Electron Paramagnetic Resonance Analysis of Different Azotobacter vinelandii Nitrogenase MoFe-Protein Conformations Generated during Enzyme Turnover:  Evidence for S = 3/2 Spin States from Reduced MoFe-Protein Intermediates

Rapid-freezing experiments elicited two transient EPR signals, designated 1b and 1c, during Azotobacter vinelandii nitrogenase turnover at 23 °C and pH 7.4. The first of the signals to form, signal 1b, exhibited g values of 4.21 and 3.76. Its formation was at the expense of the starting EPR signal (signal 1a with g values of 4.32, 3.66, and 2.01). The second signal to arise, signal 1c, with a characteristic g value of 4.69, formed very slowly and was always of low intensity. Both signals occurred independently of the substrate being reduced. Increased electron flux through the MoFe protein caused these signals to form more rapidly. Moreover, after a MoFe-protein solution had been pretreated (using conditions of extremely low electron flux) to set up an equimolar mixture of its resting state and one-electron reduced state, these signals appeared even more rapidly when this mixture was exposed to an excess of the Fe protein. We have simulated the kinetics of formation of these EPR features using the published kinetic model for nitrogenase catalysis [Lowe, D. J., and Thorneley, R. N. F. (1984) Biochem. J. 224, 887−909] and propose that they arise from reduced states of the MoFe protein and reflect different conformations of the FeMo cofactor with different protonation states.