How to determine the primary structure of a peptide with given fragments...

[XzoeyX]

...from peptide digestion.? 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.

[Kunal Pandya]

primary sequence is GARCRLMPCSKYIMLID having Disulphide bridge in between two Cysteine (C-C).

1) DTT reduce disulphide bridge while CNBr selectively cleaves methionine (M) so resulting three fragments is GARCRLM, PCSKYIM, LID,

2) DTT reduce disulphide bridge while trypsin cleaves peptide at Arginine (R) carboxyl and Lysine(K) carboxyl resulting four fragments is GAR, CR ,LMPCSK and YIMLID.

3) Only CNBr selectively cleaves at methionine (M) carboxyl result in two fragment GARCRLM PCSKYIM (Attached with disulphide bridge of two cysteine) and LID

Best Regards
Kunal Pandya