Elucidation of a MgATP signal transduction pathway in the nitrogenase iron protein: Formation of a conformation resembling the MgATP-bound state by protein engineering

The present work defines one MgATP signal transduction pathway in the nitrogenase iron (Fe) protein. Deletion of an amino acid (Leu 127) by site-directed mutagenesis in the protein chain between Asp 125, located in the ATP binding site, and Cys 132, a ligand to the [4Fe-4S] cluster, resulted in protein conformational changes resembling the MgATP-bound state in the absence of any bound nucleotides. Specifically, H-1 nuclear magnetic resonance, electron paramagnetic resonance, and circular dichroism spectroscopic properties, along with Fe chelation assays, suggested that deletion of Leu 127 in the Fe protein resulted in changes in the electronic properties of the [4Fe-4S] cluster similar to those normally observed upon MgATP binding to the wild-type Fe protein. Deletion of Leu 127 of the Fe protein lowered the redox potential of the [4Fe-4S] cluster by 112 mV compared to the wild-type Fe protein (-412 mV compared to -294 mV). A nearly identical lowering of the redox potential by 120 mV occurs in the wild-type Fe protein upon binding MgATP (-294 mV compared to -420 mV). The L127 Delta Fe protein did not contain bound nucleotides which could account for the observed conformational changes. The present results support a model in which the protein chain from Asp 125 to Cys 132 acts as one pathway for MgATP signal transduction and suggests a mechanism for this transduction to the [4Fe-4S] cluster. The L127 Delta Fe protein was found to still bind 2 MgATP or 2 MgADP molecules/Fe protein. Unlike the wild-type Fe protein, the L127 Delta Fe protein bound 2 ADP molecules/Fe protein in the absence of Mg2+. Finally, the L127 Delta Fe protein was found to bind to the MoFe protein, although the complex did not catalyze MgATP hydrolysis or substrate reduction. In concurrence with previous models, homologies between the Asp 125 to Cys 132 transduction pathway in Fe protein and the switch II region of the broad class of GTPase signal transduction proteins (G-proteins) are discussed.