## Collagen Structure: Post-Translational Modifications and Stabilization ### Correct Statements (Options 0, 1, 2) **Key Point:** Collagen is unique among proteins in containing hydroxyproline (~10% of residues) and hydroxylysine (~1% of residues), which are formed by enzymatic hydroxylation of proline and lysine residues *after* translation on the ribosome. **High-Yield:** The collagen triple helix is stabilized by: - Hydrogen bonds between chains (each chain forms H-bonds with the other two) - The Gly-X-Y repeat pattern (glycine at every third position allows tight packing) - Post-translational cross-links (see below) **Key Point:** Vitamin C (ascorbic acid) is an essential cofactor for prolyl hydroxylase and lysyl hydroxylase. Without it, collagen cannot be properly hydroxylated, leading to defective collagen and scurvy [cite:Robbins 10e Ch 3]. ### Why Option 3 Is Wrong **Warning:** Collagen molecules are NOT primarily cross-linked by peptide bonds. Instead, they are cross-linked by **covalent cross-links** formed from lysine and hydroxylysine residues: | Cross-link Type | Mechanism | Enzyme | | --- | --- | --- | | **Aldol condensation** | Lysine/hydroxylysine → aldehyde (allysine) → Schiff base | Lysyl oxidase | | **Aldol cross-links** | Two aldehydes condense | Non-enzymatic | | **Mature cross-links** | Aldol products rearrange to stable forms | Time-dependent | **Clinical Pearl:** Lysyl oxidase requires copper as a cofactor. Copper deficiency (Menkes disease) or inhibition (by β-aminopropionitrile in lathyrogens) prevents cross-linking, weakening collagen and causing connective tissue fragility. These **covalent cross-links** (not peptide bonds) are what give collagen its tensile strength and stability over time.
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