title: Structural and electronic factors in heterolytic cleavage: formation of the Co(I) intermediate in the corrinoid/iron-sulfur protein from Clostridium thermoaceticum.
abstract: We have completed the first direct structural characterization of an enzyme-bound four-coordinate Co(I) intermediate, in this case for the corrinoid/iron-sulfur protein (C/Fe-SP) from Clostridium thermoaceticum. Extended X-ray absorption fine structure and X-ray edge spectroscopy of the active Co(I) state of the C/Fe-SP indicates a four-coordinate (distorted) square-planar structure where the best fit gives average Co-N(equatorial) distances of 1.87 +/- 0.01 A, corresponding to 4.2 +/- 0.3 ligands. The X-ray edge spectrum of Co(I) C/Fe-SP contains a moderate intensity 1s-4p + "shake-down" (SD) transition and no 1s-3d peak (where SD transitions are indicative of square-planar geometries). X-ray edge results for the methyl-Co(III) form, reported earlier [Wirt, M. D., Kumar, M., Ragsdale, S. W., & Chance, M. R. (1993) J. Am. Chem. Soc. 115, 2146-2150], are consistent with a base-off methylcobamide structure. The absence of a ligated 5-methoxybenzimidazole base in the methyl-Co(III) state is important since the base-off form is predicted to predispose the Co-C bond toward heterolytic cleavage to form the four-coordinate Co(I) species concurrent with methyl transfer. Additionally, we have examined first-derivative X-ray edge spectra of Co(I) C/Fe-SP, relative to edge spectra of a cobalt foil, as an indicator of effective nuclear charge on cobalt. The Co(I) C/Fe-SP edge position at 7720.5 +/- 0.3 eV is less than, but very close to, the value seen for the corresponding free Co(I) cobalamin.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Wirt
authors.forename: M
authors.initials: M
authors.lastname: Kumar
authors.forename: J J
authors.initials: JJ
authors.lastname: Wu
authors.forename: E M
authors.initials: EM
authors.lastname: Scheuring
authors.forename: S W
authors.initials: SW
authors.lastname: Ragsdale
authors.forename: M R
authors.initials: MR
authors.lastname: Chance
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D003013:Clostridium / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D013052:Spectrometry, X-Ray Emission; D013057:Spectrum Analysis; D014965:X-Rays
chemical_list: D001426:Bacterial Proteins; C080269:C-Fe-SP protein, Moorella thermoacetica; D007506:Iron-Sulfur Proteins; D003035:Cobalt