Adenosylcobalamin-dependent ribonucleoside triphosphate reductase from Lactobacillus leichmannii. Rapid, improved purification involving dGTP-based affinity chromatography plus biophysical characterization studies demonstrating enhanced, "crystallographic level" purity

NMR observations of 13C-enriched coenzyme B12 bound to the ribonucleotide reductase from Lactobacillus leichmannii

The 13C NMR resonance and one-bond 1H-13C coupling constants of coenzyme B12 enriched in 13C in the cobalt-bound carbon have been observed in the complex of the coenzyme with the B12-dependent ribonucleotide reductase from Lactobacillus leichmannii. Neither the 13C NMR chemical shift nor the 1H-13C coupling constants are significantly altered by binding of the coenzyme to the enzyme. The results suggest that ground-state Co-C bond distortion is not utilized by this enzyme to activate coenzyme B12 for C-Co bond homolysis.

Synthesis of adenosylcobinamide 2-chlorophenyl phosphate, a zwitterionic cobinamide phosphate analog of adenosylcobalamin en route to crystallizable cobinamides

The design and implementation of a new, higher yield synthetic method for synthesizing zwitterionic cobinamide phosphates is described. Adenosylcobinamide 2-chlorophenyl phosphate, beta-AdoCbi-PAr -- a 5,6-dimethylbenzimidazole-free adenosylcobalamin analog, where a 2-chlorophenyl group replaces the ribofuranose and 5,6-dimethylbenzimidazole moieties -- is prepared in tens of milligram quantities, quantities sufficient for crystallization and enzyme trials, amounts 100-fold greater than previously available. The use of (31)P NMR spectroscopy to follow reactions directly, the use of control reactions to learn how to reduce reactant water content, and the use of reaction solvents that completely dissolved the corrinoid reactants were crucial for developing this new synthetic route. beta-AdoCbi-PAr was synthesized in 10% overall isolated yield from cyanocobinamide. Cyanocobinamide was converted to cyanocobinamide 2-chlorophenyl phosphate by direct phosphorylation with 2-chlorophenyl phosphodi-(1,2,4-triazolide) in 25% isolated yield and > or = 98% purity. Sodium borohydride reduction of cyanocobinamide 2-chlorophenyl phosphate and reaction with 5'-chloro-5'-deoxy-adenosine produced beta-AdoCbi-PAr in 42% yield and > or = 98% purity. These compounds were characterized by HPLC, (1)H and (31)P NMR, UV-visible spectroscopy, and liquid secondary ionization mass spectroscopy.

Uptake and physiological function of vitamin B12 in a photosynthetic unicellular coccolithophorid alga, Pleurochrysis carterae

The photosynthetic coccolithophoid alga, Pleurochrysis (Hymenomonas) carterae, could take up and accumulate exogenous vitamin B12, most of which was converted into the coenzyme forms of vitamin B12. Two vitamin B12-dependent enzyme activities (methylmalonyl-CoA mutase, 2.6+/-0.4 nmol/min/mg protein and methionine synthase, 85.1+/-38.9 pmol/min/mg protein) could be found in a cell homogenate of the vitamin B12-supplemented alga. Most of the methylmalonyl-CoA mutase activity and 19.2% of the vitamin B12 accumulated by the algal cells were recovered in the macromolecular fractions with Mr of 150 kDa, although the remaining vitamin B12 was found only in free vitamin B12 fractions.

Hydrogen abstraction from thiols by adenosyl radicals: chemical precedent for thiyl radical formation, the first catalytic step in ribonucleoside triphosphate reductase from Lactobacillus leichmannii

Aqueous solutions of adenosylcobalamin (AdoCbl) were thermolyzed with excess beta-mercaptoethanol under anaerobic conditions. The product studies reveal that approximately 90% Co-C bond homolysis occurs, to yield Co(II)cobalamin, 5'-deoxyadenosine, and the disulfide product from the combination of two HOCH2CH2S* radicals, 2,2'-dithiodiethanol; there is also approximately 10% Co-C bond heterolysis, yielding Co(III)cobalamin, adenine, and 2,3-dihydroxy-4-pentenal. The kinetic studies show there is a first-order dependence on AdoCbl and zero-order dependence on thiol under the higher [RSH] experimental conditions used, consistent with the rate-determining step at high [RSH] being the generation of Ado*. The kinetic results require that, in enzyme-free AdoCbl solution, adenosyl radical (Ado*) is formed as a discrete intermediate which then abstracts H* from the added thiol. The activation parameters for Co-C bond homolysis in the presence of thiol trap are the same within experimental error as the activation parameters for Co-C bond homolysis without trap, standard delta H(obs) = 29(2) kcal mol(-1) and standard delta S(obs) = -1(5) e.u. The results, in comparison to the rate of Co-C bond homolysis in ribonucleoside triphosphate reductase (RTPR), reveal that RTPR accelerates Co-C bond cleavage in AdoCbl by approximately 10(10+/-1). The recent literature evidence bearing on the exact mechanism of RTPR enzymic cleavage of the Co-C bond of AdoCbl is briefly discussed, notably the fact that this mechanism is presently controversial, but does involve at least coupled (and possibly concerted) Co-C cleavage, -S-H cleavage, and C-H (Ado-H) formation steps.