Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides: Ala-V

Question

Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides:

Ala-Val-Cys-Arg-Thr-Gly-Cys-Lys-Asn-Phe-Leu

Tyr-Lys-Cys-Phe-Arg-His-Thr-Lys-Cys-Ser

Treatment of the polypeptide with trypsin which cuts after each K and R, yields fragments with the following amino acid compositions (determined by hydrolysis):

(Ala-Arg-Arg-Cys-Cys-Phe-Val)   (Cys-Cys-Gly-Lys-Ser-Thr)       (Asn-Leu-Phe)

(His-Lys-Thr)          (Lys-Tyr)

Indicate the positions of the Disulfide bonds in the intact polypeptide: Explain how you determined it.

Explanation / Answer

Ala-Val-Cys-Arg-Thr-Gly-Cys-Lys-Asn-Phe-Leu

Tyr-Lys-Cys-Phe-Arg-His-Thr-Lys-Cys-Ser

disulphide bonds are present in cys*

Ala-Val-Cys*-Arg-Thr-Gly-Cys*-Lys-Asn-Phe-Leu

Tyr-Lys-Cys*-Phe-Arg-His-Thr-Lys-Cys*-Ser

disulphide between in 1st cys in pepetide 1 and 3rd cys in pepetide 2 disulphide between 2nd cys in peptide 1 and 1st cys in pepetide 2

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