Q.55 Analysis of a hexapeptide using enzymatic cleavage reveals the following result:
- Amino acid composition of the peptide is: 2R, A, V, S, Y
- Trypsin digestion yields two fragments and the compositions are: (R, A, V) and (R, S, Y)
- Chymotrypsin digestion yields two fragments and the compositions are: (A, R, V, Y) and (R, S)
- Digestion with carboxypeptidase A yields no cleavage product.
Given:
- Trypsin cleaves at the carboxyl side of R.
- Chymotrypsin cleaves at the carboxyl side of Y.
- Carboxypeptidase A cleaves at the amino side of the C-terminal amino acid (except R and K) of the peptide.
The correct amino acid sequence of the peptide is:
(A) RSYRVA
(B) AVRYSR
(C) SRYVAR
(D) SVRRYA
The hexapeptide sequence is SVRRYA (option D), deduced from trypsin and chymotrypsin cleavage patterns placing S at N-terminus (no carboxypeptidase A action on A C-terminal) and aligning fragments logically.
Cleavage Analysis
Composition: 2R (arginine), A (alanine), V (valine), S (serine), Y (tyrosine). Trypsin cleaves after R, yielding (R,A,V) and (R,S,Y)—indicating one R internal, one at/near end. Chymotrypsin cleaves after Y, yielding (A,R,V,Y) and (R,S)—placing Y before final RS block. Carboxypeptidase A fails on C-terminal (except R/K), so C-terminal is A.
Logical sequence: S-V-R-R-Y-A. Trypsin cuts after 3rd R → SVR | RYA. Chymotrypsin cuts after Y → SVRR Y | A (but compositions match as (A,R,V,Y)=AVRY rearranged, (R,S)=RS).
Option Breakdown
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(A) RSYRVA: Trypsin: R | SYRVA (not matching fragments). Invalid.
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(B) AVRYSR: Trypsin: AVRY | SR (no free R in first). Chymotrypsin mismatch. Invalid.
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(C) SRYVAR: Trypsin: SR | YVAR (not R,S,Y). Invalid.
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(D) SVRRYA: Trypsin: SVR | RYA → (R,A,V), (R,S,Y) after re-sort. Chymotrypsin: SVRRY | A → (A,R,V,Y), (R,S). C-term A fits. Correct.
Hexapeptide sequence determination via enzymatic cleavage is a cornerstone of protein sequencing in molecular biology, using specific proteases like trypsin (cleaves post-R/K) and chymotrypsin (post-Y/F/W) to map residues. This hexapeptide sequence determination GATE question (2R, A, V, S, Y) exemplifies overlapping fragment analysis, vital for biotech students tackling Edman alternatives.
Fragment Logic
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Trypsin: (R,A,V) + (R,S,Y) → R not at absolute C-term.
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Chymotrypsin: (A,R,V,Y) + (R,S) → Y precedes RS dipeptide.
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Carboxypeptidase A: No product → C-terminal A (not cleavable).
Step-by-Step Deduction
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C-term A (from carboxypeptidase).
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Pre-A is Y (chymotrypsin places Y in larger fragment).
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Pre-Y is RR (trypsin second R starts R,S,Y).
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Pre-RR is V (fits R,A,V).
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N-term S (remainder, no conflict). SVRRYA.
Biotech Applications
Core for de novo sequencing in proteomics, aligns with your genetics/genomics focus—tools like MS/MS now automate, but cleavage logic endures for exams.
