Hydroxyproline and hydroxylysine occur principally in
**Core Concept**
Hydroxyproline and hydroxylysine are key amino acids found in collagen, a primary structural protein in connective tissue. These modifications are crucial for collagen's stability and function.
**Why the Correct Answer is Right**
Hydroxyproline and hydroxylysine are formed through the action of specific enzymes, prolyl hydroxylase and lysyl hydroxylase, respectively. These enzymes catalyze the post-translational modification of proline and lysine residues in collagen chains. The addition of hydroxyl groups to these amino acids enhances the stability of the collagen triple helix and facilitates cross-linking between collagen molecules. This process is essential for maintaining the mechanical properties of connective tissue.
**Why Each Wrong Option is Incorrect**
**Option A:**
This option would be incorrect because hydroxyproline and hydroxylysine are not primarily found in elastin.
**Option B:**
This option would be incorrect because while keratin does contain modified amino acids, hydroxyproline and hydroxylysine are not characteristic of keratin.
**Option C:**
This option would be incorrect because although fibronectin is a glycoprotein found in connective tissue, hydroxyproline and hydroxylysine are not its primary components.
**Clinical Pearl / High-Yield Fact**
Collagen's unique structure and stability are critical for maintaining tissue integrity and elasticity. The presence of hydroxyproline and hydroxylysine in collagen is essential for its function in connective tissue.
**Correct Answer:** C. Collagen