Question

You wish to determine the sequence of a polypeptide that has the following amino acid composition.

1 Ala 4 Arg 2 Asn 3 Asp 4 Cys 3 Gly 1 Gln 4 Glu1 His 1 Lys 1 Met 1 Phe 2 Pro 4 Ser 2 Tyr 1 Trp

What is the maximum number of peptides you can expect if you cleave the polypeptide with cyanogen bromide ?

What is the maximum number of peptides you can expect if you do a quick digestion of the polypeptide with chymotrypsin?

Analysis of the intact polypeptide reveals that there are no free sulfhydryl groups.

There are how many disulfide bonds likely to be present?

how many different arrangements of disulfide bonds are possible?

Answer 1

1. Cyanogen bromide cleaves at C terminal of Methionine residues where it encounters. So two peptides will generate.

2. Chymotrypsin cleaves at C terminus of aromatic amino acid residues. So 4 fragments will be generated.

3. Only 2 disulfide bonds will be present in this sequence because a total of 4 Cys residues will form 2 disulfide bonds (one bond with two Cys residues).

4. Only 3 possibilities are there to form disulfide bonds. Two possibilities arise if 4 Cys residues make 2 disulfide bonds in a linear fashion and one possibility if middle 2nd and 3rd Cys residues form only one disulfide bond. Keep in mind that Methionine is also a sulfonated amino acid but it does not make any disulfide bond in the protein.