Activation of the H+-ATP synthase in the photosynthetic bacterium Rhodobacter capsulatus

The regulation of the membrane-bound H+-ATPase from the photosynthetic bacterium Rhodobacter capsulatus was investigated. In the presence of uncouplers the rate of ATP hydrolysis was about 40 mM ATP/M bacteriochlorophyll (Bchl)/s. Without uncouplers this rate increased and if, additionally, the chromatophores were illuminated, it was almost doubled. If uncouplers were added shortly after illumination, the rate increased to 300- 350 mM ATP/M Bchl/s. Obviously, energization of the membrane leads to the formation of a metastable, active state of the H+-ATPase. The maximal rate of ATP hydrolysis can be measured only when first all H+-ATPases are activated by Δμ(H+) and when the Δμ(H+) is abolished in order to release its back pressure on the hydrolysis rate. The half-life time of the metastable state in the absence of Δμ(H+) is about 30 s. It is increased by 3 mM P(i) to about 80 s and it is decreased by 1 mM ADP to about 15 s. Quantitatively, the fraction of active H+-ATPases shows a sigmoidal dependence on pH(in) (at constant pH(out)) and the magnitude of Δφ determines the maximal fraction of enzymes which can be activated: ΔpH and Δφ are not equivalent for the activation process.