Full mechanism, in text

Passing on methyl groups

1 · Homocysteine accumulates

After methionine gives up its methyl group it becomes , a potentially toxic intermediate, and its build-up in blood is linked to cardiovascular events, stroke and dementia.When cells metabolize protein, after the sulfur-containing amino acid methionine (Met) is used (as a methyl donor) it is converted to (Hcy) — a potentially toxic intermediate that must be processed rapidly.

Long-term Hcy accumulation in blood → hyperhomocysteinemia → positively correlated with cardiovascular events, stroke, and dementia.

Two routes to handle Hcy:
· Remethylation back to Met (requires B12 + folate) ← this scene
· Transsulfuration to cystathionine → cysteine (requires B6) ← the other exit.

2 · MTR converts Hcy → Met

Methionine synthase uses B12 as the intermediate carrier, taking a methyl group from folate and handing it to to remake methionine, so both must be present.Methionine synthase (MTR) carries out the core reaction in the cytosol:

+ 5-methyl-THF → Met + THF *(with B12 as cofactor)*

B12 (cobalamin) on MTR binds one methyl group → forming methylcobalamin (methyl-B12) → transfers the methyl to Hcy → generates Met → B12 picks up the next methyl group.

Folate (as 5-methyl-THF) is the source of the methyl. B12 is the intermediate carrier. Both must be present at the same time — this is the chemical root of 'B12 + folate are partners'.

Clinical insight: when B12 is deficient, 5-methyl-THF accumulates (folate trap) → folate gets stuck at this step. Large amounts of folate can still correct the anemia, but not the nerve damage.

3 · SAM · universal methyl donor

The new methionine is at once turned into , the universal methyl donor that supplies nearly every methylation reaction, from DNA to neurotransmitters, and then cycles back to .Freshly made Met does not stay — it is immediately adenylated (an adenosine moiety from added) to form (S-adenosylmethionine) — the body's universal methyl donor.

SAM is the methyl source for nearly all methylation reactions:
· DNA methylation (CpG islands, gene-expression regulation)
· Histone methylation (chromatin on/off switching)
· Phosphatidylcholine synthesis (cell membranes, neural sphingomyelin)
· Neurotransmitter synthesis (dopamine → epinephrine, system)
· Creatine synthesis (muscle energy reserve)
· DNA repair.

After the reaction, SAM hands off the methyl group and becomes SAH (S-adenosyl-) → hydrolyzed → back to Hcy → enters the next cycle.

So B12 + folate = the 'turning gears' of the whole methylation system — if these two are inadequate, methylation in nearly every cell across the body is throttled.

4 · Downstream: one deficit, many gaps

Low from too little B12 or folate hits nerves, mood, vessels, fetal development and blood, and adequate folate can mask the anemia while nerve damage continues.Imagine the pool is chronically low (B12 / folate deficiency):

Nervous system:
· Myelin sphingomyelin synthesis decreases → slow nerve conduction → numbness, poor balance
· / dopamine synthesis decreases → low mood, depressive tendency (this is the chemical chain behind 'depression correlates with B12 / folate').

Circulatory system:
· accumulates → endothelial damage → elevated cardiovascular risk.

Reproductive / developmental:
· Folate insufficiency in early pregnancy → neural tube defects (NTD) → so the US Preventive Services Task Force (USPSTF) recommends that anyone planning or able to become pregnant take 400-800 µg of folic acid daily
· B12 is also involved, but folate is the established clinical intervention target.

Blood:
· Slowed DNA synthesis → impaired division of red-cell precursors → megaloblastic anemia — red blood cells grow large but mature incompletely.

Key warning: adequate folate alone + B12 deficiency → anemia is masked (folate can maintain nucleotide synthesis) but nerve damage continues → by the time symptoms appear it may be irreversible.

This is why the US / some European countries fortify grains with folate + also recommend B12 screening, and not the other way around (just supplementing folate alone).

Full mechanism, in text

1 · Homocysteine accumulates

2 · MTR converts Hcy → Met

3 · SAM · universal methyl donor

4 · Downstream: one deficit, many gaps

Methylation cycle · B12 + folate

Full mechanism, in text

1 · Homocysteine accumulates

2 · MTR converts Hcy → Met

3 · SAM · universal methyl donor

4 · Downstream: one deficit, many gaps

Methylation cycle · B12 + folate