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 delta mu H+ and when the delta mu 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 delta mu H+ is about 30 s. It is increased by 3 mM Pi 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 pHin (at constant pHout) and the magnitude of delta psi determines the maximal fraction of enzymes which can be activated: delta pH and delta psi are not equivalent for the activation process.