two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. These use the adoB12 (adenosylcobalamin) form of the vitamin. Methyltransferases Methyl (–CH3) group transfers between two molecules. These use the MeB12 (methylcobalamin) form of the vitamin. Dehalogenases Some species of anaerobic bacteria synthesize B12-dependent dehalogenases, which have potential commercial applications for degrading chlorinated pollutants. The microorganisms may either be capable of de novo corrinoid biosynthesis or are dependent on exogenous vitamin B12. In humans, two major coenzyme B12-dependent enzyme families corresponding to the first two reaction types, are known. These are typified by the following two enzymes: Methylmalonyl coenzyme A mutase (MUT) is an isomerase enzyme which uses the AdoB12 form and reaction type 1 to convert L-methylmalonyl-CoA to succinyl-CoA, an important step in the catabolic breakdown of some amino acids into succinyl-CoA, which then enters energy production via the citric acid cycle. This functionality is lost in vitamin B12 deficiency, and can be measured clinically as an increased serum methylmalonic acid (MMA) concentration. The MUT function is necessary for proper myelin synthesis. Based on animal research, it is thought that the increased methylmalonyl-CoA hydrolyzes to form methylmalonate (methylmalonic acid), a neurotoxic dicarboxylic acid, causing neurological deterioration. Methionine synthase, coded by MTR gene, is a methyltransferase enzyme which uses the MeB12 and reaction type 2 to transfer a methyl group from 5-methyltetrahydrofolate to homocysteine, thereby generating tetrahydrofolate (THF) and methionine. This functionality is lost in vitamin B12 deficiency, resulting in an increased homocysteine level and the trapping of folate as 5-methyl-tetrahydrofolate, from which THF (the active form of folate) cannot be recovered. THF plays an important role in DNA synthesis so reduced availability of THF results in ineffective production of cells with rapid turnover, in particular red blood cells, and also intestinal wall cells which are responsible for absorption. THF may be regenerated via MTR or may be obtained from fresh folate in the diet. Thus all of the DNA synthetic effects of B12 deficiency, including the megaloblastic anemia of pernicious anemia, resolve if sufficient dietary folate is present. Thus the best-known "function" of B12 (that which is involved with DNA synthesis, cell-division, and anemia) is actually a facultative function which is mediated by B12-conservation of an active form of folate which is needed for efficient DNA production. Other cobalamin-requiring methyltransferase enzymes