Vitamin B12–Catalysed Reactions

Vitamin B₁₂ (cobalamin [Cbl]) serves as a cofactor for Cbl-dependent enzymes such as adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) in a range of important biological processes which include nervous system coordination, red blood cell biosynthesis, DNA synthesis and regulation. MeCbl undergoes heterolysis of the Co³⁺-C bond to yield the supernucleophilic Co⁺ state and a methyl cation, which is required for enzymatic methyl transfer reactions. Vitamin B₁₂ enzymes can be classified into four major groups: B₁₂-bindingand B₁₂-transporting proteins, methyltransferases, corrinoid dehalogenases and Cbl-dependent enzymes. Members of all groups of the vitamin B₁₂ enzymes are essential to microorganisms, while B₁₂-binding proteins and methyltransferases play crucial roles in human and animal metabolism. The AdoCbl-catalysed rearrangement reaction is initiated by homolysis of the Co-C bond to generate the Ado radical and the Co²⁺ state, which can be stabilised by mutual interaction between AdoCbl and the surrounding enzyme.