Determine the AA sequence of the brain peptide enkephalin from the information g

Question

Determine the AA sequence of the brain peptide enkephalin from the information given below. Briefly explain how your structure is consistent w/ each piece of information.

(a)complete hydrolysis of the peptide by 1M HCL at 110 degree celcius followed by AA analysis indicated the presence of gky, leu, phe and tyr in a 2:1:1:1 molar ratio

(b)treatment of the peptide with FDNB followed by complete hydrolysis and chromatography indicated the presence of the dinitrophenyl derivative of tyrosine. No free tyrosine was found.

(c) partial hydrolysis of the peptide by chymotrypsin followed by chromatography yielded leu, tyr and a shorter peptide. Complete hydrolysis of the latter followed by AA analysis indicated the presence of gly and phe in a 2:1 ratio.

Explanation / Answer

The amino acid sequence is:
N-Tyrosine-glycine-glycine-phenylalanine-leucine-C
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a. the hydrolysis with HCl breaks the peptide bonds and thus we know that there are 5 amino acids with two being glycine.
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b. Treatment with FDNB labels the N terminal amino acid. As the dinitrophenyl derivative of tyrosine is found it proves that tyrosine is the N terminal amino acid.
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c. chymotrypsin cleaves at the carboxy terminal of aromatic amino acids. As there are two aromatic amino acids-tyrosine and phenylalanine, the results will give us the sequence of the peptide.
The analysis of the shorter peptide gives us the sequence - 2 glycine and one phenyl alanine. This can be present only in this sequence:
Gly-gly-phe
If the sequence was Gly-phe-gly or phe-gly-gly this peptide would not contain two glycine and one phenyl alanine.

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So we have the N terminal and the sequence of the shorter peptide.As leucine is a free amino acid we can say that it is at the end of the peptide.
This covers all the points in the digestion too.
chymotrypsin cleaves after tyrosine and after phenylalanine.
tyr----gly-gly-phe---leu
Hence this sequence is established.

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