For the purposes of USMLE there are only two enzymes that require B12 as co-factor, these are:
- The conversion of homocysteine into methionine via the enzyme homocysteine methyl transferase which uses N5 methyl THF as another substrate converting it to THF. Therefore, deficiency of vitamin B12 would result in megaloblastic anemia because in role number one above it is necessary for the formation of THF which would then serve in purines and thiamine formation reactions and without these you won't have enough nucleotides to be incorporated in DNA synthesis, so the cell cytoplasm get's larger than the nucleus and hence the megaloblast.
- The conversion of Methyl-Malonyl Co A to Succinyl Co A via the enzyme Methylmalonyl Mutase, this is the last step in the beak down of odd carbon fatty acids. Therefore, deficiency of vitamin B12 would result in accumulation of malonyl co in urine and accumulation of odd carbon fatty acids. The latter accumilate in the sheaths of nerves and results in the classic neuropathy seen in vitamin B12.