Coenzyme B12 (adenosylcobalamin; AdoCbl) is a biologically active form of vitamin B12 . It belongs to the corrinoid group of compounds, which contain a corrin macrocycle, and is produced only by certain bacteria and archaea. It is a cofactor for various enzymes including mutases, eliminases, aminomutases, and a reductase. These enzymes catalyze reactions that generate free radicals through release of the adenosyl group, allowing usually unreactive molecules to become reactive. Only one of these enzymes, methylmalonyl CoA mutase, is found in mammals, therefore, the other enzymes must be taken in through the diet. Genetic mutations in enzymes that synthesize AdoCbl lead to AdoCbl deficiency and methylmalonic aciduria. Formulations containing AdoCbl are used to treat patients with this disorder.