Amino Acid analysis of an oligopeptide revealed the presence of the following am

Question

Amino Acid analysis of an oligopeptide revealed the presence of the following amino acid residues:

(NOT written in any specific order)

Lys Gly Asp Met Val Trp

The following facts were also observed: a) Edman's degradation yielded the PTH-Asp b) Carboxypeptidase treatment resulted in free Gly c) Trypsin treatment yielded a dipeptide and a tetrapeptide d) Chymotrypsin treatment yielded a pentapeptide e) Reaction with CNBr resulted in two tripeptides. One tripeptide was shown to contain Trp, Val and another residue.

What is the possible amino acid sequence of this hexapeptide? Also, explain how you deduced the sequence and how did each of the facts given in the observations help you put together the order of the amino acids in this hexapeptide?

Explanation / Answer

Edman's degrdation tells us the N terminal amino acid. So we know it as Asp.

Carboxypeptidase treatment reveals the COOH terminal of the peptide. So we know it as glycine.

Trypsin cleaves at the COOH terminal of Arg and Lys. This gives us the second amino acid as Lys.

Chymotrypsin treament cleaves at the COOH teminal of Trp. So we get the second last amino acid as Trp.

CnBr cleaves after methionine. so it is the third amino acid.

So the whole sequence is:

Asp-Lys-Met-Val-Trp-Gly.

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