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GATE 2025 BT – Question 38

General Biology · Biochemistry: Biomolecules, biological membranes, channels and pumps, molecular motors, action potential and transport · 2 marks · Multiple choice

An octapeptide composed of these L-amino acids – Lys, Thr, Ser, Met, Arg, Trp, Tyr, Glu – was subjected to analyses with the following outcomes:

P. The N-terminal sequencing analysis by Sanger's method yielded 'Ser' at the N-terminus

Q. Chymotrypsin treatment gave a pentapeptide, a 'Tyr' containing dipeptide and a free 'Glu'

R. Cyanogen bromide treatment gave two tetrapeptides

S. Trypsin treatment gave two tripeptides and a dipeptide

Which one of the following is the correct octapeptide sequence?

  1. Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu
  2. Ser-Arg-Lys-Met-Tyr-Thr-Trp-Glu
  3. Ser-Met-Lys-Arg-Thr-Tyr-Trp-Glu
  4. Ser-Arg-Met-Lys-Trp-Thr-Tyr-Glu

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Show answer and explanation

Correct answer: (A) Ser-Tyr-Arg-Met-Lys-Thr-Trp-Glu

Explanation

Check option A. Chymotrypsin cuts after Tyr and Trp: Ser-Tyr | Arg-Met-Lys-Thr-Trp | Glu, which is a Tyr-containing dipeptide, a pentapeptide and a free Glu. Cyanogen bromide cuts after Met: Ser-Tyr-Arg-Met | Lys-Thr-Trp-Glu, two tetrapeptides. Trypsin cuts after Arg and Lys: Ser-Tyr-Arg | Met-Lys | Thr-Trp-Glu, two tripeptides and a dipeptide. It starts with Ser. The other options fail at least one of these tests.