**2. The metabolism of vitamin B12**

The major metabolic pathway of vitamin B12 formation is shown in **Figure 1**.

**Figure 1.** The mechanism of vitamin B12.

Vitamin B12 is essential for DNA synthesis in cells. It has two different forms in cells.

Deoxyadenosyl B12 converts methylmalonyl CoA to succinyl CoA. It also transfers methyl groups from methyltetrafolate to synthesized methionine. Transferring a methyl group from methyltetrafolate forms tetrahydrofolate. If there is a lack of vitamin B12, there is no receptor to transfer a methyl group from methyltetrafolate. Then the methylfolate is trapped and tetrahydrofolate that is needed to support DNA synthesis is decreased [2].
