The Metabolism of Glycogenin Animals

Glycogen breakdown near an α(1→6) branchpoint.Following sequential removal of terminal glucoseresidues by glycogen Phosphorylase , glucoseresidues near a branch are removed in a two-stepprocess that requires a bifunctional “debranchingenzyme.” First, the transferase activity of theenzyme shifts a block of three glucose residuesfrom the branch to a nearby nonreducingend, to which they are reattached in α(1→4)linkage. The single glucose residue remaining atthe branch point, in α(1→6) linkage, is thenreleased as free glucose by the enzyme’s α(1→6)glucosidase activity.

Reaction catalyzed by phosphoglucomutase.

The reaction begins with theenzyme phosphorylated on a Serresidue. In step 1 , the enzymedonates its phosphoryl group(green) to glucose 1-phosphate,producing glucose1,6 bisphosphate.In step 2 , the phosphoryl group atC-1 of glucose 1,6-bisphosphate(red) is transferred back to theenzyme, re-forming thephosphoenzyme and producingglucose 6-phosphate.

The Sugar Nucleotide UDP-Glucose Donates Glucosefor Glycogen Synthesis

To initiate glycogen synthesis, the glucose 6-phosphate is converted to glucose 1-phosphate in the phosphoglucomutase reaction:

The product of this reaction is converted to UDPglucose by the action of UDP-glucose pyrophosphorylase, in a key step of glycogen biosynthesis:

Branch synthesis in glycogen. The glycogen-branching enzyme (also called amylo (1→4) to (1→6) transglycosylase or glycosyl-(4→6)-transferase) forms a new branch point during glycogen synthesis.

Glycogenin Primes the Initial SugarResidues in GlycogenGlycogen synthase cannot initiate a newglycogen chain de novo. It requires aprimer, usually a preformed(α1→4) polyglucose chain or branchhaving at least eight glucose residues.How is a new glycogen moleculeinitiated? The intriguing proteinglycogenin is both the primer on whichnew chains are assembled and theenzyme that catalyzes their assembly