Click here to see the overall reaction first.

The fundamental biochemical perturbation in folate deficiency shows up with the absence of the methylene form, the precise form of the compound is N⁵, N¹⁰-methylene tetrahydrofolate. The key reaction is the methylation of deoxyuridine monophosphate (dUMP) to generate thymidylate (dTMP) which is needed for DNA synthesis. One of the obvious consequences of this deficiency is megaloblastic anemia, that is, a deficit in the generation of mature red blood cells. The transfer includes the CH2 group plus a hydrogen from the cofactor itself, that is, the cofactor is effectively oxidized in the process.

1. thymidylate synthase: dUMP + methylene-THF --- > dTMP + dihydrofolate (DHF)

This means DHF must be re-reduced to THF

2. dihydrofolate reductase DHF + NADPH ----- > THF+ NADP

The deleterious effects of folate deficiency are seen in the rapidly growing red blood cell metabolism due to the thymidylate synthase reaction. Under conditions where you might want to inhibit rapidly growing cells, as in cancer chemotherapy, you would inhibit dihydrofolate reductase (DHFR), with, for examıle, methotrexate. Some of the side-effects of methotrexate treatment will resemble folate deficiency.

THF gets its methylene group from serine. The cyclic process involving methyl-THF is as follows:

Click here to see where this fits into the overall reaction.

The most important carrier of methyl groups is S-adenosyl-methionine (SAM). The S adenosyl bond is not a common functional group but the involvement of ATP tells you that an active compound has been generated. Some of the important reactions in which SAM is involved are:

• Methylation of DNA and RNA. DNA- and RNA-methylases use SAM as a source of methyl groups. A major target of methylases is the 5 position of cytosine of DNA. The degree of methylation correlates with transcriptional activity. (Globin genes, for example, are highly methylated in non-erythroid cells but not in erythroid cells.
• The conversion of epinephrine to norepinephrine is also catalyzed by an N-methyl transferase that uses SAM. Note that because methionine is an essential amino acid, if it is limiting, choline could also become a nutritional requirement

The product after the methyl group is transferred is S-adenosyl-homocysteine. Hydrolysis gives homocysteine (and adenosine).  

In order to regenerate the methionine, homocysteine must be methylated. The methylating agent is the methyl form of THF:

c. The synthesis of methionine requires vitamin B12 ( cobalmin). In vitamin B12 deficiency there is a buildup of the substrate, N5-methyl-THF which cannot be re cycled. This causes a deficiency of the other forms of THF which are needed for other reactions, particularly in DNA synthesis. The loss of THF forms due to the inability to use methyl-THF is referred to as "the methyl trap."

Overview of key reactions involving one-carbon transfer.

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