111. 15 to 30% of collagen residues are 4-hydroxyproline residues (Hyp). When rats are fed with 𝐶14 labeled Hyp, the collagen produced by them is not radioactive, but when they are fed with 𝐶14 labeled Proline, the collagen produced is radioactive. What is the most plausible reason?
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 𝐶
14 labelled Hyp is degraded in rats.

Detailed Explanation:

Collagen, a structural protein in animals, contains a significant proportion of modified amino acids, one of the most important being 4-hydroxyproline (Hyp). Hydroxyproline is derived from proline during the synthesis of collagen, and it plays a critical role in the stability and rigidity of the collagen triple helix. Approximately 15-30% of collagen residues are 4-hydroxyproline residues.

In the given scenario, rats were fed C14-labeled hydroxyproline and C14-labeled proline, with the outcome being that the collagen produced by the rats was not radioactive when fed labeled Hyp, but it was radioactive when labeled proline was used.

The most plausible reason for this observation is:

C. Proline residues are hydroxylated once they are incorporated into collagen in rats.

This means that when the rats were fed C14-labeled proline, the proline was incorporated into collagen first, and then it was hydroxylated into 4-hydroxyproline (Hyp) in the collagen structure. The radioactive label from the proline was therefore present in the collagen as C14-labeled hydroxyproline. On the other hand, when the rats were fed C14-labeled hydroxyproline, the free labeled Hyp was not incorporated directly into the collagen, possibly due to degradation or non-incorporation into the collagen matrix, explaining the lack of radioactivity in the produced collagen.

Thus, the essential biochemical process is the hydroxylation of proline to form hydroxyproline during collagen biosynthesis, and this is crucial for understanding the labeling experiment.