# DBT JRF 2022 — Question 19

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Collagen & Hydroxyproline Biosynthesis · Difficulty: Medium

## Question

15 to 30% of collagen residues are 4-hydroxyproline residues (Hyp). When rats are fed with ¹⁴C labeled Hyp, the collagen produced by them is not radioactive, but when they are fed with ¹⁴C labeled Proline, the collagen produced is radioactive. What is the most plausible reason?

## Options

- **A.** Hyp is an essential amino acid in rats.
- **B.** Free Hyp is dehydroxylated to form Proline in the rats.
- **C.** Proline residues are hydroxylated once they are incorporated into collagen in rats.
- **D.** Free ¹⁴C labelled Hyp is degraded in rats.

## Answer

Option C

## Explanation

4-Hydroxyproline (Hyp) and collagen biosynthesis: Key experimental observations: 1. Feeding ¹⁴C-Hyp → collagen produced is NOT radioactive. 2. Feeding ¹⁴C-Pro → collagen IS radioactive (contains labelled hydroxyproline). Explanation: • Free hydroxyproline (Hyp) CANNOT be directly charged onto tRNA and incorporated into proteins by ribosomes. There is no tRNA for Hyp; the genetic code only specifies proline. • Proline is incorporated into collagen polypeptide normally. AFTER translation, prolyl hydroxylase (a vitamin C-dependent enzyme) hydroxylates specific proline residues in the -X-Pro-Gly- sequence. • When free ¹⁴C-Hyp is ingested, it is degraded as a regular metabolite and NOT salvaged back into collagen. This also explains why vitamin C deficiency causes scurvy (defective collagen, as prolyl hydroxylase requires ascorbate as a cofactor). Answer: Proline residues are hydroxylated once incorporated into collagen (Option 3).

Source: https://mypyqbuddy.com/questions/15-to-30-of-collagen-residues-are-4-hydroxyproline-6a104cd5920734408bc0c722