Synthesis of pyrophosphate coupled to the reverse energy‐linked transhydrogenase reaction in <i>Rhodospirillum rubrum</i> chromatophores

Abstract

Chromatophores from the photosynthetic bacterium Rhodospirillum rubrum contain a membrane-bound transhydrogenase catalyzing the transfer of a hydride ion between NADH and NADP⁺. The reverse reaction, i.e. reduction of NAD⁺ by NADPH, can furnish sufficient energy (ΔμH⁺) to drive the phosphorylation of inorganic orthophosphate (P(i)) to pyrophosphate (PP(i)). The rate of PP(i), synthesis is 50 nmol PP(i) formed/min per μmol Bchl which is 5% of the rate of light-induced PP(i), synthesis. PP(i), synthesis is inhibited by both the H+-PPase inhibitor fluoride and the specific transhydrogenase inhibitor palmitoyl-CoA. The effects of both DCCD and uncouplers on the system provide additional evidence that the ΔμH⁺ generated by the reverse transhydrogenase reaction drives PP(i), synthesis. The rate of PP(i), synthesis can be partially inhibited by the addition of NADP⁺, a substrate of the forward energy-consuming reaction. The ΔμH⁺ generated can also be used to drive ATP synthesis by the H⁺-ATPase, but at a lower rate than the PP(i), synthesis.