Inter-relationship between photoreduction and photophosphorylation by photosynthetic bacteria R. rubrum

Abstract

Membrane preparation of the non-sulfur purple bacterium R. rubrum catalyzes several light dependent photochemical reactions which include photoreduction, photophosphorylation and photooxidation. All of these reactions are found to be dependent on the presence of several exgeneous electron donor. By increasing the concentration of exogeneous electron donor, the rate profiles of the bacterial photochemical reactions showed a typical hyperbolic saturation curve. In the presence of antimycin A or HQNO a marked inhibition was observed for the photophosphorylation, but in the case of light dependent oxygen uptake, rather stimulation effect was observed. The changes in the ratios of NAD$^+$/NADH and ATP/ADP + Pi affects all the rates of photochemical reactions. Especially by the addition of phoshpate pool, ADP + Pi the activity of photoreduction was reduced markedly, on the while the presence of excess NAD$^+$ resulted in the reduction of the rate of photophosphorylation. This result along with other inhibition study by several electron transfer inhibitors and uncouplers suggest that two photochemical reaction systems involving ferredoxin-NAD$^+$-oxidoreductase and cytochrome oxidoreductase respectively are coupled at a common reaction center in the bacterial chromatophore and thus in turn, their activities are affected each other by the ratios of NADH/NAD$^+$ and ATP/ADP + Pi. Furthermore, based on the inhibition of these two photochemical reaction by exess of molecular oxygen, it is also suspected that the overall rate of light dependent chemical reactions might be controlled by the activity of photooxidase which is believed to be locallized in the same reaction center.