Sequence Determination of the Brain Peptide Leucine Enkephalin A group of peptid

Question

Sequence Determination of the Brain Peptide Leucine Enkephalin A group of peptides that influence nerve transmission in certain parts of the brain has been isolated from normal brain tissue. These peptides are known as opioids, because they bind to specific receptors that also bind opiate drugs, such as morphine and naloxone. Opioids thus mimic some of the properties of opiates. Some researchers consider these peptides to be the brains own pain killers. Using the information below, determine the amino acid sequence of the opioid leucine enkephalin. Explain how your structure is consistent with each piece of information.

(a) Complete hydrolysis by 6 M HCl at 110 C followed by amino acid analysis indicated the presence of Gly, Leu,

Phe, and Tyr, in a 2:1:1:1 molar ratio.

(b) Treatment of the peptide with 1-fluoro-2,4-dinitrobenzene followed by complete hydrolysis and chromatography indicated the presence of the 2,4-dinitrophenyl derivative of tyrosine. No free tyrosine could be found.

(c) Complete digestion of the peptide with chymopepsin followed by chromatography yielded free tyrosine and leucin plus a tripeptide containing Phe and Gly in a 1:2 ratio.

Explanation / Answer

1. the treatment with DNFB yields tyrosine, suggests the first amino acid in the sequence to be tyrosine

2. Digestion with chymotrypsin yields, free tyrosine, tripeptide (Gly(2 units), Phe) and leucine

As chymotrypsin cleaves the peptide from the carboxyl end of tyrosine or phenylalanine or tryptophan.

the sequence of peptide possible is

Tyr-Gly-Gly-Phe-Leu

which on chymotrypsin treatment gives Tyr, Leu and Gly-Gly-Phe

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