Glycogenin

Glycogenin is a small cytosolic glycosyltransferase that acts as the primer for glycogen synthesis in animals. It initiates glycogen particle formation by attaching glucose residues from UDP-glucose to a specific tyrosine residue within the protein, a reaction known as autoglycosylation. In muscle and liver cells, glycogenin first glycosylates itself, forming a short glucose chain that serves as the primer for glycogen synthase.

Glycogen synthase then extends the chain, and branching enzyme introduces α-1,6 linkages to generate a mature, highly branched glycogen particle. Glycogenin thereby positions the growing glycogen molecule for efficient elongation and branching by the core glycogen-synthesis machinery.

Glycogenin belongs to the GT8 family of glycosyltransferases. In humans, two genes encode glycogenin proteins: GYG1 (glycogenin-1) and GYG2 (glycogenin-2). GYG1 is expressed in liver and skeletal muscle; GYG2 is expressed in heart and other tissues, with some overlap. The enzyme resides in the cytosol and functions in cooperation with glycogen synthase (GYS1) and branching enzyme (GBE1) to produce glycogen.

Defects in GYG1 or GYG2 can disrupt glycogen initiation and are associated with glycogen storage diseases and related myopathies, reflecting the essential role of glycogenin as the priming step for glycogen assembly.