Determine the amino acid sequence of an octapeptide with a composition of 2 ala,

Question

Determine the amino acid sequence of an octapeptide with a composition of 2 ala, 1 arg, 2 tyr, 1 val, 1 asp, 1 met. ( NH4+ was detected in the acid hydrolysis.)

a) Partial acid hydrolysis yields the product whose structure is shown below.

b) Chymotrypsin treatment renders 2 tetrapeptides, each containing an ala.

c) Trypsin treatment of one of the tetrapeptides produces 2 dipeptides.

d) Treatment with CNBr of the same tetrapeptide gives a tripeptide and free tyr.

e) Treatment with Sanger's reagent of the other tetrapeptide generates DNP - asp.

f) The tetrapeptide that doesn't have methionine ends in tyrosine

g) Treatment of the original octapeptide with Sanger's reagent generates DNP-ala.

The peptide sequence (using single letter amino acid codes) is _____________________.

Explanation / Answer

Chymotrypsin cleaves at aromatics like Y. To produce two tetrapeptides from an octapeptide
with two tyrosines, one must be the C-terminus and the other at position 4. Thus:
_ _ _ Y _ _ _ Y
Since trypsin cleaves, in this case, after R, we infer that R is at position 2 or 6.
_ R _ Y _ _ _ Y
_ _ _ Y _ R _ Y
That CNBr cleavage of the tetrapeptides produces free Y indicates the sequence MY adjacent to
R. Thus,
_ R M Y _ _ _ Y
_ _ _ Y _ R M Y
End group analysis of the second chymotrypsin tetrapeptide indicates Asp = D, but the presence
of NH4+ suggests asparagine, N (The end in question must be the N-terminus.) Thus:
_ R M Y N _ _ Y
N _ _ Y _ R M Y
We can now place AV (from (a)) into these sequences:
_ R M Y N A V Y
N A V Y _ R M Y

Treatment of the original octapeptide with Sanger's reagent generates DNP-ala.

Therefore, it is A R M Y N A V Y

The octapeptide sequence is ARMYNAVY.

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