Mössbauer, EPR, and magnetization studies of the Azotobacter vinelandii Fe protein. Evidence for a [4Fe-4S]1+ cluster with spin S = 3/2.

We have studied the Fe protein (Av2) of the Azotobacter vinelandii nitrogenase system with Mössbauer and EPR spectroscopies and magnetic susceptometry. In the oxidized state the protein exhibits Mössbauer spectra typical of diamagnetic [4Fe-4S]2+ clusters. Addition of Mg.ATP or Mg.ADP causes a pronounced decline in the quadrupole splitting of the Mössbauer spectra of the oxidized protein. Our studies show that reduced Av2 in the native state is heterogeneous. Approximately half of the molecules contain a [4Fe-4S]1+ cluster with electronic spin S = 1/2 and half contain a [4Fe-4S]1+ cluster with spin S = 3/2. The former yields the characteristic g = 1.94 EPR signal whereas the latter exhibits signals around g = 5. The magnetization of reduced Av2 is dominated by the spin S = 3/2 form of its [4Fe-4S]1+ clusters. These results explain a long standing puzzle, namely why the integrated spin intensity of the g = 1.94 EPR signal is substantially less than 1 spin/4 Fe atoms. In 50% ethylene glycol, 90% of the clusters are in the spin S = 1/2 form whereas, in 0.4 M urea, 85% are in the S = 3/2 form. In 0.4 M urea, the EPR spectrum of reduced Av2 exhibits well defined resonances at g = 5.8 and 5.15, which we assign to the S = 3/2 system. The EPR and Mössbauer studies yield a zero-field splitting of 2D approximately equal to -5 cm-1 for this S = 3/2 state.