5. You have been asked to deduce the sequence of a heptapeptide (7 amino acid re

Question

5. You have been asked to deduce the sequence of a heptapeptide (7 amino acid residues) from the following information. Cleavage with a certain reagent or enzyme yielded 3 peptides, with sequences as follows Tyr-Lys Glu-Ser Ala-Phe Arg What was the cleavage reagent or enzyme used? Cleavage with another reagent or enzyme also yielded 3 peptides, with sequences as follows: Lys-Glu-Ser Ala-Phe Arg-Tyr What was the cleavage reagent or enzyme used? What is the sequence of the heptapeptide? (Use 1-letter abbreviations.)

Explanation / Answer

Here is the list of some common chemicals or enzyme used in protein sequencing:

1.trypsin-cleaves at C terminal of positively charged aa

2.Pepsin-cleaves at N terminal of aromatic aa

3.Chymotrypsin-cleaves at C terminal of aromatic aa

4.CNBr-cleaves at C terminal of met

If we look at the fragments given in 1st part,it can be assumed that probably the enzyme/chemical is acting at arg or lys so the agent can be trypsin.

So we can assume the sequence to be ala-phe-arg-tyr-lys-glu-ser

Same if we look at fragments given in 2nd part we can say that probably the reagent/enzyme is acting on aromatic aa.The enzyme can be chymotrypsin.

By comaring fragments from both part,the sequence will be:

Ala-phe-arg-tyr-lys-glu-ser

A-F-R-Y-K-E-S

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