-1,4 linkage by glycogen synthase.
• A branch is created by the breaking of an a-1,4 link
and the formation of an a-1,6 link.
• A block of residues, typically 7 in number, is
transferred to a more interior site.
• The block of 7 or so residues must include the
nonreducing terminus and come from a chain at
least 11 residues long.
• The new branch point must be at least 4 residues
away from a preexisting one.
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 Branching is important because it increases the
solubility of glycogen.
 Branching creates a large number of terminal residues,
the sites of action of glycogen phosphorylase and
synthase.
 Thus, branching increases the rate of glycogen synthesis
and degradation.
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Glycogen Synthase Is the Key Regulatory Enzyme
in Glycogen Synthesis
• Glycogen synthase is phosphorylated at multiple
sites by protein kinase A (PKA) and several other
kinases.
• The resulting alteration of the charges in the protein
lead to its inactivation
• Phosphorylation has opposite effects on the
enzymatic activities of glycogen synthase and
phosphorylase
Net charge after
posphorylation
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• Phosphorylation converts the active a form of the
synthase into inactive b form.
• The phosphorylated b form requires a high level of
the allosteric activator G6-P for activity
• The a form is active whether or not G6-P is present
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Glycogen Is an Efficient Storage Form of
Glucose
• One ATP is hydrolyzed incorporating glucose 6phosphate into glycogen
-ATP
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Glycogen
90%
10%
branch
G1-P
Glucose
G1-P
-1 ATP
-1 ATP
G6-P
+31 ATP
Pyruvate
The complete oxidation of glucose
6-phosphate yields about 31
molecules of ATP.
Storage consumes slightly more
than one molecule of ATP per
molecule of glucose 6-phosphate;
so the overall efficiency of storage
is nearly 97%.
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Glycogen Breakdown and Synthesis Are
Reciprocally Regulated
• By a hormone-triggered cAMP cascade acting
through protein kinase A
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Insulin Stimulates Glycogen Synthesis by
Activating Protein Phosphatase 1
• When blood-glucose levels are high, insulin stimulates
the synthesis of glycogen by triggering a pathway that
activates protein phosphatase 1
Different site from PKA
phosphorylation in response
to epinepfrine
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Glycogen Metabolism in the Liver Regulates
the Blood-Glucose Level
• After a meal rich in carbohydrates, blood-glucose
levels rises, leading to an increase in glycogen
synthesis in the liver
• Insulin is the primary signal for glycogen synthesis
• The liver senses the concentration of glucose in the
blood, (~80 to 120 mg/100ml).
• The liver takes up or releases glucose accordingly.
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• The amount of liver phosphorylase a decreases
rapidly when glucose is infused.
• After a lag period, the amount of glycogen synthase
a increases, which results in the synthesis of
glycogen.
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