Glikojenin

Glycogenin, sometimes rendered as glikojenin in some languages, is a cytosolic enzyme that serves as the primer for glycogen biosynthesis in animals and fungi. In humans, two glycogenin genes have been described: GYG1, encoding glycogenin-1, and GYG2, encoding glycogenin-2. The protein initiates glycogen synthesis by autoglucosylation: it transfers glucose from UDP-glucose to a specific tyrosine residue on itself, forming a covalently attached glucose at the tyrosine. This reaction repeats to build a short glucose chain that acts as the starter primer for glycogen synthase.

Once a sufficient primer length is reached, glycogen synthase extends the chain and branching enzyme introduces branches, enabling the nascent glycogen particle to grow. In this process, glycogenin primarily facilitates the initial priming and is then succeeded by the elongation and branching activities of other glycogen-synthesizing enzymes.

Biochemically, glycogenin functions as a glycosyltransferase with an active site that recognizes UDP-glucose. It is generally located in the cytosol and associates with developing glycogen particles during the onset of glycogenesis. The presence of glycogenin ensures that glycogen synthesis begins with a defined primer rather than starting from free glucose units.

Genetic mutations in GYG1 or GYG2 can disrupt normal primer formation and have been linked to inherited disorders affecting glycogen storage and muscle function in some cases, underscoring the essential role of glycogenin in proper glycogen biosynthesis.