The P Cluster of the Azotobacter vinelandii Nitrogenase Complex: Effects of Substitution at the Cluster-bridging Residue, a-Cys88
The major focus of the research in our laboratory is the investigation of the role of the nitrogenase component, theMoFe protein, in the catalytic mechanism of biological nitrogen fixation. This dissertation research centers on the role(s) of the P cluster, one of the two unique FeS clusters of the MoFe protein, in the electron transfer mechanism of nitrogenase.
Prior to the solution of the x-ray crystal structure of the Azotobacter vinelandii MoFe protein, it was had been determined which of the highly conserved cysteinyl residues of this protein were likely P cluster ligands. After elucidation of the crystal structure, it became evident that cysteine-88 of the a-subunit (a-Cys88) and cysteine-95 of the b-subunit (b-Cys95) could play important roles in maintaining and/or perturbing the conformation of the double-cubane structure by virtue of their bridging positions. It was found that three out of ten bacterial strains with substitutions at the a-Cys88 ligand retained significant catalytic activity. We investigated the effects of these substitutions on the overall structural, kinetic and spectroscopic parameters. The results of prior studies suggested a role for the P clusters in accepting, storing, and then delivering the electrons received from the Fe protein. Therefore, we asked whether a-Cys88 substitution resulted in perturbed functioning of the overall catalytic mechanism and more importantly what these differences reveal about the normal mechanism of nitrogenase.
Alterations in the bridging cysteine a-88 affected the rate of substrate reduction which can be explained in part by production of cluster-less MoFe protein. Electron flux, NaCl concentration, and reductant concentration titration assays revealed a significant uncoupling of the ATP hydrolysis rate from the substrate reduction rate in the a-88Cys-to-Gly MoFe protein. Rapid kinetic analysis revealed decreased electron tranfer rates in all three of the a-Cys88 altered MoFe proteins when compared to the wild-type MoFe protein. The intermolecular electron transfer rate was lowered in the a-88Cys-to-Asp MoFe protein, while the intramolecular electron transfer rate was limiting in the a-88Cys-to-Gly and a-88Cys-to-Thr MoFe proteins. These results indicated a role for the a-88 position in controlling electron flow through the P cluster.
Another significant finding centers on the spectroscopic signals derived from one of these a-88Cys substituted MoFe proteins. The a-88Cys-to-Gly MoFe protein possesses a unique S=1/2 EPR signal in the native, dithionite-reduced state that was shown to be due to a one-electron-oxidized P cluster. This new paramagnetic center was evidence for the dramatic perturbation of the electromagnetic properties of the P cluster by the a-88Cys-to-Gly substitution. Additionally, both MÃ¶ssbauer and magnetic circular dichroism spectroscopies have also demonstrated significant changes in the electromagnetic environments of the P clusters of these a-88Cys altered MoFe proteins and that each substitution affected the P cluster differently. The novel EPR signal was exploited in order to follow the sequence of electron transfer events in the nitrogenase reaction.
Finally, altered nitrogenase component proteins were combined and analyzed in an attempt to distinguish which particular step(s) are perturbed in the overall enzymatic reaction.