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Improvement of solubility and stability of the antimicrobial peptide nisin by protein engineering

Nisin is a 3.4-kDa antimicrobial peptide that, as a result of posttranslational modifications, contains unsaturated amino acids and lanthionine residues. It is applied as a preservative in various food products. The solubility and stability of nisin and nisin mutants have been studied. It is demonstrated that nisin mutants can be produced with improved functional properties. The solubility of nisin A is highest at low pH values and gradually decreases by almost 2 orders of magnitude when the pH of the solution exceeds a value of 7. At low pH, nisin Z exhibits a decreased solubility relative to that of nisin A; at neutral and higher pH values, the solubilities of both variants are comparable. Two mutants of nisin Z, which contain lysyl residues at positions 27 and 31, respectively, instead of Asn-27 and His-31, were produced with the aim of reaching higher solubility at neutral pH. Both mutants were purified to homogeneity, and their structures were confirmed by one- and two-dimensional 1H nuclear magnetic resonance. Their antimicrobial activities were found to be similar to that of nisin Z, whereas their solubilities at pH 7 increased by factors of 4 and 7, respectively. The chemical stability of nisin A was studied in the pH range of 2 to 8 and at a 20, 37, and 75 degrees C. Optimal stability was observed at pH 3.0. Nisin Z showed a behavior similar to that of nisin A. A mutant containing dehydrobutyrine at position 5 instead of dehydroalanine had lower activity but was significantly more resistant to acid-catalyzed chemical degradation than wild-type nisin Z.