FORMATION OF A SQUARE-PLANAR CO(I) B-12 INTERMEDIATE - IMPLICATIONS FOR ENZYME CATALYSIS

被引：45

作者：

WIRT, MD ^{[1
]}

SAGI, I ^{[1
]}

CHANCE, MR ^{[1
]}

机构：

[1] GEORGETOWN UNIV,WASHINGTON,DC 20057

来源：

BIOPHYSICAL JOURNAL | 1992年 / 63卷 / 02期

关键词：

D O I：

10.1016/S0006-3495(92)81605-3

中图分类号：

Q6 [生物物理学];

学科分类号：

071011 ;

摘要：

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) B-12 that demonstrate a four-coordinate (distorted) square-planar configuration. Comparison of EXAFS solutions for Co(I) and Co(II) B-12 (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) B-12 complex. This paper presents the first direct evidence that formation of the Co(I) B-12 intermediate must involve breaking of the Co-N (DMB) bond.

引用

页码：412 / 417

页数：6

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