A corrinoid 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; serves as a cofactor in isomerase-, methyltransferase-, and dehalogenase-catalyzed reactions, including the conversion of methylmalonyl-CoA to succinyl-CoA and the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine