Question

Which one of the following pairs of amino acids is NOT incorporated in a polypeptide chain?

• A. ( 4-Hydroxyproline and ( gamma)-carboxyglutamate)
• B. ( gamma)-Carboxyglutamate and desmosine)
• C. ( Ornithine and citrulline)
• D. ( 4-Hydroxyproline and 5-hydroxylysine)

Hint:

Remember, only the 20 standard amino acids and their post-translationally modified forms are part of polypeptides. Metabolic intermediates like ornithine and citrulline are exceptions.

Answer 1

The Correct Option is C

Step 1: Understanding polypeptide incorporation. Amino acids that are directly encoded by the genetic code are incorporated into polypeptides during translation. Some modified amino acids, such as 4-hydroxyproline and ( gamma)-carboxyglutamate, are formed post-translationally but are part of polypeptides.
Step 2: Evaluating the given pairs. - (A) 4-Hydroxyproline and ( gamma)-carboxyglutamate: Both are post-translational modifications found in proteins. - (B) ( gamma)-Carboxyglutamate and desmosine: These are also post-translational modifications and part of polypeptides. - (C) Ornithine and citrulline: These amino acids are not incorporated into polypeptides. They are intermediates in metabolic pathways (urea cycle and arginine biosynthesis). - (D) 4-Hydroxyproline and 5-hydroxylysine: Both are post-translational modifications found in proteins like collagen.
Step 3: Conclusion. The pair Ornithine and Citrulline are NOT incorporated into polypeptide chains, as they function outside the context of protein synthesis.