Type I collagen (fibrillar collagen) is synthesised intracellularly as procollagen and undergoes extracellular processing. Which enzyme cleaves the C- and N-terminal propeptides of procollagen to yield tropocollagen, and deficiency of which cofactor leads to inadequate hydroxylation of proline and lysine residues?

• A Procollagen peptidases cleave propeptides; vitamin C (ascorbic acid) is required for prolyl and lysyl hydroxylase ✓
• B Lysyl oxidase cleaves propeptides; copper is the cofactor
• C Collagenase cleaves propeptides; zinc is the cofactor
• D Carboxypeptidase cleaves propeptides; vitamin K is the cofactor

Explanation

Procollagen N- and C-proteinases (procollagen peptidases) cleave the terminal propeptides to produce tropocollagen, which then self-assembles into fibrils. Within the rough ER, prolyl hydroxylase and lysyl hydroxylase require vitamin C (ascorbic acid) as a cofactor to hydroxylate proline and lysine residues; these hydroxyresidues are essential for cross-linking and triple-helix stability. Vitamin C deficiency (scurvy) impairs hydroxylation, destabilising the collagen helix, leading to poor wound healing, bleeding gums, and perifollicular haemorrhages. Lysyl oxidase (copper-dependent) cross-links tropocollagen extracellularly and is unrelated to propeptide cleavage.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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