Issue 9, 2006

The enzymatic activation of coenzyme B12

Abstract

The enzymatic “activation” of coenzyme B12 (5′-deoxyadenosylcobalamin, AdoCbl), in which homolysis of the carbon–cobalt bond of the coenzyme is catalyzed by some 109- to 1014-fold, remains one of the outstanding problems in bioinorganic chemistry. Mechanisms which feature the enzymatic manipulation of the axial Co–N bond length have been investigated by theoretical and experimental methods. Classical mechanochemical triggering, in which steric compression of the long axial Co–N bond leads to increased upward folding of the corrin ring and stretching of the Co–C bond is found to be feasible by molecular modeling, but the strain induced in the Co–C bond seems to be too small to account for the observed catalytic power. The modeling study shows that the effect is a steric one which depends on the size of the axial nucleotide base, as substitution of imidazole (Im) for the normal 5,6-dimethylbenzimidazole (Bzm) axial base decreases the Co–C bond labilization considerably. An experimental test was thus devised using the coenzyme analog with Im in place of Bzm (Ado(Im)Cbl). Studies of the enzymatic activation of this analog by the B12-dependent ribonucleoside triphosphate reductase from Lactobacillus leichmannii coupled with studies of the non-enzymatic homolytic lability of the Co–C bond of Ado(Im)Cbl show that the enzyme is only slightly less efficient (3.8-fold, 0.8 kcal mol−1) at activating Ado(Im)Cbl than at activating AdoCbl itself. This suggests, in agreement with the modeling study, that mechanochemical triggering can make only a small contribution to the enzymatic activation of AdoCbl. Another possibility, electronic stabilization of the CoII homolysis product by compression of the axial Co–N bond, requires that enzymatic activation be sensitive to the basicity of the axial nucleotide. Preliminary studies of the enzymatic activation of a coenzyme analog with a 5-fluoroimidazole axial nucleotide suggest that the catalysis of Co–C bond homolysis may indeed be significantly slowed by the decrease in basicity.

Graphical abstract: The enzymatic activation of coenzyme B12

Article information

Article type
Perspective
Submitted
12 Dec 2005
Accepted
26 Jan 2006
First published
07 Feb 2006

Dalton Trans., 2006, 1123-1133

The enzymatic activation of coenzyme B12

K. L. Brown, Dalton Trans., 2006, 1123 DOI: 10.1039/B517599M

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