Difficulty: Easy
Correct Answer: Dopamine
Explanation:
Introduction / Context:
Amino acids can be grouped into standard proteinogenic amino acids (directly encoded by the genetic code), post-translationally modified amino acids found in proteins, and non-protein amino acids that occur in metabolism but are not incorporated into polypeptide chains. Distinguishing these categories is essential for understanding protein structure, metabolism, and analytical methods.
Given Data / Assumptions:
Concept / Approach:
Proteinogenic amino acids are the 20 standard residues (plus selenocysteine and pyrrolysine in special cases). Post-translational modifications (e.g., hydroxylysine, hydroxyproline) are present within proteins after enzymatic modification. Non-protein amino acids or amino-acid–related compounds may be biosynthesized and physiologically important, yet they are not used by ribosomes to build proteins.
Step-by-Step Solution:
Verification / Alternative check:
Analytical biochemistry texts list dopamine among biogenic amines rather than amino acids. In contrast, hydroxylysine and cystine are routinely detected during protein hydrolysis and structural studies.
Why Other Options Are Wrong:
Common Pitfalls:
Equating “not one of the 20” with “non-protein.” Some nonstandard residues (e.g., hydroxylysine) are indeed in proteins after modification, whereas small amines like dopamine are not incorporated into polypeptides.
Final Answer:
Dopamine
Discussion & Comments