Question

In the presence of DTT, a peptide digested with CNBr yielded three fragments with the sequences: LID, GARCRLM, PCSKYIM. Also in the presence of DTT, the same peptide digested with trypsin yielded four fragments of which three are: GAR, LMPCSK, and YIMLID. Without DTT, CNBr digestion yielded two fragments and treatment with DTT alone has been shown to have no effect on the peptide's molecular weight. Please deduce the primary structure of the peptide

Answer 1

Primary sequence of the given peptide is GARCRLMPCSKYIMLID. This peptide has Disulphide Bridges in between two Cysteine amino acids (C-C).

This sequence is deduced from the given information:

1) DTT reduce Disulphide Bridge, while CNBr breaks the bond at methionine (M) residue. So, resulting three fragments are GARCRLM, PCSKYIM, and LID.

2) DTT reduce disulphide bridge, while trypsin breaks the bond at Arginine (R) carboxyl and Lysine(K) carboxyl, which results in four fragments GAR, CR, LMPCSK and YIMLID.

3) CNBr breaks the bond at methionine (M) carboxyl that results in only two peptide fragments: GARCRLM, and PCSKYIM (Attached with Disulphide Bridge of two cysteine amino acids) and LID.