Nucleotide dependent conformational changes in the nitrogenase Fe protein
Nitrogenase is a complex metal-containing enzyme that catalyzes the conversion of nitrogen gas to ammonia. During nitrogenase catalysis the Fe protein and the molybdenum-iron protein associate and dissociate in a manner resulting in the hydrolysis of two molecules of MgATP and the transfer of at least one electron to the MoFe protein. The role of nucleotide binding and hydrolysis in nitrogenase catalysis is one of the most fascinating aspects of nitrogenase function. The Fe protein upon binding to MgATP undergoes a huge conformational change which is important for subsequent steps of nitrogenase reaction mechanism. Therefore structural characterization of the Fe protein bound to MgATP will provide a basis on how MgATP binding promotes the complex formation whereas hydrolysis to MgADP leads to the dissociation of the macromolecular complex structure. Towards these ends we have conducted structural studies on a site-directed variant of the Fe protein which is a close mimic of the MgATP conformational state. Structural characterization of this Leu127 deletion variant revealed a distinctly new conformation of the Fe protein which arises from the rigid body reorientation of the homodimeric Fe protein subunits with respect to each other. The structure not only provides the first basis on rationalizing the initial docking interactions between the component proteins but also helps us to dissect the conformational changes on the Fe protein which occur upon nucleotide binding from those conformational changes that are imposed on the Fe protein by the MoFe protein during complex formation. Having this structure in hand, we have developed several other experimental approaches like Mass spectrophotometry and Small Angle X-ray Scattering/Diffraction (SAXS) techniques to probe the relationship between the Leu127 deletion variant a close structural mimic of MgATP bound "on state" and the actual MgATP bound state which is more difficult to probe crystallographically. These studies will help us to compare the different nucleotide bound states (MgADP and MgATP) of the Fe protein in solution that will help to predict the level of conformational change that is induced in the Fe protein that makes it compatible for binding to the MoFe protein in the nitrogen catalysis cycle.