FeMo protein EXAFS

Kenneth O. Hodgson, Richard H. Hohn, et al. EXAFS (extended X-ray absorption fine structure) study of structure of FeMo protein component of nitrogenase... 

The structural features of the FeMo-cofactor model are generally consistent with the results of analytical and spectroscopic (EXAFS, ENDOR, and Mossbauer) studies of the cofactor. The composition of the nonprotein part of the FeMo-cofactor model, lMo 7Fe 9S l homocitrate, is within the range of values that have been reported (48, 53-55). The absence of protein-bound bridging ligands between the two clusters in FeMo-cofactor is consistent with the ability to extract the intact cofactor from MoFe-protein. EXAFS studies of the Mo environment in both the MoFe-protein and the isolated cofactor indicate that 2-30(N) and 3-5S are directly coordinated to Mo, with 3-4Fe present in the second coordination shell of Mo (104,105) the crystallographic model contains 30(N), 3S, and 3Fe. Studies of Fe EXAFS... 

Fig. 2. The fe -weighted EXAFS data associated with the iron K-edge of the iron-molybdenum cofactor (FeMoco) extracted from the FeMo-protein of the nitrogenase of Klebsiella pneumoniae, and its Fourier transform (19). 

Recent EPR, Mossbauer, and EXAFS studies of the FeFe protein from R. capsulatus have provided spectroscopic evidence that the metal clusters of the FeFe protein bear a certain degree of structural homology to those present in the MoFe and VFe proteins. Therefore, it is likely that the FeFe protein contains redox centers that are homologous to the P cluster and FeMo or FeVa cofactor, respectively. The FeMo or FeVa cofactor homologue in the FeFe protein is termed FeFe cofactor or FeFeco. 

Fig. 5. (A) Fourier transform of Mo K-edge EXAFS spectrum of the Mo-Fe protein from nitrogenase [Adapted from S. P. Cramer, in X-Ray Absorption Principles, Applications, Techniques of EXAFS, SEXAFS and XANES (D. C. Koningsberger and R. Prins, eds.), p. 257. Wiley, New York, 1988]. The Fourier transform shows two peaks indicating that at least two major components, Mo-S (and Mo-O/N) and Mo-Fe, are required to explain the Mo EXAFS spectrum from nitrogenase. (B) Model for the FeMo-co proposed based on the Mo EXAFS data and comparison with data from model compounds [T. E. Wolff, J. M. Berg, C. Warrick, K. O. Hodgson, R. H. Holm, and R. B. Frankel, J. Am. Chem. Soc. 100, 4630 (1978)]. (C) Structure of the FeMo-co based on a more recent X-ray crystal structure [J Kim and D. C. Rees, Science 257, 1677 (1992)]. The similarities between the structures in (B) and (C) are remarkable.

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