Vitamin B12 functions in metabolism in two coenzyme forms: adenosylcobalamin and methylcobalamin. Only three vitamin B12-dependent enzymes have been discovered in animals: methylmalonyl-coenzyme A (CoA) mutase, leucine mutase, and methionine synthetase. Through these enzymes vitamin B12 is involved in the metabolism of one carbon units and the consequent synthesis of nucleic acids, the conversion of methylmalonyl-CoA to succinyl-CoA, and the methylation of homocysteine to methionine in partnership with folic acid.
