Studies on the mechanism of phosphorylation and transport of beta-galactosides by the lactose phosphotransferase system of Staphylococcus aureus. Kinetic investigations using tosyl galactosides as reversible dead-end inhibitors

Abstract

Tosyl galactosides, previously shown to be potent reversible dead-end inhibitors of the membrane-bound Enzyme IIlac of the lactose phosphotransferase system of Staphylococcus aureus, were used for an investigation of the kinetic mechanism of the sugar phosphorylation/transport reaction catalyzed by this enzyme: phospho-Factor IIIlac&sugar Enzyme IIlac lead to Factor IIIlac&sugar phosphate. Inhibition of Enzyme IIlac was studied in three different systems. Washed membranes, and washed membranes in the presence of 0.1% Triton X-100 were used for phosphorylation experiments, and whole cells were used for transport studies. When washed membranes were used to supply Enzyme IIlac, inhibition of phosphorylation by tosyl galactoside was linear non-competitive against both the sugar and phospho-Factor IIIlac substrates, with an apparent Ki of about 0.5 mM. This Ki decreased with increasing Factor IIIlac concentration. In the presence of 0.1% Triton X-100, the phosphorylation reaction was stimulated; under these conditions the inhibition became strictly competitive against sugar, and completely uncompetitive against phospho-Factor IIIlac. Apparently washed membranes can catalyze phosphorylation both via a reaction sequence in which sugar binds first and via one in which phospho-Factor IIIlac binds first, but in the presence of 0.1% Triton the reaction does not occur by the former sequence. The inability of bound phospho-Factor IIIlac to hinder the binding of tosyl galactosides suggests that the initial binding sites of the two substrates of Enzyme IIlac are separated by at least the distance of the tosyl moiety. Radioactive methyl 6-O-(p-toluenesulfonyl) beta-galactoside was not converted into a phosphorylated product in the reaction mixtures, i.e. it is a true dead-end inhibitor. Inhibition of beta-galactoside transport into whole cells by tosyl galactosides was competitive, with an apparent Ki of 5-10 mM, an order of magnitude higher than the Ki for inhibition of phosphorylation by membrane preparations. This result suggest that a significant level of unphosphorylated phospho-Factor IIIlac is present inside the cells, or that cellular levels of this compound are considerably lower than those used for in vitro sugar phosphorylation assays. Radioactive tosyl galactoside inhibitor was not transported into whole cells.