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Sulfur-containing cobalamins: X-ray absorption spectroscopic characterization

E M Scheuring1, I Sagi, M R Chance

  • 1Department of Physiology and Biophysics, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461.

Biochemistry
|May 24, 1994
PubMed
Summary

Sulfur-containing cobalamins, like glutathionylcobalamin, play a key role in vitamin B12 coenzyme formation. X-ray absorption spectroscopy confirms sulfur coordination to cobalt, revealing structural details and reduced cobalt charge in these vital compounds.

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Area of Science:

  • Biochemistry
  • Bioinorganic Chemistry
  • Structural Biology

Background:

  • Sulfur-containing cobalamins are hypothesized to be crucial intermediates in the intracellular conversion of cyanocobalamin to its active coenzyme forms.
  • Glutathionylcobalamin, in particular, is considered a potential precursor for cobalamin coenzymes.
  • Previous NMR studies suggest that glutathione coordinates to the cobalt atom via its sulfur atom in glutathionylcobalamin.

Purpose of the Study:

  • To characterize three sulfur-containing cobalamin derivatives: glutathionylcobalamin, sulfitocobalamin, and cysteinylcobalamin.
  • To provide evidence for sulfur coordination to the cobalt ion in these compounds.
  • To determine the structural parameters, including bond distances and cobalt charge, of these sulfur cobalamins.

Main Methods:

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  • X-ray absorption spectroscopy (XAS) was employed to analyze the electronic and structural properties of the sulfur-containing cobalamin derivatives.
  • Analysis of XAS data provided information on cobalt-ligand bond distances (Co-Neq, Co-S, Co-Nax) and X-ray edge positions.

Main Results:

  • X-ray absorption spectroscopy confirmed sulfur coordination to the cobalt ion in glutathionylcobalamin, sulfitocobalamin, and cysteinylcobalamin.
  • The determined Co-S bond distances ranged from 2.28 to 2.35 Å, and Co-Nax distances were between 2.13 and 2.16 Å, consistent with expected values.
  • X-ray edge positions for sulfur derivatives shifted to lower energies compared to cyanocobalamin, indicating significant electron donation from sulfur to cobalt and a reduced effective charge on the cobalt ion.

Conclusions:

  • The study provides strong evidence for sulfur coordination in glutathionylcobalamin, sulfitocobalamin, and cysteinylcobalamin through X-ray absorption spectroscopy.
  • Structural data reveal consistent Co-S and Co-Nax bond lengths across the studied sulfur cobalamins.
  • The observed electronic changes suggest that sulfur ligands significantly reduce the cobalt's effective charge, supporting their proposed role in cobalamin coenzyme biosynthesis.