On the functional unit of energy coupling in photophosphorylation by bacterial chromatophores

(1) Putative relationships between the rate of photophosphorylation, the proton-motive force and the concentration of an uncoupling molecule are considered within the framework of the delocalised chemiosmotic coupling hypothesis. The addition of a partially inhibitory titre of a specific, tight-binding H⁺-ATP synthase inhibitor is not expected, within the framework of a delocalised coupling model, to alter the form of this relationship. (2) Photophosphorylation in chromatophores from Rhodopseudomonas capsulata is potently uncoupled by the protonophore SF6847. Yet the uncoupling potency of this compound is actually further increased when the rate of phosphorylation in the absence of protonophore is decreased by the addition of the energy-transfer inhibitor venturicidin, in contrast to the expectations of a delocalised energy-coupling model. (3) Similarly, valinomycin (in the presence of nigericin) uncouples more potently when the number of active H⁺-ATP synthases is decreased by the addition of the energy -transfer inhibitors N,N′-dicyclohexyl-carbodiimide or venturicidin. (4) The pore-forming ionophore gramicidin D also uncouples photophosphorylation more potently when the number of active H⁺-ATP synthases is reduced. (5) These results are discussed in relation to the idea that the functional unit of electrical events and photophosphorylation either is, or is not, the intact membrane vesicle. (6) It is concluded that the unit of energy coupling in bacterial chromatophores is much smaller than the entire coupling membrane vesicle, and that previous analyses of this point, based on titrations with ionophores alone, may need to be re-examined.