## Collagen Cross-Linking Mechanism **Key Point:** Lysyl hydroxylase catalyzes the hydroxylation of lysine residues to hydroxylysine, which is then oxidized by lysyl oxidase to allysine (aldehyde form). Allysine undergoes spontaneous condensation to form Schiff bases and aldol cross-links, stabilizing the collagen triple helix. **High-Yield:** The sequential post-translational modifications are: 1. Lysine → Hydroxylysine (lysyl hydroxylase, requires vitamin C, Fe²⁺, α-ketoglutarate) 2. Hydroxylysine → Allysine (lysyl oxidase, requires Cu²⁺, lysine tyrosylquinone) 3. Allysine → Aldol and Schiff base cross-links (spontaneous) **Mnemonic:** **LHA** = Lysine → Hydroxylysine → Allysine **Clinical Pearl:** Deficiency of lysyl oxidase (copper deficiency, Menkes disease) or lysyl hydroxylase (vitamin C deficiency/scurvy) results in defective cross-linking, leading to fragile, poorly tensile collagen. ## Why Lysine is the Answer Lysine is the substrate for lysyl hydroxylase. Hydroxylysine itself is the product of this reaction and cannot be further hydroxylated to allysine. Proline and glycine are hydroxylated by prolyl and glycyl hydroxylases respectively, but neither forms allysine.
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