Question

An octapeptide was isolated from a fungus for use as a new antibiotic. The composition of the peptide was 2 Gly, Lys, Met, Pro, Arg, Trp, Tyr. Edman degradation of the intact peptide yields FDNB- glycine. The peptide was then treated with carboxypeptidase A, B and C, only carboxypeptidase C had any effect cleaving off Pro. The peptide was then treated with trypsin (cleaves at Arg and Lys), which gave two tripeptides and a dipeptide.   The compositions of the tripeptides were Met, Lys, Tyr, and Arg, Trp, Gly. The peptide was digested with chymotrypsin (cleaves at Trp, Phe and Tyr), which yielded two tripeptides and a dipeptide. Acid hydrolysis of the dipeptide from chymotrypsin yielded only glycine (hint acid hydrolysis had two problems).  

Answer 1

FDNB- cause glycine

So glycine is 1st amino acid

Carboxy peptidase yield proline so proline is last amino acid

Glycine -2-3-4-5-6-7-proline

Trypsin digestion cause three fragments of two with 3 amino acid, and one with 2 amino acids

3 amino acid could lysine or arginine and 5 th can be lysine or arginine

So one fragment of 3 amino acids is met-lys-try and arg -try-gly

So 1 st a.a is glycine then 2nd is tyrosine 3 rd is arginine while 4th one is met 5th aa ia tryosine 6th is lysine 7th glycine and 8th is proline.

Gly-try-arg-met-trp-lys-gly-pro

Now if above one will be probable peptide sequence then lets check the out come with chymotrypsin.

After digesting with chymotrypsin it will cut at tryosine and tryosine to yield 2 amino acid peptide(gly-tyr) and two three amino acids containing peptide (arg-met-trp and lys-gly- pro)

Which is matching with outcome mention in question.

So peptide sequence is correct .

N- Gly- Try- Arg- Met-Trp-Lys-Gly- Pro-C

Thanks..!