Spectroscopic evidence for changes in the redox state of the nitrogenase P-cluster during turnover

Biological nitrogen fixation catalyzed by nitrogenase requires the participation of two component proteins called the Fe protein and the MoFe protein. Each alphabeta catalytic unit of the MoFe protein contains an [8Fe-7S] cluster and a [7Fe-9S-Mo-homocitrate] cluster, respectively designated the P-cluster and FeMo-cofactor. FeMo-cofactor is known to provide the site of substrate reduction whereas the P-cluster has been suggested to function in nitrogenase catalysis by providing an intermediate electron-transfer site. In the present work, evidence is presented for redox changes of the P-cluster during the nitrogenase catalytic cycle from examination of an altered MoFe protein that has the beta-subunit serine-188 residue substituted by cysteine. This residue was targeted for substitution because it provides a reversible redox-dependent ligand to one of the P-cluster Fe atoms. The altered beta-188(Cys) MoFe protein was found to reduce protons, acetylene, and nitrogen at rates approximately 30% of that supported by the wild-type MoFe protein. In the dithionite-reduced state, the beta-188(Cys) MoFe protein exhibited unusual electron paramagnetic resonance (EPR) signals arising from a mixed spin state system (S = 5/2, 1/2) that integrated to 0.6 spin/alphabeta-unit. These EPR signals were assigned to the P-cluster because they were also present in an apo-form of the beta-188(Cys) MoFe protein that does not contain FeMo-cofactor. Mediated voltammetry was used to show that the intensity of the EPR signals was maximal near -475 mV at pH 8.0 and that the P-cluster could be reversibly oxidized or reduced with concomitant loss in intensity of the EPR signals. A midpoint potential (Em) of -390 mV was approximated for the oxidized/resting state couple at pH 8.0, which was observed to be pH dependent. Finally, the EPR signals exhibited by the beta-188(Cys) MoFe protein greatly diminished in intensity under nitrogenase turnover conditions and reappeared to the original intensity when the MoFe protein returned to the resting state.