cobamamide and nitromethane

We describe a novel enantioselective synthesis of aristeromycin, the carbocyclic analogue of adenosine. The seven-step synthesis is also suitable for the preparation of specifically-labelled [6'-(13)C]aristeromycin. Both the unlabelled and (13)C-labelled product was coupled to vitamin B(12) to form aristeromycylcobalamin. This carbocyclic analogue of coenzyme B(12) was examined for its coenzymic activity with several adenosylcobalamin-dependent enzymes. For glyceroldehydratase and dioldehydratase, the reaction rate (k(cat)) was 38 and 44 % of that measured with adenosylcobalamin as coenzyme. In contrast, aristeromycylcobalamin showed no detectable activity with methylmalonyl-CoA mutase and ethanolamine ammonia-lyase. Instead, it was a weak inhibitor of the former and a strong inhibitor of the latter enzyme. The slower turnover rate with glyceroldehydratase raised the hope of detecting the 6'-deoxyaristeromycyl radical intermediate. Comparison of the EPR spectra of the intermediates in the glyceroldehydratase reaction, which used adenosyl- and aristeromycylcobalamines, respectively, as coenzyme, revealed a significant shift and this suggests a different geometric position of these cofactors at the binding site during the cleavage of the carbon-cobalt bond. However, we found no evidence for the existence of a 6'-deoxyaristeromycyl radical during the reaction with [6'-(13)C]aristeromycylcobalamin. We conclude that the lifetime of this radical is still too short to be observed.

Topics: Adenosine; Carbon Radioisotopes; Catalysis; Cobamides; Electron Spin Resonance Spectroscopy; Enzyme Activation; Ethanolamine Ammonia-Lyase; Hydro-Lyases; Isotope Labeling; Kinetics; Methane; Methylmalonyl-CoA Mutase; Nitroparaffins; Propanediol Dehydratase; Stereoisomerism; Substrate Specificity