Question

A solution of a peptide of unknown sequence was divided into two samples. One sample was treated with trypsin, and the other was treated with chymotrypsin. The smaller peptides obtained trypsin treatment had the following sequences: LSYAIR, VSSK, and DGMFVK. The smaller peptides obtained by chymotrypsin treatment had the following sequences: VKLSY, AIR, and VSSKDGMF. Deduce the sequence of the original peptide.

A. VKLSYDGMFAIRVSSK

B. VSSKDGMFVKLSYAIR CORRECT ANSWER

C. LSYAIRVSSK DGMFVK

D. AIRVKLSYDGMFVSSK

E. VSSKDGMFAIRVKLSY

Please show how B. Would be the correct answer

Answer 1

See the picture below, particularly how the colors interact:

Remember trypsin cuts on K and R, while chymiotripsin does it on F, W and Y. Since the second test gives the extract AIR, we can assume it will go at the end of the chain, with no possible cut of trypsin on the R. Now according to the sequences, we must also assume that the first one is VSSK, since it interacts only on its right side. Following the colors, the only correct answer must be B.