The molecular characterisation and antimicrobial properties of amidated bovine β-lactoglobulin

Abstract Amidation of bovine β -lactoglobulin ( β -Lg) imparted antimicrobial properties to this protein. Amidated β -Lg was strongly bactericidal against resting cells of Pseudomonas fluorescens , Pseudomonas fragi and Bacillus subtilis , but had a much weaker effect against Escherichia coli , Enterococcus faecalis , Salmonella typhimurium and Listeria monocytogenes . Neither native nor amidated β -Lg was effective against the yeast Saccharomyces cerevisiae and the mould Penicillium candidum. Mass spectrometric analysis demonstrated that amidation of β -Lg converted aspartyl and glutamyl residues to asparaginyl and glutaminyl residues, respectively, and that the amidation reaction did not occur to the same extent on every β -Lg molecule. The charge change was also confirmed by SDS–PAGE, ion exchange chromatography and the change in isoionic point of β -Lg. Reverse-phase chromatography showed that amidation also led to alterations in hydrophobicity of the β -Lg molecule. The antibacterial properties of the amidated β -Lg appear to be dependent on the net positive charge and charge distribution on the molecule.