Amino acids in biochemistry — Which of the following is best classified as a non-protein (non-proteinogenic) amino acid or amino-acid–related compound that is not incorporated directly into proteins?

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:

  • Dopamine is a catecholamine neurotransmitter derived from tyrosine.
  • Hydroxylysine is a post-translational modification of lysine found in collagen.
  • Cystine is the disulfide-linked dimer of cysteine commonly present in folded proteins.
  • The task is to select the option that is a non-protein amino acid or related compound not incorporated into proteins.


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:

Classify dopamine: it lacks the carboxylate group typical of amino acids and functions as a neurotransmitter, not as a building block of proteins.Classify hydroxylysine: formed by enzymatic hydroxylation of lysine residues in collagen; it is a bona fide amino acid derivative present in proteins.Classify cystine: occurs when two cysteine residues form a disulfide bond; frequently found stabilizing protein tertiary/tertiary structures.Therefore, the non-protein option is dopamine.


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:

  • Hydroxylysine: post-translationally modified amino acid within collagen; not “non-protein.”
  • Cystine: a common protein component formed from two cysteines; present in many extracellular proteins.
  • None of these: incorrect because dopamine fits the description.


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

More Questions from Structure and Properties of Amino Acids

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion