Dissecting Electronic-Structural Transitions in the Nitrogenase MoFeProtein P-Cluster during Reduction br

The [8Fe-7S] P-cluster of nitrogenase MoFe protein mediates electron transfer from nitrogenase Fe protein duringthe catalytic production of ammonia. The P-cluster transitions between three oxidation states, PN,P+,P2+of which PN <-> P+is criticalto electron exchange in the nitrogenase complex during turnover. To dissect the steps in formation of P+during electron transfer,photochemical reduction of MoFe protein at 231-263 K was used to trap formation of P+intermediates for analysis by EPR. Incomplexes with CdS nanocrystals, illumination of MoFe protein led to reduction of the P-cluster P2+that was coincident withformation of three distinct EPR signals:S= 1/2 axial and rhombic signals, and a high-spinS= 7/2 signal. Under dark annealing theaxial and high-spin signal intensities declined, which coincided with an increase in the rhombic signal intensity. Afit of the time-dependent changes of the axial and high-spin signals to a reaction model demonstrates they are intermediates in the formation of theP-cluster P+resting state and defines how spin-state transitions are coupled to changes in P-cluster oxidation state in MoFe proteinduring electron transfer.