Nucleotide-assisted [Fe_4S_4] redox state interconversions of the Azotobacter vinelandii Fe protein and their relevance to nitrogenase catalysis

摘要

The [Fe_4S_4] cluster of the nitrogenase Fe protein from Azotobacter vinelandii can exist in three redox states: oxidized [Fe_4S_4]~(2+), dithionite reduced [Fe_4S_4]~(1+), and two forms of the all ferrous [Fe_4S_4]~0, S = 4 and 0. Operation of the [Fe_4S_4]~(2+)/ [Fe_4S_4]~(1+) redox couple transfers one electron to the MoFe protein during catalysis with hydrolysis of two MgATPs. In contrast, the [Fe_4S_4]~(2+)/[Fe_4S_4]~0 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: 2[Fe_4S_4]~(1+) + 4 MgATP = [Fe_4S_4]~(2+)(MgATP) _2 + [Fe_4S_4]~0(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 [Fe_4S_4]~0(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 [Fe_4S_4]~(1+) cluster state. The known cooperative behavior of the Fe protein in the [Fe_4S_4]~(1+) 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.