SULFITE STIMULATES THE ATP HYDROLYSIS ACTIVITY OF BUT NOT PROTON TRANSLOCATION BY THE ATP SYNTHASE OF RHODOBACTER-CAPSULATUS AND INTERFERESWITH ITS ACTIVATION BY DELTA(MU)OVER-TILDA(H+)

Sulfite stimulates the rate of ATP hydrolysis by the ATP synthase in c hromatophores of Rhodobacter capsulatus. The stimulated activity is in hibited by oligomycin. The activation takes place also in uncoupled ch romatophores. The activation consists in an increase of about 12-15-fo ld of the V-max for the ATP hydrolysis reaction, while the K-m for MgA TP is unaffected at 0.16 +/- 0.03 mM. The dependence of V-max on the s ulfite concentration follows a hyperbolic pattern with half maximum ef fect at IZ mM. Sulfite affects the ability) of the enzyme in transloca ting protons. Concomitant measurements of the rate of ATP hydrolysis a nd of ATP-induced protonic flows demonstrate that at sulfite concentra tions of greater than 10 mM the hydrolytic reaction becomes progressiv ely uncoupled from the process of proton translocation. This is accomp anied by an inhibition of ATP synthesis, either driven by light ol by artificially induced ionic gradients. ATP synthesis is totally inhibit ed at concentrations of at least 80 mM. Sulfite interferes with the me chanism of activation by Delta<(mu)over tilde>(W). Low concentrations of this anion (less than or equal to 2 mM) prevent the activation by D elta<(mu)over tilde>(H). At higher concentrations a marked stimulation of the activity prevails, regardless of the occurrence of a Delta<(mu )over tilde>(H), across the membrane. Phosphate at millimolar concentr ations can reverse the inhibition by sulfite. These experimental resul ts can be simulated by a model assuming multiple and competitive equil ibria for phosphate or sulfite binding with two binding sites for the two ligands (for sulfite K-1S = 0.26 and K-2S = 37 mM, and for phospha te K-1P = 0.06 and K-2P = 4.22 mM), and in which the state bound only to one sulfite molecule is totally inactive in hydrolysis. The competi tion between phosphate and sulfite is consistent with the molecular st ructures of the two ligands and of the enzyme.