Methylcob alamin

Cobalamin-catalyzed reactions are generally classified into two groups methylcob-alamin-dependent reactions (Table 1, entry 1 to 3) and coenzyme Bi2-dependent rearrangements (Table 1, entry 4 to 11). The first group includes the biosynthesis of methionine from homocysteine, the reduction of CO2 to acetic acid via an acetyl-CoA pathway, and the biosynthesis of CH4 also via an acetyl-CoA pathway. ... 

The presence of Au(I) may likewise catalyze the reaction of Au(III) with methylcob(III)alamin Wood has included this couple under his Redox Switch category. Like PtCl l-, the reaction of AuC14 with CH3B12 is enhanced by the presence of Br" (46). It remains to be proven that the Au reaction does involve methyl radical transfer, presumably with subsequent oxidation of B12r [a cob(II)alamin compound] to the observed aquocobalamin. However, the available evidence for the formation of Au(II) intermediates is more extensive and convincing (200) than for Pt(III) intermediates (201). 

Figure 3 Absorption spectra of cobalamins in three oxidation states. The spectra are of (—) methylcob(lll)alamin, (-)...

Methylcorrinoids are competent for the efficient methylation of alkyl radicals. Thermolysis of 2 -bis(ethoxycarbonyl)propylcobalamin and methylcobalamin at 70 °C led to formation of cob(II)alamin and the organic products 2-ethyl-2-methylmalonic acid diethyl ester and 2,2-dimethylmalonic acid diethyl ester. The former product was generated with quantitative deuterium incorporation from CDsCobjllljalamin. The proposed mechanism involves homolytic substitution on methylcob(III)alamin by the 2 -bis(ethoxycarbonyl)propyl radical, resulting in net methyl-radical abstraction, a process calculated to be highly exothermic (A7/ -201 kJmoK ). The stereochemical course of the reaction should result in net inversion at the methyl carbon, although this has not been demonstrated. The reaction may serve as a precedent for several biosynthetic methylations, such as the antibiotic thienamycin synthesis. ... 

In aqueous solution and at room temperature the base-on (hexacoordinate) methylcob(III)alamin is more stable by about 4 kcal/mol than the base-off COo -aquo-COje-methylcob(III)alamin [135]. From NMR studies, the latter has been estimated to still be more stable in water, by around 7 kcal/mol, than the corresponding base-off and dehydrated form of Co -methylcob(III)alamin, which has a pentacoordinate Co -methyl-Co(III)-center [136]. 

Figure 2, Catalysis and reactivation of methionine synthase. Methionine for,motion occurs via two half reactions in which cobalamin serves as the intermediate methyl carrier. Reactivation is depicted in the right-hand portion of the diagram. An electron donor and AdoMet convert the inactive cob(II)alamin form of the enzyme to methylcob(III)alamin. In E. colU flavodoxin serves as the reductant for this priming reaction (7).

In contrast, the activation conformation appears to be unpopulated in methylated MetH. The methylcob(III)alamin enzyme retains the spectral signature of base-on cobalamin in the presence of flavodoxin and flavodoxin is not bound (K is > 70 pM) 4 ). These observations imply that AG for dissociation of histidine from cobalt is much more positive in methylcobalamin MetH than in the cob(II)alamin form of the enzyme. 

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