Formation of a square-planar Co(I) B12 intermediate. Implications for enzyme catalysis.
X-ray edge and extended x-ray absorption fine structure (EXAFS) techniques provide powerful tools for analysis of local molecular structure of complexes in solution. We present EXAFS results for Co(I) B12 that demonstrate a four-coordinate (distorted) square-planar configuration. Comparison of EXAFS solutions for Co(I) and Co(II) B12 (collected previously; Sagi et al. 1990. J. Am. Chem. Soc. 112:8639-8644) suggest that modulation of the Co-N bond to the axial 5,6-dimethylbenzimidazole (DMB), in the absence of changes in Co-N (equatorial) bond distances, may be a key mechanism in promoting homolytic versus heterolytic cleavage. As Co-C bond homolysis occurs, the Co-N (DMB) bond becomes stronger. However, for heterolytic cleavage to occur, earlier electrochemical studies (D. Lexa and J. M. Saveant. 1976. J. Am. Chem. Soc. 98:2652-2658) and recent studies of methylcobalamin-dependent Clostridium thermoaceticum (Ragsdale et al. 1987. J. Biol. Chem. 262:14289-14297) suggest that removal of the DMB ligand (before Co-C bond cleavage) favors formation of the four-coordinate square-planar Co(I) species while inhibiting formation of the five-coordinate Co(II) B12 complex. This paper presents the first direct evidence that formation of the Co(I) B12 intermediate must involve breaking of the Co-N (DMB) bond.
Wirt
M D
MD
Department of Chemistry, Georgetown University, Washington, DC 20057.
Sagi
I
I
Chance
M R
MR
D003038:Cobamides / Q000737:chemistry*; D005583:Fourier Analysis; D008958:Models, Molecular; D008968:Molecular Conformation; D013050:Spectrometry, Fluorescence / Q000379:methods; D013057:Spectrum Analysis / Q000379:methods; D014805:Vitamin B 12 / Q000737:chemistry*; D014961:X-Ray Diffraction / Q000379:methods
D003038:Cobamides; C000913:cobamamide; D014805:Vitamin B 12