Sulfur-Containing Cobalamins: X-ray Absorption Spectroscopic Characterization

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Abstract
Sulfur-containing cobalamins are thought to have a special role in the intracellular conversion of cyanocobalamin to its coenzyme forms through a Co(I) intermediate. Glutathionylcobalamin is especially interesting as a possible precursor of cobalamin coenzymes [Wagner et al. (1969) Ann. N.Y. Acad. Sci. 112, 580; Pezacka et al. (1990) Biochem. Biophys. Res. Commun. 169, 443]. Recent NMR data [Brown et al. (1993) Biochemistry 32, 8421] strongly support the hypothesis that glutathione coordinates ito the cobalt through the sulfur atom in glutathionylcobalamin. In this study three-sulfur containing cobalamin derivatives (glutathionylcobalamin, sulfitocobalamin, and cysteinylcobalamin) have been characterized by X-ray absorption spectroscopy. We give evidence for the sulfur coordination in these compounds and present the corresponding structural information. The Co-Neq distances are also distances in the sulfur-containing cobalamins are very close to one another (1.90 +/- 0.01 A). The Co-S and Co-Nax distances are also similar (Co-S: 2.28-2.35 A and Co-Nax: 2.13-2.16 A) and in the expected range. The X-ray edge positions for the sulfur derivatives shift to lower energies with respect to cyanocobalamin. This indicates strong electron donation from the sulfur to the cobalt and suggests that the effective charge on the cobalt ion in sulfur cobalamins is largely reduced from +3.

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