The bond valence sum (BVS) method is further applied to metalloenzyme active sites. When a particular coordination model is assumed, the BVS method allows for oxidation states of metal ions in metalloproteins to be determined from metal-ligand bond distances measured using extended X-ray absorption fine structure (EXAFS) analysis. Thus, the BVS can be used to determine the compatibility between a given coordination model and a particular oxidation state. A new procedure for calculating ro values on which BVS's are based is presented. This procedure allows for calculation of ro values on heteroleptic complexes and was used to determine a new set of ro distances using complexes that more closely model the active sites of interest. In particular, the new distances allow for calculations involving vanadium, molybdenum, and nickel. New calculations using EXAFS data on CO dehydrogenase, NiFe hydrogenase, manganese catalase, sulfite oxidase, MoFe nitrogenase, and VFe nitrogenase are presented. The interplay between oxidation state and coordination geometry can be quantitatively assessed using the BVS method.Determining the structures of metal ion binding sites is critical in understanding the mechanism of action of metalloenzymes. Extended X-ray absorption fine structure (EXAFS) provides a means for determining the metal-ligand bond lengths for a given metal ion in a protein. l-3 X-ray absorption near-edge spectroscopy (XANES) can also provide some information on the oxidation stateof the metal Recently, we reportedon theapplication of the bond valence sum (BVS) method to the analysis of metalligand bond lengths determined by EXAFS in metalloenzymes.5 In particular, when a structural model is deduced from EXAFS and other spectroscopic methods, the BVS can be used to confirm the compatibility of that model with a given oxidation state of the metal ion. Conversely, if the oxidation state is known, BVS can be used to test structural models. We have demonstrated that this method is effective in a variety of mononuclear and polynuclear metal ions in a wide range of oxidation states for a series of metalloenzymes.Bond valences (s) are calculated according to eq 1, where B *Abstract published in Advance ACS Abstracts, August 15, 1993. Koningsberger, D. C., Prins, R., Eds.; X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; John Wiley and Sons: New York, 1988. Stavrapoulos, P.; Muetterties, M. C.; Carrib, M.; Holm, R. H. J. Am. Chem. SOC. 1991, 113, 8485. Bastian, N . R.; Diekert, G.; Niederhoffer, E. C.; Teo, B.-K.; Walsh, C. T.; Orme-Johnson, W. H. J. Am. Chem. SOC. 1988, 110, 5581. Tan, G. 0.; Ensign, S. A.; Ciurli, S.; Scott, M. J.; Hedman, B.; Holm, R. H.; Ludden, P. W.; Korzun, 2. R.; Stephens, P. J.; Hodgson, K. 0. Proc. Narl. Acad. Sci. U.S.A. 1992, 89, 4427. Baidya, N.; Olmstead, M.; Mascharak, P. K. Inorg. Chem. 1991, 30, 929. Baidya, N.; Olmstead, M. M.; Mascharak, P. K. J . Am. Chem. SOC. 1992, 114, 9666. The Bioinorganic Chemistry ofNickel; Lancaster, L. R., Jr., Ed.; VCH Publishers...
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