Broadly speaking, vitamin B12 is important because it’s a necessary part of many chemical reactions in the body.
Vitamin B12 and folate work as cofactors in the synthesis of the amino acid methionine via the enzymes methionine synthase and L-malonyl-CoA mutase. A deficiency of vitamin B12 or of folate disrupts this process and leads to the production of abnormally large red blood cells, resulting in megaloblastic anemia.[1]
Vitamin B12 is also used by methyl-malonyl-CoA mutase to produce succinyl CoA. Interrupting this reaction leads to the neurological symptoms of B12 deficiency.[2]
B12 is also used in the production of the myelin sheaths that surround nerves and help with nerve signal transmission. If the sheath is damaged, the signals can become disrupted or abnormal, causing pain or loss of sensation. In vitamin B12 deficiency, peripheral neuropathy can occur, which is the result of damage to the myelin sheaths in the nerves of the hands and the feet.[1]
Vitamin B12 is part of one-carbon metabolism, the metabolic process that produces DNA in the body. It helps enzymes work in the production of the building blocks of DNA called purines and pyrimidines.[2]