Question

You wish to determine the sequence of a polypeptide that has the following amino acid composition: Ala: 1 Arg: 4 Asn: 2 Asp: 3 Cys: 4 Gly: 3 Gln: 1 Glu: 4 His: 1 Lys :1 Met: 1 Phe: 1 Pro: 2 Ser: 4 Tyr: 2 Trp: 1 What is the maximum number of peptides you can expect if you cleave the polypeptide with: A) Trypsin B) Chymotrypsin C) CNBr D) There are no free sulfhydryl groups in the peptide. How many disulfide bonds do you expect to find? E) How many different possible arrangements of disulfide bonds are possible in this peptide?

Answer 1

What is the maximum number of peptides you can expect if you cleave the polypeptide with:

(A) Trypsin: Carboxylic end of Lys or Arg is cleaved by Trypsin except when proline is not the immediate residue.

So, the maximum number of peptide could be 5 (4 Arg and 1 Lys).

(B) Chymotrypsin

Chymotrypsin cuts at the C-terminal of tyrosine, tryptophan, and phenylalanine.

So, in this case maximum number of smaller peptides possible = 4 (2 Tyr, 1Trp and 1 Phe)

(C) CNBr

CNBr cuts at the C-terminal of methionine. So, maximum number possible = 2 (1 Met )

(D) There are no free sulfhydryl groups in the peptide. How many disulfide bonds do you expect to find?

A disulfide bond is formed from two Cys residues. So there will be 2 disulfide bonds if it is intra-molecular and 4 disulfide bonds can be formed through inter-molecular bonding.

(E) How many different possible arrangements of disulfide bonds are possible in this peptide?

It can be linear or bridging depending on the postions of the Cys residues.