Sulfite stimulates the rate of ATP hydrolysis by the ATP synthase in chromatophores of Rhodobacter capsulatus. The stimulated activity is inhibited by oligomycin. The activation takes place also in uncoupled chromatophores. The activation consists in an increase of about 12–15-fold of the Vmax for the ATP hydrolysis reaction, while the Km for MgATP is unaffected at 0.16±0.03 mM. The dependence of Vmax on the sulfite concentration follows a hyperbolic pattern with half maximum effect at 12 mM.
Sulfite affects the ability of the enzyme in translocating protons. Concomitant measurements of the rate of ATP hydrolysis and of ATP-induced protonic flows demonstrate that at sulfite concentrations of greater than 10 mM the hydrolytic reaction becomes progressively uncoupled from the process of proton translocation. This is accompanied by an inhibition of ATP synthesis, either driven by light or by artificially induced ionic gradients. ATP synthesis is totally inhibited at concentrations of at least 80 mM.
Sulfite interferes with the mechanism of activation by ΔH+. Low concentrations of this anion (≤ 2 mM) prevent the activation by ΔH+. At higher concentrations a marked stimulation of the activity prevails, regardless of the occurrence of a ΔH+ across the membrane. Phosphate at millimolar concentrations can reverse the inhibition by sulfite.
These experimental results can be simulated by a model assuming multiple and competitive equilibria for phosphate or sulfite binding with two binding sites for the two ligands (for sulfite K1S= 0.26 and K2S= 37 mM, and for phosphate K1P= 0.06 and K2S= 4.22 mM), and in which the state bound only to one sulfite molecule is totally inactive in hydrolysis. The competition between phosphate and sulfite is consistent with the molecular structures of the two ligands and of the enzyme.