Glycogen synthase cannot initiate synthesis de novo; elongation of a new glycogen molecule requires a pre-existing protein primer. Which protein serves this function and forms the initial glycosidic bond to itself?
- A Glycogenin, which autocatalytically attaches glucose to one of its own tyrosine residues ✓
- B Glycogen phosphorylase, which primes the chain through reverse phosphorolysis
- C Amylo-(1,4 to 1,6)-transglycosylase, which creates the first branch point
- D UDP-glucose pyrophosphorylase, which donates the primer glucose residue
Correct answer: A. Glycogenin, which autocatalytically attaches glucose to one of its own tyrosine residues
Explanation
Glycogenin acts as both primer and catalyst: it glucosylates a specific tyrosine hydroxyl on itself, then extends an oligosaccharide chain of about 8 to 12 residues before glycogen synthase takes over elongation. Phosphorylase only degrades glycogen, the branching enzyme acts on pre-existing chains, and UDP-glucose pyrophosphorylase merely synthesizes the donor nucleotide sugar and never attaches glucose to protein.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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