Rc. Tittsworth et Bj. Hales, DETECTION OF EPR SIGNALS ASSIGNED TO THE 1-EQUIV-OXIDIZED P-CLUSTERS OF THE NITROGENASE MOFE-PROTEIN FROM AZOTOBACTER-VINELANDII, Journal of the American Chemical Society, 115(21), 1993, pp. 9763-9767
Component 1 (the MoFe protein) of conventional nitrogenase from Azotob
acter vinelandii contains two types of metal clusters, called M-center
s and P-clusters. The M-centers, or FeMo-co, are paramagnetic in the a
s-isolated form of the protein, exhibiting an EPR signal typical of a
rhombic S = 3/2 system. The P-clusters are diamagnetic in the as-isola
ted form of the protein but become paramagnetic upon chemical or poten
tiometric oxidation. We have undertaken a controlled oxidative titrati
on of component 1 with thionine solution and observed EPR signals beli
eved to originate from paramagnetic P-clusters. We present EPR spectra
associated with half-integer S = 5/2 and S = 1/2 spin states and prop
ose that these spectra arise from the 1-equiv-oxidized form of the P-c
luster. Spectral analysis and theoretical models are presented which s
upport the argument that P-cluster oxidation occurs 1 equiv at a time
and that the 1-equiv-oxidized P-clusters are stable species in the oxi
dative titration of Av1.