Question

Need help with this organic CHEM problem.

Determines the sequence of the following peptide (octapeptide)

Complete hydrolysis: 2Arg, Leu, Lys, Met, Phe, Tyr

Edman’s test: PTH – Leu

Carboxypeptidase A: Ser

Treatment with BrCN:

1. Arg, Phe, Ser

2. Arg, Leu, Lys, Met, Tyr

Treatment with Trypsin:

1. Arg

2. Ser

3. Arg, Met, Phe

4. Leu, Lys, Tyr

Answer 1

The sequence to the above octapeptide is:
Leu-Tyr-Lys-Arg-Met-Phe-Arg-Ser

Through Edman's test, it is clear that the amino acid in fragment PTH-Leu occupies the N terminal of amino acid chain where as the action of carboxypeptidase A gives Serine as fragment which confirms its position as C terminal of peptide chain.

Now, CNBr attacks on the -S-CH₃ residue of methionide and fragment it with its N terminal chain, breaking the Carboxyl group end. Thus, On N terminal side of Met lies, Arg, Leu, Lys, Tyr and on the C terminal of Met lies the amino acids, Arg, Phe, Ser.

Attack of Trypsin on the peptide chain targets the carboxyl group side of Lysine and Arginine. This confirms the order giving the fragments Leu-Tyr-Lys, Arg, Met-Phe-Arg, Ser.

Thus, the sequencing:

Leu-Tyr-Lys-Arg-Met-Phe-Arg-Ser