Dissecting Electronic-Structural Transitions in the Nitrogenase MoFe Protein P-Cluster during Reduction
Bryant Chica, Jesse L. Ruzicka, Lauren M. Pellows, Hayden Kallas, Effie C. Kisgeropoulos, Gregory E. Vansuch, David W. Mulder, Katherine A. Brown, Draženka Svedružić, John W. Peters, Gordana Duković, Lance C. Seefeldt, Paul W. King
Abstract
The [8Fe-7S] P-cluster of nitrogenase MoFe protein mediates electron transfer from nitrogenase Fe protein during the catalytic production of ammonia. The P-cluster transitions between three oxidation states, PN, P+, P2+ of which PN↔P+ is critical to 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. In complexes with CdS nanocrystals, illumination of MoFe protein led to reduction of the P-cluster P2+ that was coincident with formation of three distinct EPR signals: S = 1/2 axial and rhombic signals, and a high-spin S = 7/2 signal. Under dark annealing the axial and high-spin signal intensities declined, which coincided with an increase in the rhombic signal intensity. A fit of the time-dependent changes of the axial and high-spin signals to a reaction model demonstrates they are intermediates in the formation of the P-cluster P+ resting state and defines how spin-state transitions are coupled to changes in P-cluster oxidation state in MoFe protein during electron transfer.