Glycogen Metabolism

alpha 1-4 linkage

links glucose molecules in glycogen

alpha 1-6 linkage

branches in glycogen every 8-14 residues

cytosol granules

storage location of glycogen in liver and muscle (MW 6 to 1600×10⁶)

glycogenin

protein that the reducing end of glycogen is covalently linked

reducing end

hemiacetal carbon (attached to two O)

advantage of branching

• grow/degrade chain fast

• more soluble

• pack glucose in small volume

glycogen granule

has enzymes for glycogen synthesis and degradation

phosphoglucomutase

• converts glucose-6-phosphate (G6P) → glucose-1-phosphate (G1P)

• initiates glycogen synthesis

UDP-glucose pyrophosphorylase

activates glucose-1-phosphate (G1P) using UTP → UDP-glucose + PP_i

glycogenin

adds first glucose to tyrosine -OH of glycogenin protein

glycogen synthase

• adds sugars to glucose chain

• UDP-glucose + [glycogen]_n → UDP + [glycogen]_n+1

1,5-gluconolactone

glycogen synthase inhibitor

glycogen synthase

catalyzes the formation of alpha 1-4 glycosidic bonds in glucose

glycogen synthase

transfers glycosyl units from UDP-glucose to C4 -OH at non reducing end of glycogen strand

amylo-(1,4→1,6)-transglycosylase

branching enzyme that forms glycogen branches

amylo-(1,4→1,6)-transglycosylase

7 residue chains transferred to C6 OH group of glucose

glycogenolysis

breakdown of glycogen (catabolism)

digestive breakdown of glycogen

unregulated; almost all ingested food is absorbed and metabolized

breakdown of tissue glycogen

regulated and controlled

glycogen phosphorylase (GP)

cleaves glucose from non-reducing ends of glycogen

glycogen debranching enzyme

removes branches and makes more glucose accessible to glycogen phosphorylase (GP)

phosphoglucomutase

converts glucose-1-phosphate (G1P) → glucose-6-phosphate (G6P)

phosphorylysis

used phosphate to break bonds (rather than water)

muscle

where glycogen breakdown to G6P is used for glycolysis

liver

where glycogen breakdown to G6P is converted to glucose

low ATP

when glycogen phosphorylase is activated and glycogen synthase is inhibited

high ATP

when glycogen phosphorylase in inhibited and glycogen synthase is activated

phosphoprotein phosphatase-1 (PP1)

• dephosphorylates glycogen synthase (activating it)

• dephosphorylates glycogen phosphorylase (deactivating it)

glycogen synthase inactivated

glycogen synthase is phosphorylated

glycogen phosphorylase inactivated

glycogen phosphorylase dephosphorylated