Question

Deduce the sequence of a heptapeptide that contains the amino acids Gly, His, Tyr, Ser, Ala, Met, and Arg, from the following experimental data. Edman degradation cleaves Ala from the heptapeptide, and carboxypeptidase forms His and a hexapeptide. Treatment of the heptapeptide with chymotrypsin forms a hexapeptide and a single amino acid. Treatment of the heptapeptide with trypsin forms a pentapeptide and a dipeptide. Partial hydrolysis forms His, Ala, Tyr, and the tripeptides Ser-Gly-Met and Gly-Met-Arg.

7 amino acids total

Answer 1

Answer:

Since Edman degradation cleaves Ala and Carboxypeptidase cleaves His from the heptapeptide So we can conclude that Ala is the N terminal residue and His is the C terminal Amino acid because in Edman degradation amino terminal residue is always cleaved and carboxypeptidase enzymes always cleaves the C terminal residue.

So the sequence is like Ala- AA2-AA3-AA4-AA5-AA6-His (AA= Amino Acid)

Chymotrypsin is a protease enzyme that cleaves on the C-terminal Aromatic amino acids like phenylalanine (F), tryptophan (W), and tyrosine (Y) on peptide chains. Since Treatment of the heptapeptide with chymotrypsin forms a hexapeptide and a single amino acid. So the AA6 must be the aromatic amino acid Tyr. Chymotrypsin cleaves the C terminal end of Tyr and frees the single amino acid His. So the sequence becomes

Ala- AA2-AA3-AA4-AA5-Tyr-His

Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine. Treatment of the heptapeptide with trypsin forms a pentapeptide and a dipeptide.So the AA5 will be surely Arg.  Trypsin cleaves the Carboxyl end of Arg and frees thedipeptide Tyr- His. So the sequence becomes

Ala- AA2-AA3-AA4-Arg-Tyr-His

It is also given that Partial hydrolysis forms His, Ala, Tyr, and the tripeptides Ser-Gly-Met and Gly-Met-Arg. So In the final heptapeptide sequence the AA2-AA3-AA4 is clearly Ser-Gly-Met.

SO the Hexapeptide Is,

Ala- Ser-Gly-Met-Arg-Tyr-His