The path of electron transfer to nitrogenase in a phototrophic alpha-proteobacterium

The phototrophic alpha-proteobacterium, Rhodopseudomonas palustris, is a model for studies of regulatory and physiological parameters that control the activity of nitrogenase. This enzyme produces the energy-rich compound H₂, in addition to converting N₂ gas to NH₃. Nitrogenase is an ATP-requiring enzyme that uses large amounts of reducing power, but the electron transfer pathway to nitrogenase in R. palustris was incompletely known. Here, we show that the ferredoxin, Fer1, is the primary but not sole electron carrier protein encoded by R. palustris that serves as an electron donor to nitrogenase. A flavodoxin, FldA, is also an important electron donor, especially under iron limitation. We present a model where the electron bifurcating complex, FixABCX, can reduce both ferredoxin and flavodoxin to transfer electrons to nitrogenase, and we present bioinformatic evidence that FixABCX and Fer1 form a conserved electron transfer pathway to nitrogenase in nitrogen-fixing proteobacteria. These results may be useful in the design of strategies to reroute electrons generated during metabolism of organic compounds to nitrogenase to achieve maximal activity.