Synthesis of pyrophosphate coupled to the reverse energy-linked transhydrogenase reaction in Rhodospirillum rubrum 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 (Δ \\ ̃ gmH +) 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 Δ \\ ̃ gmH + 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 Δ \\ ̃ gmH + generated can also be used to drive ATP synthesis by the H +-ATPase, but at a lower rate than the pp i synthesis.