Problem 22
Extracts from the bacterium Bacillus brevis contain a peptide with antibiotic properties. This peptide forms complexes with metal ions and seems to disrupt ion transport across the cell membranes of other bacterial species, leading to bacterial death. The structure of the peptide has been determined from a series of observations. a. Complete acid hydrolysis of the peptide, followed by amino acid analysis, yielded equimolar amounts of Leu, Orn, Phe, Pro, and Val. Orn is ornithine, an amino acid not present in proteins but present in some peptides. Ornithine has the structure b. The molecular weight of the peptide is approximately 1,200 Da. c. The peptide failed to undergo hydrolysis when treated with the enzyme carboxypeptidase. This enzyme catalyzes the hydrolysis of the carboxyl- terminal residue of a polypeptide unless the residue is Pro or, for some reason, does not contain a free carboxyl group. d. Treatment of the intact peptide with 1-fluoro-2,4dinitrobenzene, followed by complete hydrolysis and chromatography, yielded only free amino acids and the derivative shown here. e. Partial hydrolysis of the peptide followed by chromatographic separation and sequence analysis yielded these di- and tripeptides (the amino-terminal amino acid is always the first amino acid): Leu-Phe Phe-Pro Orn-Leu Val-Orn Val-Orn-Leu Phe-Pro-Val Pro-Val-Orn Given this information, deduce the amino acid sequence of the peptide antibiotic. Show your reasoning. When you have arrived at a structure, demonstrate that it is consistent with each experimental observation.
Problem 23
A peptide with the primary structure Lys-Arg-Pro-Leu-Ile-Asp-Gly-Ala must be synthesized by the methods developed by Merrifield. Calculate the percentage of the peptides synthesized that will be full length and have the correct sequence if the addition of each amino acid residue is \(96 \%\) efficient. Do the calculation a second time but assume a \(99 \%\) efficiency for each cycle.
