HbA1c is glycosylated haemoglobin formed by non-enzymatic glycation. The predominant site of glycation and the chemical nature of the bond initially formed is:

• A Lysine-16 of the alpha-chain forms a stable peptide bond with glucose
• B Serine hydroxyl groups on the beta-chain react with glucose via O-linked glycosylation by a transferase enzyme
• C Cysteine-93 of the beta-chain forms a disulfide-linked glucose adduct
• D N-terminal valine of the beta-chain forms a reversible aldimine (Schiff base) which then undergoes Amadori rearrangement to a stable ketoamine (HbA1c) ✓

Explanation

Glycation of haemoglobin is non-enzymatic. Glucose condenses with the free amino group (alpha-NH2) of the N-terminal valine residues of beta-chains to form an unstable aldimine (Schiff base), which spontaneously rearranges to the stable Amadori product (1-amino-1-deoxy-D-fructose derivative) — this is HbA1c. Since haemoglobin stays in RBCs for 120 days, HbA1c reflects average blood glucose over the preceding 8–12 weeks, making it the gold standard for glycaemic monitoring in diabetes.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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