On the novel catalytically-independent antimicrobial function of hen egg-white lysozyme: a conformation-dependent activity.

Dependency of the antimicrobial activity on the conformation of lysozme was examined by the means of gradual thermal inactivation at neutral pH and different temperatures. We found that heating of lysozyme at increasing temperatures for 20 min in pH 6.0 results in progressive loss of enzyme activity, while greatly promotes its antimicrobial action to the Gram-negative bacteria without a detrimental effect on the inherent bactericidal activity to Gram-positive ones, suggesting action independent of catalytic function. The most potent bactericidal conformation of lysozyme to either Gram-negative or -positie bacteria was that retaining approximately 50% of the native enzyme activity (HL80/6). HL80/6 showed several fold increase in surface hydrophobicity, with exposed two thiol groups, and 17% deamidation. Spectrophotometric analysis of HL80/6 revealed slight changes in its secondary structures, but considerable global conformational changes as a result of the formation of beta conformation, via cyclic imide, at the three aspartylglycyl sequences of lysozyme molecule. Direct damage to the bacertial membranes by HL80/6, was demonstrated by using ELISA and liposomal membrane model. Furthermore, the antimicrobial activity of HL80/6 was inhibited by the divalent cations Ca2+ and Mg2+ suggesting that HL80/6 interacts at a divalent cation binding site on the bacterial membrane and subsequently permeabilize it. The results introduce an interesting structure-antimicrobial relationship that the antimicrobial action of lysozyme is independent of its catalytic function. In addition, it is worth emphasizing that the naturally-occurring conformational transition of lysozyme at physiological temperatures can be a biologically relevant event to switch its antimicrobial specificity to include the food-borne pathogens and heralding fascinating opportunities for application in formulated food systems.