Glükogeenina

Glükogeenina, known in English as glycogenin, is a small cytosolic glycosyltransferase that functions as the primer for glycogen synthesis in many eukaryotes. It initiates glycogen particle formation by attaching the first glucose residues, enabling subsequent elongation by glycogen synthase.

The mechanism of action involves autoglycosylation: glycogenin transfers glucose from UDP-glucose to a tyrosine residue on itself, generating a short glucan primer typically comprising several glucose units. This primer provides the substrate for glycogen synthase, which extends the chain, while branching enzyme introduces α-1,6 branches to form mature glycogen. In this way, glükogeenina sets the starting point for a growing glycogen molecule rather than relying solely on free UDP-glucose and synthase alone.

In humans, there are two glycogenin isoforms, glycogenin-1 and glycogenin-2, encoded by the GYG1 and GYG2 genes, respectively. They show tissue-specific expression patterns, with glycogenin-1 predominantly contributing to glycogen initiation in skeletal muscle and heart, and glycogenin-2 playing a more prominent role in liver tissue. The presence and activity of these isoforms help determine tissue distribution and efficiency of glycogen assembly.

Clinical significance is recognized when glükogeenina function is compromised. Defects or mutations in GYG1 or GYG2 can disrupt glycogen initiation, leading to abnormal glycogen storage and related neuromuscular symptoms, cardiomyopathy, or metabolic myopathies. Such conditions illustrate the essential role of glycogenin in normal glycogen metabolism and energy storage.

Research on glükogeenina continues to explore its regulatory interactions with glycogen synthase and branching enzyme, as well as its involvement in tissue-specific glycogen metabolism and related diseases.