Question

A pentapeptide that contains histidine, two glycines, tryptophan, and alanine is incubated with the protease chymotrypsin. A dipeptide and a tripeptide result, and the dipeptide contains tryptophan while the tripeptide contains histidine and two glycines. Brief treatment with carboxypeptidase A yields free histidine. What is the sequence of the peptapeptide?

Answer options:

GGWAH

GGHWA

AWGGH

GGHAW

GWAGH

Answer 1

Ans. Step 1: Chymotrypsin digests the peptide bond at COO- terminus of a Tyr (Y), Trp (W) and Phe (F) in the peptide.

Chymotrypsin treatment yields a dipeptide with Trp residue. So, the second position of the pentapeptide is Trp.

So, 2^nd residue is Trp.

Step 2: The pentapeptide consists of one His, one Ala, one Trp and two glycine. The tripeptide has one histidine and two glycine.

So, the dipeptide must contain one Ala and one Trp residue.

The 2^nd position (C-ter) residue is Trp (see step 1). So, the 1^st residue (at N-ter) must be alanine.

So, 1^st residue is alanine. Or, alanine is the N-terminal residue.

Step 3: Carboxypeptidase A cleaves the peptide bond at C-terminal of aromatic or aliphatic residues.

In the tripeptide, His has an aromatic side chain. The side chain of glycine (-H atom) is neither aromatic nor aliphatic.

To yield a FREE histidine residue, the His-residue must be the C-terminal residue.

Step 4: We have,

            Dipeptide (from N- to C-ter) = A-W

            Tripeptide (From N- to C-ter): G, G, H

             C-terminal residue = His

Thus, overall pentapeptide sequence (from N- to C-ter) = AWGGH

Thus, correct option is C. AWGGH