glikogenin

Glycogenin, also spelled glikogenin in some languages, is a small cytosolic protein that acts as the primer for glycogen synthesis in animals and many other eukaryotes. It catalyzes the first step of glycogen particle formation by autoglucosylation: it transfers glucose from UDP-glucose to a specific tyrosine residue on itself, producing a short glucose chain that remains attached to the protein and serves as a primer for glycogen synthase.

Once the primer is formed, glycogen synthase extends the chain by adding further glucose units, and branching enzyme introduces alpha-1,6 branches to build mature glycogen particles.

Structure and genes: In mammals there are two glycogenin isozymes, glycogenin-1 and glycogenin-2, encoded by GYG1 and GYG2. Glycogenin-1 is widely expressed, whereas glycogenin-2 shows more restricted expression, notably in heart and certain muscles. The protein is cytosolic and associates with the glycogen particle surface near the non-reducing ends.

Biochemical properties: Glycogenin uses UDP-glucose as the glucose donor and is a glycosyltransferase with autoglucosylation activity. The autoglucosylation step creates a short oligosaccharide primer, typically several glucose units long, that remains attached to glycogenin and is extended by glycogen synthase.

Clinical relevance: Mutations in GYG1 or alterations in glycogenin function have been linked to rare human diseases affecting glycogen storage, underscoring the enzyme’s essential role in normal glycogen metabolism.