Question

A sample of a peptide of unknown sequence was treated with trypsin; another sample of the same peptide was treated with chymotrypsin.

The sequences (N-terminal to C-terminal) of the smaller peptides produced by trypsin digestion were as follows:

Cys—Thr—Ala—Ser—Ile—Lys

Arg—Thr—Arg—Gln—Arg

Gln—Phe—Ala—Val—Tyr—Asn—Lys

Phe—His—Cys—Asn

The sequences of the smaller peptides produced by chymotrypsin digestion were as follows:

Asn—Lys—Cys—Thr—Ala—Ser—Ile—Lys—Phe

Arg—Thr—Arg—Gln—Arg—Gln—Phe

His—Cys—Asn

Ala—Val—Ty

Answer 1

Chymotrypsin hydrolyzes the peptide bonds that follow large hydrophobic residues, e.g. Phenylalanine, Tyrosine, Tryptophan.

Trypsin hydrolyzes the peptide bonds that follow positively charged residues, e.g. Lysine and Arginine. If Arg-Thr fragment present trypsin won't cleave that bond.

Both cleave at Carboxyl Side.

Answer: 22 amino acid sequence is

Arg—Thr—Arg—Gln—Arg—Gln—Phe—Ala—Val—Tyr—Asn—Lys—Cys—Thr—Ala—Ser—Ile—Lys—Phe—His—Cys—Asn

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