Nitrogenase reduction by electron carriers: Influence of redox potential on activity and the [formula omitted] ratio

Abstract

The variation with redox potential of nitrogenase activity and the ratio of ATP hydrolyzed per two electrons transferred were measured using two systems: the dithionite/bisulfite couple at pH 7.4; and H 2, hydrogenase, and ferredoxin at pH 8.5. In both cases, the variation in nitrogenase activity with redox potential followed a theoretical Nernst plot for a two-electron process with an apparent midpoint potential of about −470 mV. The ratio ATP 2e − was about 4 under highly reducing conditions. However, above the apparent midpoint potential, the ratio ATP 2e − increased drastically, reaching values as high as 20. These data imply that a low redox potential must be maintained for efficient nitrogen fixation in vitro and in vivo.