Regarding the biochemistry and molecular biology of collagen in wound healing, all of the following are correct EXCEPT:

Explanation

## Collagen Structure, Synthesis, and Remodeling in Wound Healing ### Collagen Types and Timeline **Key Point:** Type III collagen is deposited early in wound healing but is NOT completely replaced by Type I collagen by the end of the inflammatory phase. Remodeling occurs over weeks to months. ### Collagen Synthesis and Maturation ```mermaid flowchart TD A[Fibroblast synthesizes procollagen]:::action --> B[Hydroxylation of Pro & Lys<br/>requires Vitamin C]:::action B --> C[Triple helix formation]:::outcome C --> D[Secretion of Type III & Type I collagen]:::action D --> E[Lysyl oxidase cross-linking<br/>Lys → Aldehyde]:::action E --> F[Collagen maturation & remodeling<br/>weeks to months]:::action F --> G[Type III → Type I ratio decreases<br/>Tensile strength increases]:::outcome ``` ### Analysis of Each Option | Statement | Accuracy | Explanation | |-----------|----------|-------------| | Type I collagen predominates in mature scars | ✓ Correct | Type I is ~80% of mature scar; provides tensile strength | | Lysyl oxidase oxidizes Lys/Hyl residues | ✓ Correct | Forms aldehyde cross-links (allysine); requires Cu²⁺ cofactor | | Vitamin C hydroxylates Pro/Lys | ✓ Correct | Ascorbic acid is essential; deficiency → scurvy, poor healing | | Type III completely replaced by day 5 | ✗ **WRONG** | Type III is laid down early but remodeling continues for weeks–months | ### Why Option 3 is Wrong **High-Yield:** Type III collagen is deposited during the proliferative phase (days 5–21) but is NOT completely replaced by Type I collagen by the end of the inflammatory phase (day 5). In fact: 1. **Early wound (days 5–14):** Type III:Type I ratio is ~3:1 2. **Intermediate wound (weeks 2–4):** Type III:Type I ratio is ~1:1 3. **Mature scar (>3 months):** Type III:Type I ratio is ~1:10 **Clinical Pearl:** The gradual replacement of Type III with Type I collagen is a hallmark of the remodeling phase (weeks to years). This process is driven by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Premature scar revision (<3 months) risks disrupting this remodeling and should be avoided unless necessary. ### Verification of Correct Options **Option 0:** Type I collagen (80% of mature scar) provides tensile strength. Type III is more compliant but weaker. **Option 1:** Lysyl oxidase (LOX), a copper-dependent enzyme, catalyzes the oxidation of lysine and hydroxylysine residues to aldehydes (allysine, hydroxyallysine), which then condense to form Schiff bases and aldol cross-links. This is essential for collagen mechanical strength. **Option 2:** Vitamin C (ascorbic acid) is a cofactor for prolyl and lysyl hydroxylase. Without it, proline and lysine residues are not hydroxylated, preventing triple helix formation and leading to defective collagen (scurvy). **Mnemonic: VLC** — Vitamin C, Lysyl oxidase, Collagen cross-linking (the three critical biochemical steps for collagen maturation).