REDOX PROPERTIES AND EPR SPECTROSCOPY OF THE P-CLUSTERS OF AZOTOBACTER-VINELANDII MOFE PROTEIN

In Azotobacter vinelandii MoFe protein the oxidation of the P clusters to the S = 7/2 state is associated with a redox reaction with E(m,7.5) = +90 +/- 1 0 mV (vs the normal hydrogen electrode), n = 1. A concomitant redox process is observed for a rhombic S = 1/2 EPR signal with g = 1.97, 1.88 and 1.68. This indicates that both S = 1/2 and S = 7/2 signals are associated with oxidized P clusters occurring as a physical mixture of spin states. The maximal intensity of the S = 1/2 and S = 7/2 signals in the mediated equilibrium redox titration is similar if not identical to that of solid-thionine-treated samples. Summation of the spin concentration of the S = 1/2 spin state (0.25 +/- 0.03 spin/alpha2beta2) and the S = 7/2 spin state (1. 3 +/- 0.2 spin/alpha2beta2) confirms that the MoFe protein has absolutely no more than two P clusters. In spectra of enzyme fixed at potentials around -100 mV a very low-intensity g = 12 EPR signal was discovered. In parallel-mode EPR the signal sharpened and increased > 1 0-fold in intensity which allowed us to assign the g = 12 signal to a non-Kramers system (presumably S = 3). In contrast with the non-Kramers EPR signals of various metalloproteins and inorganic compounds, the sharp absorption-shaped g = 12 signal is not significantly broadened into zero field, implying that the zero field splitting of the non-Kramers doublet is smaller than the X-band microwave quantum. The temperature dependence of this g = 12 EPR signal indicates that it is from an excited state within the integer spin multiplet. A bell-shaped titration curve with E(m,7.5) = -307 +/- 30 mV and +81 +/- 30 mV midpoint potentials is found for the g = 12 EPR signal. We propose that this signal represents an intermediate redox state of the P clusters between the diamagnetic, dithionite-reduced and the fully oxidized S = 7/2 and S = 1/2 state. Redox transitions of two electrons (-307 +/- 30 mV) and one electron (+90 +/- 10 mV) link the sequence S = 0 half arrow right over half arrow left S = 3 half arrow right over half arrow left (S = 7/2 and S = 1/2). We propose to name the latter paramagnetic oxidation states of the P clusters in nitrogenase P(OX1) and P(OX2), and to retain P(N) for the diamagnetic native redox state. The magnetic circular dichroism and Mossbauer data on thionine-oxidized MoFe protein have to be re-evaluated bearing in mind that the oxidized P clusters can exist in two redox-states. Finally, an account is given of the EPR spectroscopic properties of S = 9/2 and other systems obtained upon superoxidation of the MoFe protein.