Oxidative phosphorylation by ADP + P_i-loaded membrane vesicles of alkaliphilic Bacillus firmus OF4

ATP synthesis in ADP + P_i-loaded membrane vesicles of the facultative alkaliphile Bacillus firmus OF4 at an external pH of 10.5 did not depend upon the presence of cell wall polymers, e.g. as a proton barrier or sequestration device. Upon energization with ascorbate plus phenazine methosulfate, vesicles at pH_out = 7.5 generated an electrochemical proton gradient (Δp) of -160 mV, acid and positive out, whereas at pH_out = 10.5, the Δp was -40 mV, alkaline and positive out. Nonetheless, ATP synthesis was more rapid at the more alkaline pH value, especially in the presence of 200 mM K₂SO₄, which markedly lowered the surface potential. No synthesis was observed upon abolition of the Δp. Respiration-derived transmembrane potentials (Δψ) energized ATP synthesis much better than an equally large diffusion potential. The diffusion potential failed to energize ATP synthesis above pH 9.5. When Δp, all in the form of a Δψ, was titrated downward at either pH 7.8 or 9.5, ATP synthesis by the latter vesicles was much less adversely affected in the Δp range of -150 to -50 mV, supporting the existence of a sparing, non-chemiosmotic energy component at high pH.