Concerning the Intermediacy of Organic Radicals in Vitamin B12-Dependent Enzymic Reactions

The vitamin B₁₂ coenzyme adenosylcobalamin assists the enzymic catalysis of molecular rearrangements of the type ???? in which the migrating group X can be OH, NH_2 or a suitable substituted carbon atom such as C(<???>CH_2)CO_2H. This paper discusses evidence for the participation of organic radicals as intermediates in these reactions. Theoretical and model studies supporting the intermediacy of radicals in the reactions catalysed by the enzymes diol dehydratase and α-methyleneglutarate mutase are described. For the model studies, alkyl radicals, alkylcobaloximes (alkyl represents, for example, ethoxycarbonyl substituted, but-3-enyl and cyclopropylmethyl) and also dihydroxyalkylcobalamins have been investigated. The Co-C_α-C_β angle of 125^circ in adenosylcobalamin is shown to be an 'especial' angle by analysis of the crystal structures of R- and S-2,3-dihydroxypropylcobalamin.