Structural Characterization of Four Redox States of the P-cluster in Molybdenum Nitrogenase via Electrochemical Control of Crystals.

We report X-ray crystallographic structures of the Azotobacter vinelandii nitrogenase MoFe protein showing the 8Fe-7S electron-transfer P-cluster in four redox states. Using electrochemical poising of protein crystals, together with in crystallo EPR spectroscopic verification of the redox state, we obtain structures showing the P-cluster at PN, P1+, P2+, and P3+ levels. This provides a detailed structural characterization of P-cluster rearrangement between the catalytically relevant PN and P1+ levels and the first experimental confirmation that the S = 7/2 P3+ state is structurally similar to P2+. These studies pave the way for future understanding of the structure-function relationship in nitrogenase catalysis.