Discussion
Which of the following is not a natural amino acid ?
- A.
- B.
- C.
- D.
Answer: B
The one that is not a natural (protein) amino acid is (B), piperidine-2-carboxylic acid (pipecolic acid).
The twenty amino acids coded by DNA and used to build proteins include proline (A), the five-membered cyclic amino acid, tryptophan (C), with its indole side chain, and tyrosine (D), with its p-hydroxyphenyl side chain.
(B) is the six-membered ring homologue of proline. It does occur in living things, as a product of lysine breakdown and in some plants, but it is not one of the protein amino acids, which is the sense of 'natural amino acid' the paper uses. Its ring is one carbon larger than proline's, which is the detail to look for.
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