Question

Determine the correct amino acid sequence of a heptapeptide (a small peptide containing seven amino acids) on the basis of the following data. Note: when the amino acid sequence is not given, a comma separates the amino acids.
(i) Amino acid composition: (Ala, Arg, Leu, Lys, Phe and 2Val). It means that this heptapeptide contains one Ala, one Arg, one Leu, one Lys, one Phe and two Val.
(ii) N-terminal analysis of the heptapeptide: Leu
(iii) Trypsin digestion (cleavage) of the heptapeptide yielded two fragments: (Lys, Phe, Val, Val) and (Ala, Arg, Leu)
(iv) Carboxypeptidase A digestion of the heptapeptide: no digestion
(v) Chymotrypsin digestion of the heptapeptide produced two fragments: (Ala, Arg, Leu, Phe) and (Lys, Val, Val)
The correct amino acid sequence of this heptapeptide from N-terminal to C-terminal is:________.
a. Leu-Ala-Arg-Phe-Val-Val-Lys
b. Leu-Ala-Arg-Val-Phe-Val-Lys
c. Leu-Ala-Phe-Arg-Val-Val-Lys
d. Arg-Ala-Lys-Val-Phe-Val-Leu
e. Leu-Ala-Lys-Val-Phe-Val-Arg

Answer 1

Answer:

a. Leu-Ala-Arg-Phe-Val-Val-Lys .

Explanation:

Hello.

In this case, considering the given data, we notice that the amino acid has a N-terminal Leu, that is why we first discard d. Moreover, considering the data (iii) and (v) we can infer that there are going to be two fragments by which it is possible to reason the proper sequence:

(iii): the amino acid has one section composed by Lys-Phe-Val-Val and another one by Ala, Arg, Leu that is why e is also discarded.

(v): the other two fragments are Ala, Arg, Leu, Phe, that is why b is discarded.

(iii) and (v): given the aforementioned fragments, we can combine them and therefore discard c, that is why the correct sequence is:

a. Leu-Ala-Arg-Phe-Val-Val-Lys .

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