Kinetic and thermodynamic studies on ligand substitution reactions and base-on/base-off equilibria of cyanoimidazolylcobamide, a vitamin B12 analog with an imidazole axial nucleoside.
ABSTRACT Ligand substitution reactions of the vitamin B12 analog cyanoimidazolylcobamide, CN(Im)Cbl, with cyanide were studied. Cyanide substitutes imidazole (Im) in the alpha-position more slowly than it substitutes dimethylbenzimidazole in cyanocobalamin (vitamin B12). The kinetics of the displacement of Im by CN- showed saturation behaviour at high cyanide concentration; the limiting rate constant was found to be 0.0264 s(-1) at 25 degrees C and is characterized by the activation parameters: DeltaH(not =) = 111 +/- 2 kJ mol(-1), DeltaS(not =) = +97 +/- 6 J K(-1) mol(-1), and DeltaV(not =) = +9.3 +/- 0.3 cm3 mol(-1). These parameters are interpreted in terms of an I(d) mechanism. The equilibrium constant for the reaction of CN(Im)Cbl with CN- was found to be 861 +/- 75 M(-1), which is significantly less than that obtained for the reaction of cyanocobalamin with CN- (viz. 10(4) M(-1)). pKbase-off for the base-on/base-off equilibrium was determined spectrophotometrically and found to be 0.99 +/- 0.05, which is about 0.9 pH units higher than that obtained previously in the case of cyanocobalamin. In addition, the kinetics of the base-on/base-off reaction was studied using a pH-jump technique and the data obtained revealed evidence for an acid catalyzed reaction path. The results obtained in this study are discussed in reference to those reported previously for cyanocobalamin.
- SourceAvailable from: Rudi van EldikAdvances in Physical Organic Chemistry - ADVAN PHYS ORGAN CHEM. 01/2006; 41:1-78.
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ABSTRACT: Phenolyl cobamides are unique members of a class of cobalt-containing cofactors that includes vitamin B(12) (cobalamin). Cobamide cofactors facilitate diverse reactions in prokaryotes and eukaryotes. Phenolyl cobamides are structurally and chemically distinct from the more commonly used benzimidazolyl cobamides such as cobalamin, as the lower axial ligand is a phenolic group rather than a benzimidazole. The functional significance of this difference is not well understood. Here we show that in the bacterium Sporomusa ovata, the only organism known to synthesize phenolyl cobamides, several cobamide-dependent acetogenic metabolisms have a requirement or preference for phenolyl cobamides. The addition of benzimidazoles to S. ovata cultures results in a decrease in growth rate when grown on methanol, 3,4-dimethoxybenzoate, H(2) + CO(2), or betaine. Suppression of native p-cresolyl cobamide synthesis and production of benzimidazolyl cobamides occurs upon the addition of benzimidazoles, indicating that benzimidazolyl cobamides are not functionally equivalent to the phenolyl cobamide cofactors produced by S. ovata. We further show that S. ovata is capable of incorporating other phenolic compounds into cobamides that function in methanol metabolism. These results demonstrate that S. ovata can incorporate a wide range of compounds as cobamide lower ligands, despite its preference for phenolyl cobamides in the metabolism of certain energy substrates. To our knowledge, S. ovata is unique among cobamide-dependent organisms in its preferential utilization of phenolyl cobamides.Journal of bacteriology 02/2013; · 2.69 Impact Factor
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ABSTRACT: Vitamin B(12) is an organometallic compound with important metabolic derivatives that act as cofactors of certain enzymes, which have been grouped into three subfamilies depending on their cofactors. Among them, methylmalonyl-CoA mutase (MCM) has been extensively studied. This enzyme catalyzes the reversible isomerization of L-methylmalonyl-CoA to succinyl-CoA using adenosylcobalamin (AdoCbl) as a cofactor participating in the generation of radicals that allow isomerization of the substrate. The crystal structure of MCM determined in Propionibacterium freudenreichii var. shermanii has helped to elucidate the role of this cofactor AdoCbl in the reaction to specify the mechanism by which radicals are generated from the coenzyme and to clarify the interactions between the enzyme, coenzyme, and substrate. The existence of human methylmalonic acidemia (MMA) due to the presence of mutations in MCM shows the importance of its role in metabolism. The recent crystallization of the human MCM has shown that despite being similar to the bacterial protein, there are significant differences in the structural organization of the two proteins. Recent studies have identified the involvement of an accessory protein called MMAA, which interacts with MCM to prevent MCM's inactivation or acts as a chaperone to promote regeneration of inactivated enzyme. The interdisciplinary studies using this protein as a model in different organisms have helped to elucidate the mechanism of action of this isomerase, the impact of mutations at a functional level and their repercussion in the development and progression of MMA in humans. It is still necessary to study the mechanisms involved in more detail using new methods.Journal of Zhejiang University SCIENCE B 06/2012; 13(6):423-37. · 1.11 Impact Factor