MgATP-independent hydrogen evolution catalysed by nitrogenase: An explanation for the missing electron(s) in the MgADP-AIF4 transition-state complex

Abstract

When the MoFe (Kp1) and Fe (Kp2) component proteins of Klebsiella pneumoniae nitrogenase are incubated with MgADP and AIF4- in the presence of dithionite as a reducing agent, a stable putative transition-state complex is produced. Surprisingly, the EPR signal associated with reduced Kp2 is not detectable, but Kp1 retains the S = 3/2 EPR signal arising from the dithionite reduced state of the MoFe cofactor centre of the protein. This is consistent with the [Fe4S4] centre of the Fe protein in the complex being oxidized, and similar observations have been made with the complex of Azotobacter vinelandii. No satisfactory explanation for the fate of the electrons lost by Kp2 has been forthcoming. However, we report here that during the preparation of the MgADP-AIF4 K. pneumoniae complex under argon, H2 was evolved in amounts corresponding to one half of the FeMoco content of the Kp1 (FeMoco is the likely catalytic site of nitrogenase with a composition Mo:Fe7:S9: homocitrate). This is surprising, since activity is observed during incubation in the absence of MgATP, normally regarded as being essential for nitrogenase function, and in the presence of MgADP, a strong competitive inhibitor of nitrogenase. The formation of H2 by nitrogenase in the absence of AIF4- was also observed in reaction mixtures containing MgADP but not MgATP. The reaction showed saturation kinetics when Kp1 was titrated with increasing amounts of Kp2 and, at saturation, the amount of H2 formed was stoichiometric with the FeMoco content of Kp1. The dependence of the rare of formation of H2 on [MgADP] was inconsistent with the activity arising from MgATP contamination. We conclude that MgATP is not obligatory for H+ reduction by nitrogenase since MgADP supports a very low rate of hydrogen evolution.