Which of these amino acids is formed from a precursor amino acid only after the latter has been incorporated into a polypeptide chain?

Question

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Hydroxyproline is formed from proline only after it has been incorporated into a polypeptide chain. This process involves the action of the enzyme prolyl hydroxylase, which adds a hydroxyl group to the side chain of proline. This modification is important for the stability and strength of collagen, which is rich in hydroxyproline residues.

Which of These Amino Acids Is Formed from a Precursor