An octapeptide was hydrolyzed and the amino acids separated. It was found to hav
ID: 786648 • Letter: A
Question
An octapeptide was hydrolyzed and the amino acids separated. It was found to have the following amino acid composition: Gly (1); Ala (2); Val (1); Lys (2); Met (1); Asp (1). After incubation with FDNB and hydrolysis, 2,4-dinitrophenylalanine was detected. Treatment with cyanogen bromide, which cleaves at M, resulted in two fragments: a pentapeptide containing two K residues and a tripeptide containing V, G, and A. (The residues are not listed in any particular order.) After treatment with trypsin, which cleaves at R and K, there were 3 fragments: two dipeptides and MGAV. What is the sequence of the peptide? Use the one-letter abbreviations for the amino acids, and do not use dashes.
Explanation / Answer
1. An octapeptide was determined to have the following amino acid composition: (Lys)2, (Gly) 1, (Phe) 2, His, Leu, Met. The native peptide was incubated with 1-fluoro-2,4-dinitrobenzene (FDNB) (Sanger Method) and then hydrolyzed; 2,4-dinitrophenylglycine was identified by HPLC. When the native peptide was exposed to Pepsin (Cleavage point at amino side of Phe, Trp, Tyr), a pentapeptide, a dipeptide and free glycine were recovered. 2,4-Dinitrophenyl-phenylalanine was recovered from the pentapeptide and also the dipeptide (Sanger Method). Incubation of the native peptide with trypsin (Cleavage points at carboxy side of Lys, Arg) gave a tetrapeptide, a tripeptide, and free Lys. 2,4-Dinitrophenyl-leucine was recovered from the tetrapeptide (Sanger Method). The native sequence was determined to be:
A) Gly