FUNCTION Glycogenin participates in the glycogen biosynthetic process along with glycogen synthase and glycogen branching enzyme. It catalyzes the formation of a short alpha (1,4)-glucosyl chain covalently attached via a glucose 1-O-tyrosyl linkage to internal tyrosine residues and these chains act as primers for the elongation reaction catalyzed by glycogen synthase.CATALYTIC ACTIVITY L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = alpha-D-glucosyl-L-tyrosyl-[glycogenin] + UDP + H(+)CATALYTIC ACTIVITY [1,4-alpha-D-glucosyl](n)-L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = [1,4-alpha-D-glucosyl](n+1)-L-tyrosyl-[glycogenin] + UDP + H(+)COFACTOR Divalent metal ions. Required for self-glucosylation. Manganese is the most effective.ACTIVITY REGULATION Inhibited by palladium ions.PATHWAY Glycan biosynthesis; glycogen biosynthesis.SUBUNIT Part of the GYS1-GYG1 complex, a heterooctamer composed of a tetramer of GYS1 and 2 dimers of GYG1, where each GYS1 protomer binds to one GYG1 subunit (via GYG1 C-terminus); the GYS1 tetramer may dissociate from GYG1 dimers to continue glycogen polymerization on its own (PubMed:17055998, PubMed:22160680, PubMed:35690592, PubMed:35835870). May also form a heterooctamer complex with GYS2 (via GYG1 C-terminus) (By similarity).INTERACTION Localizes to glycogen granules (glycosomes) in the cytoplasm (By similarity). Cytosolic localization is dependent on the actin cytoskeleton (By similarity).ALTERNATIVE PRODUCTS Highly expressed in skeletal muscle and heart, with lower levels in brain, lung, kidney and pancreas.PTM Self-glycosylated by the transfer of glucose residues from UDP-glucose to itself, forming an alpha-1,4-glycan of around 10 residues attached to Tyr-195.PTM Phosphorylated.DISEASE The disease is caused by variants affecting the gene represented in this entry.DISEASE The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the glycosyltransferase 8 family. Glycogenin subfamily.
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