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 10(9)- to 10(14)-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 Co(II) 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.

Entropic Origin of Cobalt–Carbon Bond Cleavage Catalysis in Adenosylcobalamin-Dependent Ethanolamine Ammonia-Lyase

“…10,30 The entropic origin of Co-C bond cleavage catalysis in EAL represents a paradigm shift from traditional, enthalpy-based proposals. 5,14,17,18 We propose a model of “configurational catalysis,” in which ramification of protein configurations along the reaction coordinate leads to the favorable activation entropy. This model has parallels in the significant protein configurational entropy changes associated with protein-ligand interactions, 53-55 and protein unfolding, 57 and is consistent with transient kinetic and spectroscopic studies of EAL.…”

“…This represents a paradigm shift from previous enthalpy-based proposals for enzymic Co-C bond cleavage catalysis. 5,14,17,18 As depicted in Figure 5B, a significant fraction of the increased entropy at the transition state for Co-C bond cleavage is maintained in the Co(II)-substrate radical pair state. 25 Thus, the increase in entropy also makes a significant contribution to the thermodynamic driving force for radical pair separation.…”

Gold-Containing Indoles as Anticancer Agents That Potentiate the Cytotoxic Effects of Ionizing Radiation

“…1–3 However, other metals including copper, gold, and platinum possess properties such as redox reactivity, Lewis acidity, variable coordination modes, and reactivity towards biological macromolecules that can unleash lethal effects on cells. 4–6 The toxicity of these metals can, under certain conditions, be controlled and subsequently used to efficiently kill cells that are associated with pathogenic conditions such as cancer.…”

Chemoselective Deprotection of α-Indole and Imidazole Ribonucleosides

“…Semisynthesis of cobalamins [1][2][3][4][5][6] with altered axial nucleotide ligands requires cobyric acid [7] and an α-ribonucleotide [8] as precursors. Analogs of coenzyme B 12 (5 -deoxyadenosylcobalamin, AdoCbl) with altered axial nucleotide ligands, such as B3-deazaAdoCbl ( Figure 1) are of interest as probes of the function, if any, of the axial nucleotide in the enzymatic activation of AdoCbl for carbon-cobalt bond homolysis [9,10] .…”