(D) Ala-Gly

The correct answer to Q.31 is (D) Ala-Gly, as alanine and glycine lack reactive side chains for common post-translational modifications (PTMs). This MCQ tests knowledge of PTM sites in proteins, crucial for exams like CSIR NET or GATE Biotechnology.​

Correct Answer

Option (D) Ala-Gly is the pair that does not undergo post-translational modification. Alanine (Ala) has a simple methyl side chain (-CH₃), and glycine (Gly) has no side chain (just -H), lacking functional groups like hydroxyl (-OH) or amide for glycosylation, phosphorylation, or hydroxylation.

Option Analysis

(A) Asn-His: Undergoes PTM

Asparagine (Asn) in Asn-His can undergo N-linked glycosylation or hydroxylation by enzymes like FIH, especially in hypoxia response pathways. Histidine (His) supports this motif in some contexts, though less common than Ser/Thr.

(B) Tyr-Ser: Undergoes PTM

Tyrosine (Tyr) features a phenolic -OH group for phosphorylation (e.g., by tyrosine kinases in signaling). Serine (Ser) adds O-linked glycosylation or phosphorylation potential, making this pair PTM-prone.​

(C) Asn-Ser: Undergoes PTM

This is a classic N-glycosylation sequon (Asn-X-Ser/Thr, where X ≠ Pro). Asparagine links to glycans in the ER, while Ser enables O-glycosylation, common in glycoproteins.

(D) Ala-Gly: No PTM

Neither Ala nor Gly has modifiable groups—Ala’s inert alkyl chain and Gly’s hydrogen prevent attachments like phosphate, sugar, or hydroxyl. They remain unchanged post-translation.