A nonapeptide was determined to have the following amino acid composition: (Lys)₂, (Gly)₂, (Phe)₂, His, Leu, Met. The native peptide was incubated with 1-fluoro-2,4-dinitrobenzene (FDNB) and then hydrolyzed; 2,4-dinitrophenylhistidine was identified by HPLC. When the native peptide was exposed to cyanogen bromide (CNBr), an octapeptide and free glycine were recovered. Incubation of the native peptide with trypsin gave a pentapeptide, a tripeptide, and free Lys. 2,4-Dinitrophenyl-histidine was recovered from the pentapeptide, and 2,4-dinitrophenylphenylalanine was recovered from the tripeptide. Digestion with the enzyme pepsin produced a dipeptide, a tripeptide, and a tetrapeptide. The tetrapeptide was composed of (Lys)₂, Phe, and Gly. The native sequence was determined to be:

Question

Quizplus · Accepted Answer

We can use the information given about the peptide fragments obtained from various enzyme digestions to piece together the native sequence: - From FDNB labeling and HPLC, we know that there is a histidine residue in the peptide. - From CNBr cleavage, we know that one of the peptides produced is an octapeptide (8 amino acids) with one glycine residue. Since we do not see any peptides with more than 8 amino acids in the other digestions, we can infer that this octapeptide is the longest continuous stretch of amino acids in the peptide. Based on the amino acid composition provided, the octapeptide can only be His-Leu-Phe-Gly-Lys-Lys-Phe-Met or His-Phe-Leu-Gly-Lys-Lys-Phe-Met. - From trypsin digestion, we know that there is a pentapeptide (5 amino acids) with 2,4-dinitrophenyl-histidine and a tripeptide (3 amino acids) with 2,4-dinitrophenyl-phenylalanine. The pentapeptide must contain the histidine residue, and since we already have a His residue in the octapeptide, it must be either His-Leu-Phe-Gly-Lys or His-Phe-Leu-Gly-Lys. The tripeptide must contain the phenylalanine residue, and since we already have a Phe residue in the octapeptide, it must be either Phe-Met-Gly or Phe-Gly-Met. - From pepsin digestion, we know that there is a tetrapeptide (4 amino acids) with (Lys)₂, Phe, and Gly. The only option that fits with the possible sequences we have so far is Lys-Lys-Phe-Gly. Putting all these pieces together, we get the sequence: His-Leu-Phe-Gly-Lys-Lys-Phe-Gly-Met. This corresponds to choice C.

A Nonapeptide Was Determined to Have the Following Amino Acid