Nucleotide-assisted [Fe4S4] redox state interconversions of the Azotobacter vinelandii Fe protein and their relevance to nitrogenase catalysis.

The [Fe4S4] cluster of the nitrogenase Fe protein from Azotobacter vinelandii can exist in three redox states: oxidized Fe4S4, dithionite reduced Fe4S4, and two forms of the all ferrous Fe4S4, S = 4 and 0. Operation of the Fe4S4/ Fe4S4 redox couple transfers one electron to the MoFe protein during catalysis with hydrolysis of two MgATPs. In contrast, the Fe4S4/Fe4S4 couple transfers two electrons per binding event, accompanied by hydrolysis of only two MgATPs. Both reactions occur at nearly identical rates even though the number of electrons transferred differs by a factor of 2. MgATP and MgADP facilitate interconversion of the three redox states: 2Fe4S4 + 4 MgATP = Fe4S4(MgATP)2 + Fe4S4(MgATP)2, as demonstrated by the MgATP reaction. This reaction was investigated as a possible precursor reaction to provide two electrons in the form of Fe4S4(MgATP)2 for delivery to the MoFe protein to then conduct a two-electron substrate reduction. However, experiments showed that this disproportionation reaction, which readily occurs, was not viable during nitrogenase catalysis utilizing the Fe4S4 cluster state. The known cooperative behavior of the Fe protein in the Fe4S4 state taken together with a measured turnover potential of -460 mV with an n = 2 value, suggest a gating process on the MoFe protein involving a two electron step.