Stoichiometry of proton movements coupled to ATP synthesis driven by a pH gradient in Streptococcus lactis

An electrochemical potential difference for H+ was established across the plasma membrane of the anaerobe Streptococcus lactis by addition of sulfuric acid to cells suspended in potassium phosphate at pH 8 along with valinomycin or permeant anions. Subsequent acidification of the cell was measured by the distribution of salicyclic acid. A comparison between cells treated or untreated with the inhibitor N,N'-dicyclohexylcarbodiimide was used to reveal that portion of net proton entry attributable to a direct coupling between H+ inflow and synthesis of ATP catalyzed by the reversible proton-translocating ATPase of this microorganism. When the imposed electrochemical proton gradient was below 180-190 mV, proton entry was at the rate expected of passive flux, for both control cells and cells treated with the ATPase inhibitor, However, at higher driving force acidification of control cells was markedly accelerated, coincident with ATP synthesis, while acidification of cells treated with the inhibitor continued at the rate characteristic of passive inflow. This observed threshold (180-190 mV) was identified as the reversal potential for this H+ "pump". Parallel measurements showed that the free energy of hydrolysis for ATP in these washed cells was 8.4 kcal/mole (370mV). The comparison between the reversal (threshold) potential and the free energy of hydrolysis for ATP indicates a stoichiometry of 2 H+/ATP for the coupling of proton movements to ATP formation in bacteria.