Exafs Studies of Femo-cofactor and Mofe Protein - Direct Evidence For the Long-range Mo-fe-fe Interaction and Cyanide Binding To the Mo In Femo-cofactor

The biological reduction of dinitrogen to ammonia by the nitrogenase system requires the MoFe protein, which contains two iron-molybdenum cofactors (FeMoco) and two Fe-S P-clusters, and the Fe protein, which is the electron donor in catalysis. The two FeMo-cofactors are the likely sites of substrate binding and reduction, and knowledge of their detailed structure and function is central to understanding the chemistry of this complex enzyme system. Recent crystal structure studies of the MoFe protein are providing remarkable details about the MoFe protein, but questions about the structure of isolated FeMoco and reactivity of the FeMoco site remain unanswered. We report herein a new series of Mo-K edge EXAFS studies of highly concentrated isolated FeMoco, isolated FeMoco plus CN-, and the MoFe protein. Very high quality data has been obtained over a wide k-range through improved experimental techniques. In addition, new EXAFS analysis methodology (called GNXAS) based on multiple scattering formalism with further enhancements has been used to analyze the data. Several important results have emerged: for the first time a second shell of Fe atoms at approximately 5.1 angstrom from the Mo is clearly present in the EXAFS for both FeMoco and the MoFe protein. This provides direct evidence for the ''intact'' nature of extracted FeMoco and demonstrates the ability to detect and analyze such long-range absorber-scatter interactions. Second, the EXAFS results give very accurate metrical details of the FeMoco sites, and these differ from those of FeMoco from the X-ray crystal structure of the MoFe protein at its current level of refinement. Finally, using the GNXAS analysis method, it is shown that added CN- coordinates to Mo in isolated FeMoco. These results further define accurate metrical details of FeMoco within and outside of the protein and provide the methodological basis for further investigations of chemical reactivity.