Glycogen

BIOC13

Cellulose structure

β-glucose, 1-4 bonds, no branches

Amylose structure

α-glucose, 1-4 bonds, no branches

Amylopectin structure

α-glucose, 1-4 and 1-6 bonds, branch every ~20 units

Glycogen

α-glucose, 1-4 and 1-6 bonds, branch every ~10 units

(?) results in rapid rise in blood glucose

simple sugars

(amylopectin/amylose) gives slower glucose release

amylose

Cellulose purpose

cannot be digested, major component of stool and important for digestive health

Biomolecules requiring aerobic metabolism

fatty acids and proteins

carbo-loading

prepare for anaerobic metabolism by building up glycogen stores

“hitting the wall”

depleting glycogen stores, muscle output falls to 50% even if fat is available, dizziness/confusion

Glycogen synthesis

glycogen_n + UDP-glucose → glycogen_n+1 + UDP

Glycogen degradation

glycogen_n+1 + P_i → glycogen_n + G-1-P

UDP-glucose purpose

glucose donor in glycogen synthesis

Glycogen synthase

transfers glucose from UDP-glucose to C4 terminal residue of glycogen chain to form an α-1,4-glycosidic bond

Glycogenin

Synthesizes glucose oligosaccharide primer (10-20) subunits for glycogen synthase

Branching enzyme

generates branches by cleaving α-1,4-linkages to release a block of ~7 glucoses and rejoining with an α-1,6-linkage

ATP required to incorporate dietary glucose into glycogen

2

ATP yield from complete oxidation of glucose from glycogen

31

Active a form of glycogen synthase is (phosphorylated/unphosphorylated)

unphosphorylated (synthase)

Less active b form of glycogen synthase is (phosphorylated/unphosphorylated)

phosphorylated (synthase)

Glycogen synthase allosteric regulation

Binding G-6-P increases activity of both forms

Glycogen Phosphorylase

Catalyzes cleavage of α-1,4-glycosidic bond by addition of P_i, releases a G-1-P from glyocgen

Phosphoglucomutase

Converts G-1-P to G-6-P

Transferase

Shifts 3 residues near branch point to nearby chain

α-1,6-glucosidase

Cleaves α-1,6-bond at the branch point, releasing a free glucose

Active a form of glycogen phosphorylase is (phosphorylated/unphosphorylated)

phosphorylated (phosphorylase)

Less active b form of glycogen phosphorylase is (phosphorylated/unphosphorylated)

unphosphorylated (phosphorylase)

R/T state favored in b form

T state

R/T state favored in a form

R state

Default phosphorylase form in liver

a form in R state

Default phosphorylase form in muscle

b form in T state

PKA effect on glycogen phosphorylase

phosphorylates to active a form

What activates PKA

cAMP produced by adenylate cyclase as signaled by G proteins

Enzymes that inhibit glycogen synthase

PKA and glycogen synthase kinase

Hormone that signal for glycogen breakdown

glucagon and epinephrine

Hormone that signal for glycogen synthesis

insulin

PP1

Dephosphorylates glycogen synthase to active a form

PP1 effect on glycogen synthase

activates glycogen synthase

PP1 effect on phosphorylase kinase

inactivates the kinase

PP1 effect on phosphorylase

inactivates the phosphorylase

PKA effect on PP1

Regulatory G subunit that releases catalytic subunit of PP1, decreasing activity

Lag between decrease in degradation and increase in synthesis

All 10 phosphorylase (on 1 PP1) has to bind to glucose before synthesis is initiated

Glutamine in cancer

Increases proliferation