Active-site heterogeneity in Limulus hemocyanin as revealed by reaction with peroxides

Previously reported differences in the reactivities toward active-site ligands such as hydrogen peroxide indicate that the active-site geometries of the arthropod and mollusc hemocyanins are significantly different. Results are presented which demonstrate that the purified subunits composing the native hemocyanin of an arthropod show comparable active-site heterogeneity. Neither whole nor stripped samples of Limulus oxyhemocyanin are completely oxidized by reaction with hydrogen peroxide. Moreover, the addition of hydrogen peroxide to deoxy or completely oxidized samples causes partial regeneration to the oxy form. This behavior is most pronounced for one of the five major chromatographic fractions, Zone III, which retains greater than 60% of its copper-oxygen absorbance band when treated with hydrogen peroxide. Zone III is composed of three subunits (IIIA, IIIB, and IIIB'). With excess hydrogen peroxide, the deoxy and oxidized forms of Subunit IIIA can be completely regenerated to a fully functional oxy state, while Subunits IIIB and IIIB' are less than 30% regenerated. Upon long-term storage, Subunit IIIA slowly loses 340 nm absorbance, an aging process that can be reversed by hydrogen peroxide. These features of Subunit IIIA, that set it apart from other subunits of the 48-member ensemble of intact Limulus hemocyanin, are typically exhibited by mollusc hemocyanins rather than arthropod hemocyanins. These findings clearly illustrate that significantly different active-site geometries can occur within a single type of metalloprotein and suggest that comparisons between the various Limulus subunits and Subunit IIIA may prove valuable in unraveling the structural basis for the disparity in the chemical reactivity exhibited by hemocyanins of different species.