Standard vs. nonstandard amino acids — Identify the nonstandard (post-translationally derived) amino acid among the following choices.

Introduction / Context:
Nonstandard amino acids arise through post-translational modification or occur as metabolic intermediates but are not directly encoded by the universal genetic code. Recognizing them is vital for interpreting protein composition and stability.

Given Data / Assumptions:

• Cysteine, isoleucine, and histidine are among the 20 standard amino acids.
• Hydroxyproline is a hydroxylated derivative of proline found prominently in collagen.
• Question targets the “nonstandard” residue present in proteins owing to modification.

Concept / Approach:
Hydroxylation of proline to hydroxyproline is catalyzed by prolyl hydroxylase and requires vitamin C as a cofactor. This modification is essential for collagen triple-helix stability through enhanced hydrogen bonding. The others are encoded directly by codons and incorporated during translation.

Step-by-Step Solution:

Verification / Alternative check:
Protein hydrolysate analyses from collagen reveal abundant hydroxyproline; genetic code tables do not list it as a primary residue.

Why Other Options Are Wrong:

• Cysteine, isoleucine, histidine: all are standard proteinogenic amino acids.

Common Pitfalls:
Confusing “nonstandard” with “non-protein”; hydroxyproline is nonstandard yet a genuine component of proteins after modification.

Final Answer:
Hydroxyproline