Q.28 Which one of the following pairs of amino acids is NOT incorporated in a
polypeptide chain?
(A) 4–Hydroxyproline and γ–carboxyglutamate
(B) γ–Carboxyglutamate and desmosine
(C) Ornithine and citrulline
(D) 4–Hydroxyproline and 5–hydroxylysine
Ornithine and citrulline are the pair of amino acids not incorporated into polypeptide chains during ribosomal protein synthesis.
These non-proteinogenic amino acids serve metabolic roles rather than structural ones in proteins. The other pairs involve post-translationally modified residues found in specific proteins like collagen and elastin.
Option Analysis
4-Hydroxyproline and γ-carboxyglutamate: 4-Hydroxyproline forms via post-translational hydroxylation of proline in collagen, stabilizing its triple helix. γ-Carboxyglutamate (Gla) arises from carboxylation of glutamate in blood clotting factors like prothrombin, enabling calcium binding. Both integrate into mature polypeptide chains.
γ-Carboxyglutamate and desmosine: γ-Carboxyglutamate appears in Gla-domain proteins as noted. Desmosine, derived from four lysine residues via oxidative deamination and condensation in elastin, creates cross-links between chains but counts as a modified residue within the polypeptide network.
Ornithine and citrulline: Neither enters ribosomal translation due to lacking codons; they function as urea cycle intermediates for ammonia detoxification. Ornithine links carbamoyl phosphate to citrulline, but both remain free or modified post-translationally without chain incorporation.
4-Hydroxyproline and 5-hydroxylysine: Both result from hydroxylation of proline and lysine in collagen propeptides, essential for glycosylation and cross-linking in the extracellular matrix. They persist in the final polypeptide structure.
Standard protein synthesis uses 20 canonical amino acids via ribosomal translation. However, certain modified amino acids appear in polypeptides through post-translational changes, while others like those in metabolic pathways never join chains. This CSIR NET Life Sciences question tests distinction between truly non-incorporated amino acids and modified ones in proteins such as collagen and elastin.
Post-Translational vs Non-Proteinogenic Amino Acids
Post-translational modifications (PTMs) alter residues after chain assembly. Examples include:
-
Hydroxylation yielding 4-hydroxyproline and 5-hydroxylysine in collagen for stability
-
Carboxylation forming γ-carboxyglutamate in coagulation factors
-
Cross-linking to desmosine in elastin from lysine
Non-proteinogenic amino acids like ornithine and citrulline lack tRNA recognition and serve in urea cycle without polypeptide integration.
Why Ornithine and Citrulline Stand Out
| Amino Acid Pair | Incorporation Status | Protein Example | Mechanism |
|---|---|---|---|
| 4-Hydroxyproline & γ-Carboxyglutamate | Incorporated (PTM) | Collagen, Prothrombin | Hydroxylation, Carboxylation |
| γ-Carboxyglutamate & Desmosine | Incorporated (PTM/Cross-link) | Coagulation factors, Elastin | Carboxylation, Lysine condensation |
| Ornithine & Citrulline | NOT Incorporated | None (Urea cycle) | Metabolic intermediates |
| 4-Hydroxyproline & 5-Hydroxylysine | Incorporated (PTM) | Collagen | Hydroxylation |
Ornithine and citrulline participate in arginine biosynthesis but evade ribosomal machinery, making option (C) correct for CSIR NET.
