A transmembrane electrical potential generated by respiration is not equivalent to a diffusion potential of the same magnitude for ATP synthesis by Bacillus firmus RAB.

ATP synthesis by starved whole cells of alkalophilic Bacillus firmus RAB was energized by addition of DL-malate or by imposition of a valinomycin-mediated K+ diffusion potential. At pH 9.0, the transmembrane electrical potentials produced by these two means were similar in magnitude, at close to -170 mV. While N,N'-dicyclohexylcarbodiimide-sensitive ATP synthesis occurred upon the addition of DL-malate, no ATP was synthesized in response to a diffusion potential. In contrast, Na+-dependent accumulation of alpha-aminoisobutyric acid was energized equally well by DL-malate or a diffusion potential at pH 9.0. Even at pH 7.0, DL-malate was more efficacious than a diffusion potential in energizing ATP synthesis as assessed by determining the phosphorylation potentials generated at transmembrane electrical potential values of different magnitudes. Both modes of energization did, however, result in N,N'-dicyclohexylcarbodiimide-sensitive ATP synthesis at pH 7.0. The results of these studies are consistent with a model of energy coupling in which one pathway of protons between the respiratory proton pumps and the H+-translocating ATPase is direct or localized. Whereas an artificially imposed electrochemical gradient of protons can energize ATP synthesis under certain experimental conditions, in the proton-poor milieu that is optimal for growth of alkalophilic bacilli, the direct proton pathway may be crucial.