Internal spin trapping of thiyl radical during the complexation and reduction of cobalamin with glutathione and dithiothrietol

Ramasamy, Somasundaram ; Kundu, Tapan K. ; Antholine, William ; Manoharan, Periakaruppan T. ; Rifkind, Joseph M. (2012) Internal spin trapping of thiyl radical during the complexation and reduction of cobalamin with glutathione and dithiothrietol Journal of Porphyrins and Phthalocyanines, 16 (01). pp. 25-38. ISSN 1088-4246

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Official URL: http://www.worldscientific.com/doi/abs/10.1142/S10...

Related URL: http://dx.doi.org/10.1142/S1088424611004051

Abstract

The activation of cobalamin requires a reduction from cobalamin(III) to cobalamin(II). The reduction by glutathione and dithiothreitol was followed using visible spectroscopy and electron paramagnetic resonance. In addition the oxidation of glutathione was monitored. Glutathione first reacts with oxidized cobalamin(III). The binding of a second glutathione required for the reduction to cobalamin(II) is presumably located in the dimethyl benzimidazole ribonucleotide ligand cavity. The reduction of cobalamin(III) by dithiothreitol, which contains two thiols, is much faster even though no stable cobalamin(III) complex is formed. The reduction, by both thiol reagents, results in the formation of thiyl radicals, some of which are released to form oxidized thiol products and some of which remain associated with the reduced cobalamin. In the reduced state the intrinsic lower affinity for the benzimidazole base, coupled with a trans effect from the initial glutathione bound to the β-axial site and a possible lowering of the pH results in an equilibrium between base-on and base-off complexes. The dissociation of the base facilitates a closer approach of the thiyl radical to the cobalamin(II) α-axial site resulting in a complex with ferromagnetic exchange coupling between the metal ion and the thiyl radical. This is a unique example of "internal spin trapping" of a thiyl radical formed during reduction. The finding that the reduction involves a peripheral site and that thiyl radicals produced during the reduction remain associated with the reduced cobalamin provide important new insights into our understanding of the formation and function of cobalamin enzymes.

Item Type:Article
Source:Copyright of this article belongs to Society of Porphyrins and Phthalocyanines.
Keywords:Cobalamin; Glutathione; Dithiothreitol; Electron Paramagnetic Resonance; Visible Spectroscopy; Thyil Radicals
ID Code:95777
Deposited On:07 Dec 2012 11:56
Last Modified:07 Dec 2012 11:56

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