Methylfolate (5-MTHF).

5-MTHF hands its methyl group to cobalamin, which passes it to homocysteine. Without B12 the methylfolate accumulates and cannot re-enter the cycle.

Cobalamin is the carrier that accepts the methyl group from 5-MTHF. The two are functionally inseparable at that enzyme.

MTHFR uses FAD to make 5-MTHF from its precursor, and riboflavin status affects that enzyme's stability. Supplying the finished 5-MTHF bypasses the step riboflavin governs.

PLP drives serine hydroxymethyltransferase, which loads one carbon units onto folate, and the transsulfuration exit from homocysteine. It works upstream and downstream of 5-MTHF.

Betaine homocysteine methyltransferase remethylates homocysteine without folate or B12. Running the betaine route spares 5-MTHF for other one carbon uses.

Choline is oxidised to betaine and feeds the same remethylation step, so choline and folate demand move in opposite directions. Low choline intake raises the call on folate and the other way round.

Intestinal folate deconjugase and methionine synthase both depend on zinc. Zinc status affects folate uptake and its use at the methylation step.

SAM is the product of the methylation cycle and it inhibits MTHFR as a regulatory brake. Supplying both loads one cycle at its input and output.

Serine hydroxymethyltransferase transfers a carbon from serine onto tetrahydrofolate. Serine supply governs how many one carbon units the folate pool carries.

The glycine cleavage system loads a one carbon unit onto tetrahydrofolate. Glycine and serine interconvert around the same folate carrier.

Synthetic folic acid uses the same intestinal folate carriers as 5-MTHF and must first be reduced by dihydrofolate reductase. A large folic acid dose competes with the already reduced form at the gut wall.

Reduced folates oxidise readily and ascorbate slows that loss in the stomach and in formulation. It preserves the form that is active.

Methionine synthase transfers the methyl group from 5-methyltetrahydrofolate to homocysteine, regenerating methionine. Methylfolate is the methyl donor in that reaction and methionine is the product. Supplying methionine directly and supplying the folate that regenerates it act on the same cycle from opposite ends.

Homocysteine has two fates: remethylation to methionine, which needs 5-MTHF and B12, or transsulfuration to cystathionine and then cysteine, which needs vitamin B6. Methylfolate pushes traffic toward remethylation. Cysteine sits downstream of the other branch, so the two describe a fork rather than a queue.

Taurine is the end point of the transsulfuration branch that runs from homocysteine through cysteine. Folate-dependent remethylation diverts homocysteine away from that branch. The relationship is a shared substrate pool, not an additive effect.

Nicotinamide is cleared in part by methylation to N1-methylnicotinamide, using a methyl group from S-adenosylmethionine. High nicotinamide intake therefore draws on the same methyl pool that folate and B12 regenerate. The competition is for methyl groups, and it is dose dependent.

Guanidinoacetate methyltransferase consumes a large share of the body's S-adenosylmethionine to make creatine. Dietary creatine reduces how much endogenous synthesis is needed, which lowers demand on the methyl pool that methylfolate helps regenerate. The interaction is on methyl group economy rather than on folate itself.

N-acetylcysteine supplies cysteine, the product of the transsulfuration branch of homocysteine metabolism. Methylfolate acts on the remethylation branch. Both touch homocysteine handling from different sides, which is why they appear together in methylation-focused formulations.

Folate and iron are both required for normal red blood cell formation, folate for the DNA synthesis that lets precursors divide and iron for haemoglobin itself. A shortfall in either shows up in the same tissue by a different route. The pairing is standard in prenatal and general formulations.

EGCG inhibits dihydrofolate reductase in laboratory assays, the enzyme that reduces dietary and synthetic folate toward the active pool. Methylfolate enters downstream of that step, so it is less exposed to the inhibition than folic acid is. The finding is in vitro and the magnitude at dietary intakes in people has not been established.

Pyridoxal 5-phosphate is the cofactor for serine hydroxymethyltransferase, which loads one-carbon units onto tetrahydrofolate, and for cystathionine beta-synthase on the transsulfuration branch. Without adequate B6 the folate cycle receives fewer one-carbon units and homocysteine has one fewer exit. The cofactor relationship is settled.

S-adenosylmethionine, formed from methionine and ATP, allosterically inhibits methylenetetrahydrofolate reductase, the enzyme that makes 5-MTHF. That feedback stops the cell from committing folate to methylation when methyl groups are already plentiful. Supplemental 5-MTHF bypasses the regulated step entirely, which is worth stating plainly.

Methylenetetrahydrofolate reductase is a flavoprotein that holds FAD, made from riboflavin. Where riboflavin status is low, that enzyme runs less well and less 5-MTHF is produced from the folate that is present. Supplying 5-MTHF directly makes the cell less dependent on that particular step.

Methionine adenosyltransferase, which converts methionine and ATP into S-adenosylmethionine, requires magnesium, and ATP itself is handled as a magnesium complex. Methyl group transfer downstream of methylfolate therefore depends on adequate magnesium. This is general cofactor biochemistry rather than a folate-specific finding.

The calcium salt of L-5-methyltetrahydrofolate is one of the standard stabilised forms, with calcium serving as the counter-ion that gives the molecule a workable crystalline solid. The calcium here is a formulation element in milligram-scale quantities, not a nutritional contribution. It does not change how the folate behaves once dissolved.