Methyl corrinoid derivatives

We were interested in the specificity of methyl corrinoid derivatives and tested methyl factor III and methyl factor B as substrates for methane formation. Specificity for activation of the methyl group did not reside in the lower axial ligand since the methyl group was converted readily to methane in the absence of the dimethylbenzimidazole moiety of the lower axial ligand (11). 

Figure 3. Partial structures of the immobilized corrinoid derivatives used in this study L = adenosyl, cytidyl, methyl, cyano, hydroxo, etc. IPA = l-amino-2-propanol Me2Bza = 5,6-dimethylbenzimidazole...

The second important type of organometallic reactivity of B 12-derivatives concerns the highly nucleophilic/nucleofugal Co(I)-corrins [75,91,132[. These provide the basis of the heterolytic mode of formation/cleavage of the Co - C bond, important in methyl-corrinoids in enzyme-catalyzed methyl-transfer reactions [125-127]. This mode is represented by the reaction of Co(I)-corrins with alkylating agents in the formation of the Co - CH3 bond and the nucleophile-induced demethylation of methyl Co(III)-corrins for the... 

Combining these observations, a mechanism can be proposed (Figure 6b) in which the reduced NiFe cluster A is involved in three steps the transfer of CH3 from the corrinoid/Fe-S protein, the binding of CO and formation of the CH3-CO bond, and the thioester bond of acetyl CoA. In the first stage of the assembly of acetyl CoA, CO and the methyl group can bind to the center in a random order. This is followed by the formation of an acetyl derivative. The CoA then binds to the enzyme and acetyl CoA is released. 

The metabolically important functions of the Bn-derivatives are directly concerned either with enzymatically controlled organometalhc reactions involving protein-bound adenosylcobamides (such as coenzyme B12, (3)), or methyl-Co -corrinoids (such as methylcobalamin, (4)), or with enzyme-controlled redox reactions. Studies on the underlying biologically relevant organometalhc chemistry of the Bi2-coenzymes in homogeneous (protic) solution, as well as the characterization of the enzymatic processes themselves have attracted considerable interest. ... 

The clarification of the structure of coenzyme B g stimulated attempts to synthesize the compound from vitamin B g and an activated adenosine derivative. Smith et described a chemical synthesis of the coentyme from fully reduced vitamin Big (Bi and the 5 -tosyl-2, 3 -isopropylidene derivative of adenosine. At the same time they reported the synthesis of Co-methyl-cobalamin from Bigg and methyl iodide. Soon thereafter a biological function of methylcobalamin in the methylation of homo steine was demonstrated by Guest et Only one other Co-alkyl-corrinoid compound has so far been... 

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