Discussion
Which one of the following pairs of amino acids is NOT incorporated in a polypeptide chain?
Answer: C
The pair not found in polypeptide chains is ornithine and citrulline.
Ornithine and citrulline are intermediates of the urea cycle. They have no codons and no tRNAs, and they occur as free metabolites rather than as residues of proteins.
The other pairs are residues that do occur in polypeptide chains, created by modifying amino acids after they are built in. 4-Hydroxyproline and 5-hydroxylysine are formed in collagen by vitamin C dependent hydroxylases. γ-Carboxyglutamate is formed in clotting factors such as prothrombin by vitamin K dependent carboxylation. Desmosine is the cross-link formed from four lysine side chains in elastin.
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