Electron allocation to alternative substrates of Azotobacter nitrogenase is controlled by the electron flux through dinitrogenase

Abstract

The electron flux through dinitrogenase (MoFe protein, protein containing Mo and Fe) from Azotobacter vinelandii controls the relative effectiveness of alternative substrates as electron acceptors in the nitrogenase system. The electron flux through dinitrogenase reductase (Fe protein) or the concentration of MgATP do not directly control electron allocation but rather control it via their influence on the electron flux through dinitrogenase. Kinetic properties of substrate reduction were studied as a function of the electron flux through dinitrogenase. N2 was most effective at high electron fluxes, whereas H+ was the most effective acceptor at very low rates of electron flow through dinitrogenase. The Km for acetylene was dependent on the electron flux through dinitrogenase, whereas the Km for N2 was much less sensitive to this electron flux. The lag period before the onset of acetylene reduction was proportional to the turnover time of dinitrogenase, and was approx. 12 times greater than the dinitrogenase turnover time. pH has effects on the electron allocation to substrates beyond that expected from the effect of pH on the electron flux; thus, pH may alter the relative ability of the nitrogenase enzyme system to reduce alternative substrates.