Enhanced efficiency of ATP hydrolysis during nitrogenase catalysis utilizing reductants that form the all-ferrous redox state of the Fe protein

The amount of MgATP hydrolyzed per pair of electrons transferred (ATP/2e) d uring nitrogenase catalysis (1.0 atm N-2, 30 degrees C) using titanium(IIl) citrate (Ti(III)) as reductant was measured and compared to the same react ion using dithionite (DT). ATP/2e values near 2.0 for Ti(III) and 5.0 for D T indicate that nitrogenase has a much lower ATP requirement using Ti(III) as reductant. Using reduced Azotobacter vinelandii flavoprotein (AvFlpH(2)) , a possible in vivo nitrogenase reductant, ATP/2e values near 2.0 were als o observed. When the reaction was conducted using Ti(III) under N-2, 5% CO in N-2, Ar, 5% CO in Ar, or acetylene, ATP/2e values near 2.0 were also obs erved. With Ti(III) as reductant, ATP/2e values near 2.0 were measured as a function of temperature, Fe:MoFe protein ratio, and MoFe:Fe protein ratio, in contrast to measured values of 4.0-25 when DT is used under the same co nditions. Both Ti(III) and AvFlpH2 are capable of forming the [Fe4S4](0) cl uster state of the Fe protein whereas DT is not, suggesting that ATP/2e val ues near 2.0 arise from operation of the [Fe4S4](2+)/[Fe4S4](0) redox coupl e with hydrolysis of only 2 ATPs per pair of electrons transferred. Additio nal experiments showed that ATP/2e values near 2.0 correlated with slower r ates of product formation and that faster rates of product formation produc ed ATP/2e values near 5.0. ATP/2e values of 5.0 are consistent with the ope ration of the [Fe4S4](2+)/[Fe4S4](1+) redox couple while ATP/2e values of 2 .0 could arise from operation of the [Fe4S4](2+)/[Fe4S4](0) redox couple. T hese results suggest that two distinct Fe protein redox couples may be func tional during nitrogenase catalysis and that the efficiency of ATP utilizat ion depends on which of these redox couples is dominant.