Copper modulates the heat-induced sulfhydryl/disulfide interchange reactions of beta-Lactoglobulin

Abstract : This study describes the effect of copper on the heat-denaturation/aggregation of beta-Lactoglobulin AB at neutral pH. The kinetics of disappearance of native beta-Lactoglobulin under different ionic strength and Cu(2+)/beta-Lactoglobulin molar ratio conditions were followed and the type of interactions (covalent or non-covalent) shared between non-native structures during the heating process were examined. On heating, the rate of disappearance of native beta-Lactoglobulin was accelerated by increasing the Cu(2+)/beta-Lactoglobulin molar ratio. Copper induces oxidation of the free sulfhydryl group of beta-Lactoglobulin resulting mainly in the formation of covalent dimers, which were further associated into large non-covalent aggregates under high ionic strength conditions. Characterisation of the beta-Lactoglobulin dimers reveals the existence of three different molecular species arising randomly (dimers A-A, A-B and B-B), in which tertiary structure was completely lost. The quantity of added copper constitutes a powerful way to control the heat-denaturation/aggregation process of beta-Lactoglobulin in particular regarding the relative proportion of covalent and non-covalent interactions into formed aggregates.