Electron transport to nitrogenase in Rhodospirillum rubrum

Abstract: Biological nitrogen fixation is a key step in the global nitrogen cycle. In this process, dinitrogen in the air is converted to biologically accessible ammonia, which is further assimilated in to the biosphere. Nitrogenase, the enzyme system responsible for dinitrogen reduction, is only found in prokaryotic organisms and biological nitrogen fixation is an energy-demanding process, requiring both ATP and low potential reducing equivalents. In the free-living purple non-sulfur anoxygenic phototroph Rhodospirillum rubrum, efficient electron transfer to nitrogenase is dependent on active electron transport in the chromatophore membrane.I have shown that reducing equivalents for nitrogen fixation is generated through the action of the proteins encoded by the fixABCX genes. The membrane associated protein complex encoded by these genes reduces a soluble ferredoxin, which in turn acts as the direct electron donor to nitrogenase in this organism. The heterodimeric flavoprotein FixAB has NADH dehydrogenase activity indicating that the reducing equivalents for nitrogen fixation are derived from the general metabolism of the cell. The membrane associated FixC protein is believed to drive the energetically unfavorable reduction of ferredoxin N using energy derived from the electron transfer processes in the chromatophore membrane in some unknown manner. The membrane associated protein complex encoded by fixABCX most likely constitutes the unknown link between photosynthesis and nitrogen fixation in R. rubrum.