Cobalamin (B12) and folate (B9) are two B vitamins that are essential for the folate cycle and methionine cycle, which are part of one-carbon metabolism. The folate cycle is located in the cytoplasm and mitochondria, and uses serine and glycine as inputs. The folate cycle is dependent on B12 to generate the active forms of vitamin B9. The methionine cycle requires methionine to produce cysteine, which is a key component of the trans-sulfuration pathway. The enzyme methionine synthase uses B12 as a cofactor to convert homocysteine to methionine.