glycogenbranching

Glycogen branching is the enzymatic process that creates branches in the growing glycogen molecule, resulting in a highly branched polymer. This process is carried out mainly by the glycogen branching enzyme (GBE), also known as amylo-(1,4 to 1,6)-transglucosidase, which introduces α-1,6 glycosidic branches into an existing α-1,4-linked chain during glycogenesis.

Mechanism: GBE transfers a block of glucose residues from a growing α-1,4 chain to an interior glucose residue, forming a new α-1,6 linkage and creating a branch point. The typical transferred block is about seven glucose units. This action increases the number of nonreducing ends, enhancing the rate of glycogen synthesis and subsequent mobilization when glucose is needed.

Structure and distribution: In mammals, glycogen consists of short, primarily α-1,4–linked chains with α-1,6 branches roughly every 8–12 glucose units. GBE activity is coordinated with glycogen synthase and debranching enzymes to produce a highly branched polymer, particularly in liver and muscle. Similar branched glycogen-like polymers are found in bacteria, yeast, and plants, reflecting a conserved requirement for efficient glucose storage and release.

Clinical relevance: Defects in the branching enzyme gene (GBE1) cause glycogen storage disease type IV (Andersen disease), characterized by abnormal, poorly branched glycogen that accumulates in tissues and can lead to liver dysfunction and cardiomyopathy.