All of the following statements regarding collagen structure and synthesis are correct EXCEPT:

Question

Accepted Answer

A. The N-terminal and C-terminal propeptides of procollagen are retained in mature collagen fibrils to provide structural stability. ## Collagen Structure and Post-Translational Modifications

### Correct Statements (Options 0, 1, 2)

**Key Point:** Collagen is unique among proteins in containing hydroxyproline (~10% of residues) and hydroxylysine (~1% of residues). These modified amino acids are NOT incorporated during translation but are formed **post-translationally** by enzymatic hydroxylation of proline and lysine residues in the rough endoplasmic reticulum.

**High-Yield:** The enzymes responsible are:
- **Prolyl hydroxylase** (requires vitamin C, Fe²⁺, α-ketoglutarate, O₂)
- **Lysyl hydroxylase** (same cofactors)

Without vitamin C (ascorbic acid), these hydroxylations cannot occur → defective collagen → **scurvy**.

**Key Point:** The collagen triple helix is stabilized by:
1. **Hydrogen bonds** between backbone C=O and N–H groups of adjacent chains (not side chains)
2. **Covalent cross-links** formed from lysine and hydroxylysine residues (via lysyl oxidase, which also requires vitamin C)

### The Incorrect Statement (Option 3)

**Warning:** The N-terminal and C-terminal **propeptides are CLEAVED OFF** by specific peptidases (procollagen peptidases) **before** the collagen is incorporated into fibrils. Mature collagen fibrils contain only the processed collagen triple helix, NOT the propeptides.

- **Procollagen** = tropocollagen + N-propeptide + C-propeptide
- **Mature collagen** = tropocollagen only (after enzymatic cleavage)

The propeptides are removed because they would interfere with proper fibril packing and cross-linking.

### Clinical Pearl

In **Ehlers–Danlos syndrome type VII** (arthrochalasia), mutations in procollagen peptidase prevent proper cleavage of the N-propeptide, leading to abnormal collagen fibrils and joint hypermobility.

## Summary Table

| Feature | Details |
|---------|----------|
| **Hydroxyproline/Hydroxylysine** | Post-translational modification; requires vitamin C |
| **Triple helix stability** | H-bonds between backbone atoms |
| **Vitamin C role** | Cofactor for prolyl/lysyl hydroxylase AND lysyl oxidase |
| **Propeptides** | **CLEAVED BEFORE fibril assembly** (not retained) |
| **Mature collagen** | Tropocollagen only; ~1000 amino acids per chain |

Answer

B. The triple helix of collagen is stabilized by hydrogen bonds between the backbone atoms of adjacent chains

Answer

C. Collagen contains hydroxyproline and hydroxylysine residues formed by post-translational modification of proline and lysine

Answer

D. Collagen synthesis requires vitamin C as a cofactor for prolyl and lysyl oxidase enzymes

All of the following statements regarding collagen structure and synthesis are correct EXCEPT:

Explanation

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