glycogen synthesis

stage 1

uridine diphosphate glucose (UDP-glucose) synthesis

• UDP-glucose synthesized from UTP (triphosphate) and Glucose-1-phosphate (enzyme: UDP-glucose pyrophosphorylase)

  • Produces pyrophosphate (PPi)

Rxn: Glucose-1-phosphate + UTP →(reversible) UDP-glucose + PPi

stage 2 overall

transfer glucose from UDP-glucose to growing chain

overall: catalyzed by enzyme: glycogen synthase

RXN: UDP-glucose + glycogen (n) → UDP + glycogen (n + 1)

stage 2, step 1

glycogen synthase

1. glycosyl unit from UDP-glycose transferred to hydroxyl group @ C4 of terminal glycose (a-1,4 bond)

2. requires a glycogen mol w/ a minimum of 4 residues where a glycosyl can be added

stage 2, step 2

glycogenin

1. an enzyme + its own substrate (autocatalysis) w/ two 37-kd subunits

2. each su catalyzes the addn. of a glycosyl unit from UDP-glucose → Tyr-194 of other su

3. 7 more glycosyl unit added: total of 8 glycosyl units attached to Tyr-194 for each su

stage 2, step 3

branching: needs uninterrupted (no branches) “string'“ of at least 11 glycosyl residues

1. branching enzyme transfers a “string” of ~7 residues from non-reducing end to presumptive branch pt.

• branch pt. must be more than 4 residues away from other branch pts

• link w/ a-1,6 bond