Sequencing of an unknown polypeptide has yielded the following information. What

Question

Sequencing of an unknown polypeptide has yielded the following information. What is the sequence of the polypeptide? (Be as detailed as possible!) 1) Gly, Leu, Phe, and Tyr are in a 2:1:1:1 molar ratio 2) Treatment with 1-fluoro- 2,4- dinitrobenzene (FDNB) yielded complete hydrolysis and 2,4-dinitrophenyl tyrosine and no free tyrosine 3) Digestion with chymotrypsin yielded free tyrosine and leucine, and a tripeptide of Phe and Gly Sequencing of an unknown polypeptide has yielded the following information. What is the sequence of the polypeptide? (Be as detailed as possible!) 1) Gly, Leu, Phe, and Tyr are in a 2:1:1:1 molar ratio 2) Treatment with 1-fluoro- 2,4- dinitrobenzene (FDNB) yielded complete hydrolysis and 2,4-dinitrophenyl tyrosine and no free tyrosine 3) Digestion with chymotrypsin yielded free tyrosine and leucine, and a tripeptide of Phe and Gly Sequencing of an unknown polypeptide has yielded the following information. What is the sequence of the polypeptide? (Be as detailed as possible!) 1) Gly, Leu, Phe, and Tyr are in a 2:1:1:1 molar ratio 2) Treatment with 1-fluoro- 2,4- dinitrobenzene (FDNB) yielded complete hydrolysis and 2,4-dinitrophenyl tyrosine and no free tyrosine 3) Digestion with chymotrypsin yielded free tyrosine and leucine, and a tripeptide of Phe and Gly

Explanation / Answer

In Sanger's method 2,4-FNDB forms amino acid 2,4 dinitro phenyl derivative the amino terminal residue of the plypeptide, that is, the first amino acid in the ploypeptide chain. The derivative formed is that of tyrosine so the first amino acid is tyrosine.

Tyr-X-X-X-X-X

Chymotrypsin cleaves at the C-terminal of aromatics. The onlyaromatics here are Tyr and Phe and on treatment with chymotrypsin we get Tyr and Leu, since Tyr is already a terminal amino acid, the other terminal amino acid must be leucine which is after Phe, because chymotrysin cleaves at the C- terminal of aromatics and only Tyr and Leu are obtained. This implies that the Phe is not terminal and has only Leu at its C- terminal. The structure is now

Tyr-X-X-Phe-Leu

SInce now we have only 2 Gly left, as per the question, the final polypeptide is

Tyr-Gly-Gly-Phe-Leu

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