glykogeenina

Glykogeenina, known in English as glycogenin, is a small glycosyltransferase that acts as the primer for glycogen biosynthesis in many eukaryotes, including humans. In humans two isoforms are described: glycogenin-1 (GYG1) and glycogenin-2 (GYG2). The enzyme is typically found in association with glycogen synthase and other enzymes involved in glycogen metabolism.

The core function of glykogeenina is to initiate glycogen synthesis by autoglucosylating itself. Using UDP-glucose as the donor, glycogenin catalyzes the attachment of glucose residues to a specific tyrosine residue on its own polypeptide, forming a short, covalently linked glucan primer. This primer, consisting of roughly eight to twelve glucose units, provides the initial chain that glycogen synthase can recognize and extend to form a growing glycogen molecule. After primer formation, glycogen synthase adds further glucose residues in alpha-1,4 linkages, and branching enzyme introduces alpha-1,6 branches to produce the mature, highly branched glycogen particle.

Structure and interactions: glycogenin contains a catalytic domain for UDP-glucose transfer and a separate region that facilitates interaction with glycogen synthase. The primer is covalently attached to a tyrosine residue on glycogenin, which serves as the starting point for glycogen assembly.

Clinical relevance: mutations in GYG1 or GYG2 have been linked to disorders affecting glycogen synthesis, including tissue-specific myopathies and cardiomyopathy in some cases. The clinical presentation is variable and continues to be investigated.

Distribution and evolution: glycogenin is conserved in many eukaryotes that synthesize glycogen, though some organisms employ alternative initiation mechanisms.