GLYCOGEN METABOLISM

Glycogen Structure

•Most of the glucose residues in glycogen are linked by -1,4-glycosidic bonds.

•Branches at about every tenth residue are created by -1,6-glycosidic bonds .

Glycogen is an important fuel reserve for several reasons

•Glycogen serves as a buffer to maintain blood-glucose 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

The major sites of glycogen storage

.1The liver (10% by weight)

.2The skeletal muscle (2% by weight)

•Glycogen is present in the cytosol in the form of

granules ranging in diameter from 10 to 40 nm

Glycogen degradation consists of three steps

.1The release of G1-P from glycogen

.2The remodeling of the glycogen substrate to permit

further degradation

Glycogen degradation consists of three steps

.3The conversion of G1-P into G6-P.

.1It is the initial substrate for glycolysis

.2it can be processed by the pentose phosphate pathway to

yield NADPH and ribose derivatives

.3it can be converted into free glucose for release into the

bloodstream.

Glycogen metabolism is regulated by

•Allosterically:–Allosteric responses allow the adjustment of

enzyme activity to meet the needs of the cell in which the enzymes are expressed.

•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.

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.

Glycogen Phosphorylase:the 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)

G°´ 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

Two Remodeling Enzymes

•Transferase:–Shifts a block of three glycosyl residues from

one outer branch to the other

-1,6-glucosidase (debranching enzyme)–Hydrolyzes the -1, 6-glycosidic bond,

resulting in the release of a free glucose molecule.

–Glucose is phosphorylated by hixokinase (glycolysis)

•This paves the way for further cleavage by

phosphorylase.•In eukaryotes, the

transferase and the -1,6-glucosidase

activities are present in a single polypeptide chain, in a bifunctional

enzyme

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

G1-P G6-P

G1,6-BPSerine

Serine

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

Glycogen PhosphorylasePyridoxal Phosphate integral group of the Enzyme

The Pi substrate binding site

•In human beings, 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 of the

enzyme .

Phosphorylas exists in two states

The R state, catalytic site is more accessible and a binding site for orthophosphate is well organized.

The T state is less active because the catalytic site is

partly blocked.

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

phosphorylase

B

usually inactive

ATP A

usually active

P

Phosphorylase kinase

Depending oncellular conditions

The equilibrium for phosphorylase a, favors the R-state

The equilibrium for phosphorylase b, favors the T-state

Phosphorylase a differs from b by a phosphoryl group on each subunit

The R and T states of each of the a or b forms are in equilibrium

•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

In muscles-Posphorylase b

•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 .

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

•Phosphorylase kinase in the skeletal muscle: is (

is catalytic areregultoryis calmodulin

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

cells of the pancreas when the blood-sugar level is low.

• Epinephrine binds to the -adrenergic receptor in muscle, whereas glucagon binds to the glucagon receptor in liver.

• These binding events activate the subunit of the heteromeric Gs protein.

• A specific external signal is transmitted into the cell

•Epinephrine also binds to the 7TM -adrenergic receptor in the liver, which then activates phospholipase C and, hence, initiates the

phosphoinositide cascade•The consequent rise in the level of inositol 1,4,5-

trisphosphate induces the release of Ca2+ from endoplasmic reticulum stores.

•Recall that the subunit of phosphorylase kinase is the Ca2+ sensor calmodulin.

•Binding of Ca2+ to calmodulin leads to a partial activation of phosphorylase kinase.

•Stimulation by both glucagon and epinephrine leads to maximal mobilization of liver glycogen .

Glycogen Is Synthesized and Degraded by Different Pathways

•glycogen is synthesized by a pathway that utilizes uridine diphosphate glucose (UDP-glucose) rather than G1-P as the activated

glucose donor .

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.

•UDP-glucose is synthesized from G1-P and (UTP) in a reaction catalyzed by UDP-glucose

pyrophosphorylase.

•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 .

Glycogen Synthase Catalyzes the Transfer of Glucose from UDP-

Glucose to a Growing Chain

•glycogen synthase, is the key regulatory enzyme in glycogen synthesis.

•Glycogen synthase 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 -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.

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

•Branching occurs after a number of glucosyl residues are joined in -1,4 linkage by glycogen

synthase.•A branch is created by the breaking of an -1,4

link and the formation of an -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.

A Branching Enzyme Forms -1,6 Linkages

•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.

Glycogen Synthase Is the Key Regulatory Enzyme in Glycogen

Synthesis•Glycogen synthase is phosphorylated at multiple

sites by protein kinase A 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

•Phosphorylation converts the active a form of the synthase into a usually 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

•One ATP is hydrolyzed incorporating glucose 6-phosphate into glycogen

Glycogen Is an Efficient Storage Form of Glucose

-ATP

Glycogen

Glucose

G6-P

G1-P

Pyrovate

-1 ATP

10%branch

90%

+31 ATP

G1-P -1 ATP

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%.

•By a hormone-triggered cAMP cascade acting through protein kinase A

Glycogen Breakdown and Synthesis Are Reciprocally

Regulated

•The hydrolysis of phosphorylated serine and threonine residues in proteins is catalyzed by

protein phosphatases.•Phosphatase 1 (PP1), plays key roles in regulating

glycogen metabolism.

Protein Phosphatase 1 Reverses the Regulatory Effects of Kinases

on Glycogen Metabolism

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

Glycogen Metabolism in the Liver Regulates the Blood-

Glucose Level•After a meal rich in carbohydrates, blood-glucose

levels rise, 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.

•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.

•Phosphorylase a is the glucose sensor in liver cells.

–The binding of glucose to phosphorylase a shifts its allosteric equilibrium from the active R form to the

inactive T form.–This conformational change renders the phosphoryl

group on serine 14, a substrate for PP1.–It is significant that PP1 binds tightly to

phosphorylase a but acts catalytically only when glucose induces the transition to the T form.

–Recall that the R-T transition of muscle phosphorylase a is unaffected by glucose and is

thus unaffected by the rise in blood-glucose levels

•Phosphorylase b, in contrast with phosphorylase a, does not bind the PP1.

•Consequently, the conversion of a into b is accompanied by the release of PP1, which is

then free to activate glycogen synthase

•There are about 10 phosphorylase a molecules per molecule of phosphatase.

•Hence, the activity of glycogen synthase begins to increase only after most of phosphorylase a is converted into b.

TypeDefective enzymeOrgan affected

Glycogen in the affected organ

Clinical features

I Von

Gierke disease

Glucose 6-phosphatase or

transport system

Liver and kidney

Increased amount; normal structure.

Massive enlargement of the liver. Failure to thrive. Severe hypoglycemia, ketosis,

hyperuricemia, hyperlipemia.

II Pompe disease

-1,4-Glucosidase (lysosomal)

All organs

Massive increase in amount; normal structure.

Cardiorespiratory failure causes death, usually before age 2.

III Cori disease

Amylo-1,6-glucosidase

(debranching enzyme)

Muscle and liver

Increased amount; short outer branches.

Like type I, but milder course.

IV Andersen disease

Branching enzyme

                                    

Liver and spleen

Normal amount; very long outer branches.

Progressive cirrhosis of the liver. Liver failure causes death, usually

before age 2.

V McArdle disease

PhosphorylaseMuscleModerately increased amount; normal structure.

Limited ability to perform strenuous exercise because of painful muscle

cramps. Otherwise patient is normal and well developed.

VI Hers

disease

PhosphorylaseLiverIncreased amount.Like type I, but milder course.

VIIPhosphofructokinaseMuscleIncreased amount; normal structure.

Like type V.

VIIIPhosphorylase kinase

LiverIncreased amount; normal structure.

Mild liver enlargement. Mild hypoglycemia.

Note: Types I through VII are inherited as autosomal recessives. Type VIII is sex linked.