Energy-linked Reactions in Photosynthetic Bacteria

Chromatophores of Rhodospirillum rubrum catalyze a Pi-PPi exchange reaction in the dark. This reaction was inhibited by uncouplers of phosphorylation and ADP, and was stimulated by oligomycin. An energized state is therefore participating in this reaction. Methylene diphosphonate and fluoride which are inhibitors of the membrane-bound inorganic pyrophosphatase also inhibited and thus the reaction appears to be catalyzed by the pyrophosphatase. The Km for Pi was high (48 mm). For comparison, the PPi-Pi exchange reaction catalyzed by yeast inorganic pyrophosphatase was studied. Very high Pi and Mg2+ concentrations were required and, as expected, uncouplers had no effect on the reaction with the yeast enzyme. Under similar conditions the ratio of the exchange reaction to the hydrolytic reaction was considerably greater in chromatophores than with the yeast enzyme and this probably reflects the contribution of the energized state. The mechanism of the exchange in chromatophores is postulated to be due to the dynamic reversal of the energylinked hydrolytic reaction [see PDF for equation] where ∼x represents the energized state in general.