Vitamin B12 is a water-soluble vitamin with roles in brain and nervous system functioning and blood formation through regulation of DNA synthesis, cellular metabolism, fatty acid metabolism, and amino acid metabolism. It belongs to the corrinoid group of compounds, which contain a corrin macrocycle, and is produced only by certain bacteria and archaea. Reference to vitamin B12 usually encompasses a group of cobalt-containing compounds (or cobalamins), which include cyanocobalamin, hydroxocobalamin, 5'-deoxyadenosylcobalamin (AdoB12), and methylcobalamin (MeB12). These forms serve as cofactors in isomerase-, methyltransferase-, and dehalogenase-catalyzed reactions. For example, methylmalonyl coenzyme A mutase acts on AdoB12 to convert methylmalonyl-CoA to succinyl-CoA, an important step before entry into the Kreb’s cycle. Additionally, methionine synthase uses MeB12 to transfer a methyl group from 5-methyltetrahydrofolate to homocysteine to generate tetrahydrofolate and methionine, which is important for DNA synthesis.
