Question

You have a peptide whose sequence is secretly known, that you want to analyze. The peptide sequence is D-M-K-T-L-A-R-S-M-E-I-D-Q

You have three reagents that cleave polypeptides, CNBr, chymotrypsin, and trypsin into smaller peptides. If you could purify these peptides and sequence them by Edman, but only get four amino acids.

1) what end of the protein does the Edman reaction cleave residues from?

n-terminus or c-terminus

2) what are all the Edman sequences (up to 4 amino acids) of the peptide products from cyanogen bromide cleavage?

3) what are all the Edman sequences (up to 4 amino acids) of the peptide products from chymotrypsin cleavage?

4) what are all the Edman sequences (up to 4 amino acids) of the peptide products from trypsin cleavage?

Answer 1

1) Edman's reagent is used in sequencing of amino acids. It cleaves one amino acid each from the amino terminal or n-terminus.

2) Cyanogen bromide cleaved the peptide bond from the C terminus of the aminoacid methionine. The cyanogen bromide cleavage produces the following sequences,

• D-M
• K-T-L-A-R-S-M
• E-I-D-Q

Edmans reaction will produce D, K, T, and L.

3) Chymotrypsin cleaves on the C terminal of phenyl alanine, tryptophan and tyrosine. There are no such amino acids in the given peptide structure. So, the Edmans reaction will produce D, M, K and T.

4) Trypsin cleaves at C terminus of lysine and arginine. So, the peptide sequences are,

• D-M-K
• T-L-A-R
• S-M-E-I-D-Q

The Edmans reaction will produce D, T, S