As "coenzyme" forms, methylcobalamin and adenosylcobalamin are the forms required by certain B12-dependent enzymes and reactions within the body. Vitamin B12 is required for proper red blood cell formation, neurological function, and DNA synthesis. Vitamin B12 functions as a cofactor for methionine synthase and L-methylmalonyl-CoA mutase. Methionine synthase catalyzes the conversion of homocysteine to methionine. Methionine is required for the formation of SAMe (S-adenosylmethionine), an important methyl donor.