Amino Acids And Proteins

Question

A peptide on complete hydrolysis yields glycine, alanine, and valine in equal amounts, and partial hydrolysis gives the fragments Gly-Ala and Ala-Val but never Gly-Val. What is the most consistent amino acid sequence of this tripeptide?

RankGuru · Accepted Answer

Sequencing a peptide from overlapping fragments is a classic logical exercise that uses the fact that peptide bonds run directionally from the free amino end to the free carboxyl end. Complete hydrolysis tells us the tripeptide contains one each of glycine, alanine, and valine, so we must order these three residues. The partial-hydrolysis fragment Gly-Ala shows glycine is directly bonded before alanine, and the fragment Ala-Val shows alanine is directly bonded before valine. Overlapping these two pieces at the shared alanine gives the unambiguous order Gly-Ala-Val. The explicit absence of any Gly-Val fragment confirms glycine and valine are not adjacent, which is consistent only with alanine lying between them. The sequence Ala-Gly-Val would require an Ala-Gly fragment, which was not observed, so it is rejected; reading Val-Ala-Gly reverses the established direction and contradicts the fragments; and Gly-Val-Ala would require a Gly-Val fragment that is explicitly absent. This reasoning reflects the NCERT idea that primary structure is a defined sequence. As a check, the deduced sequence reproduces exactly the two observed fragments and excludes the forbidden one, so it is fully self-consistent.

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