Beta-melanotropin is a peptide hormone containing 18 amino acids. Following trea

Question

Beta-melanotropin is a peptide hormone containing 18 amino acids. Following treatment with a proteolytic enzyme, the following four peptide fragments were isolated.

      #1) WECK                      #2) SDLGPYK                #3) MEHFR              #4) QE

a)What proteolytic enzyme was most likely used?

a) carboxypeptidase A           b) trypsin                 c) chymotrypsin            d) V8 protease             

Treatment of the original 18 amino acid polypeptide with cyanogen bromide yielded two fragments; one peptide with 8 amino acids and one peptide with 10 amino acids.

b) Deduce (write out) the amino acid sequence of the original peptide:

Please explain the why and how for these questions respectively!!

Thank you so much!!!

Explanation / Answer

a) Trypsin acts on C terminal of basic amino acids like Lys and Arg, so the original sequence can be

SDLGPYK WECK MEHFR QE

or SDLGPYK MEHFR WECK QE

But the second part says after cyanide bromide cleavage gives 2 parts i.e. 8 amino acids and 10 amino acids long. Cyanide bromide acts on C terminal of Methionine residue. This combination is possible in second arrangement i.e. SDLGPYK MEHFR WECK QE. The fragments would be SDLGPYKM and EHFRWECKQE.

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