Glycogen formation, or glycogenesis, requires a primer protein to start the process. The correct enzyme is glycogenin, essential for initiating the glucose chain.

Correct Answer

(2) Glycogenin

Glycogenin acts as a self-glucosylating primer protein that starts glycogen synthesis by attaching the first glucose molecules to itself via tyrosine residue. Without glycogenin, glycogen synthase cannot extend the chain, halting initiation. This makes it indispensable for de novo glycogen formation in liver and muscle cells.

Option Breakdown

(1) Glucokinase

Glucokinase phosphorylates glucose to glucose-6-phosphate in liver, a preliminary step for glycogenesis. It does not initiate glycogen chain formation; glycogen can start without it using alternative hexokinases in other tissues.​

(2) Glycogenin

Correct. This glucosyltransferase primer covalently links UDP-glucose to its tyrosine, forming the initial α-1,4-glucose oligomer (about 8-12 residues). Glycogen synthase then elongates it; absence prevents any new glycogen particle formation.​

(3) Glycogen Phosphorylase

This enzyme catalyzes glycogen breakdown (glycogenolysis), releasing glucose-1-phosphate from α-1,4 bonds. It opposes synthesis and is inactive during glycogenesis; its absence does not block initiation.

(4) Glycogen Kinase

No such enzyme exists in standard glycogen metabolism (likely a distractor for glycogen synthase kinase 3, GSK3). GSK3 regulates glycogen synthase activity via phosphorylation but plays no direct role in initiation.​