EXAFS studies on the PN and POX states of the P-clusters in nitrogenase

Abstract

The MoFe protein of nitrogenase is an α2β2 tetramer that contains two each of two different types of metal centers, the FeMo-cofactor and the P-clusters. The function of the P-clusters is believed to be to accept electrons from the Fe protein of nitrogenase and to donate them to the FeMo-cofactor. We have studied the P-clusters of Azotobacter vinelandii nitrogenase in both the PN and POX states utilizing Fe K-edge X-ray absorption spectroscopy. Since the MoFe holoprotein contains the seven iron FeMo-cofactor centers in addition to P-clusters, we have utilized a FeMo-cofactor-deficient MoFe protein synthesized by the Δnif H strain DJ54. That MoFe protein is an α2β2 tetramer that contains P-clusters by the criteria of metal analysis, CD spectroscopy, cluster extrusion, and electrochemical reduction of the POX state. Several important results have emerged from our XAS studies. The first shell Fe-S coordination shows the same average Fe-S distance (2.26 A) in both states. The second coordination shells could only be well fit using two different Fe-Fe contributions. In both states, short Fe-Fe components with distances of 2.57 A and 2.42 A for the PN and POX states, respectively, were required to complement longer 2.75 A and 2.70 A distances. Understanding of the P-cluster structure is essential if we are to make advances in understanding the role of the P-clusters and their participation in electron transfer through the nitrogenase system.