Cobalt (B12 Component).

Methionine synthase hands the methyl group from 5-methyltetrahydrofolate to cobalamin and then to homocysteine. Without B12 the folate pool sits stuck in the methyl form, so the two are read together.

Folate and B12 share the same single reaction, so a generous folate intake can mask thin B12 in the blood picture while the coenzyme role in nerve tissue goes unmet. They belong on the same label.

B12 and folate send homocysteine back to methionine while B6 lets it leave through transsulfuration. Together they cover both exits from the same junction.

Methionine synthase reductase uses an FAD cofactor to re-reduce the cobalt centre after it oxidises. Riboflavin status therefore governs how long each cobalamin molecule stays catalytically active.

BHMT remethylates homocysteine using betaine, independent of the cobalamin-dependent enzyme. It carries load in the liver when the B12 route is limited.

Cobalamin works as two coenzymes, the methyl form for methionine synthase and the adenosyl form for methylmalonyl-CoA mutase in mitochondria. Supplying both covers each site directly.

Propionyl-CoA carboxylase is biotin dependent and feeds methylmalonyl-CoA to the cobalamin-dependent mutase. Odd-chain and branched-chain carbon passes through both enzymes in sequence.

Carnitine buffers accumulating propionyl groups as propionylcarnitine when the cobalamin-dependent mutase step is slow. The two intersect on the same mitochondrial acyl pool.

Ileal uptake of the intrinsic factor and cobalamin complex by cubilin is calcium dependent. Thin luminal calcium reduces that receptor-mediated step.

High-dose ascorbate degrades cobalamin in solution by reducing and displacing the cobalt centre. Formulators separate the two in the mix or in time rather than co-dissolving them.

B12 and folate drive nuclear maturation of the red cell line while iron builds the haemoglobin inside it. Supplying only one leaves the other as the ceiling on normal red cell formation.

Cobalt is the metal ion at the centre of the corrin ring, and cobalamin is the only form in which cobalt has a recognised function in human biochemistry. Take the cobalt out and there is no vitamin. That relationship is definitional rather than a discovered interaction.

Methylcobalamin is cobalamin carrying a methyl group on the upper axial position of the cobalt atom, and it is the cofactor methionine synthase uses to hand a methyl group to homocysteine. The cobalt is the atom the methyl group binds to and leaves from. Nothing about that step works without the metal centre.

Hydroxocobalamin carries a hydroxyl group on the cobalt, and cells convert it to the methyl and adenosyl forms as needed. It is the form closest to what occurs naturally in tissue. Again the cobalt atom is the reactive centre that accepts and releases the upper ligand.

Only bacteria and archaea can build the corrin ring and insert cobalt into it; no plant or animal makes B12. Some gut and food-fermentation bacteria produce cobalamin and related corrinoids from inorganic cobalt. Human colonic synthesis sits distal to the ileal receptor site that absorbs B12, so what is made there is not straightforwardly taken up.

Dietary cobalt was reported to suppress starch-driven succinate accumulation in the colon by supporting bacterial vitamin B12 synthesis, in animal work. The measurement is a colonic metabolite in animals, not a human outcome. It illustrates that cobalt availability shapes what the microbial community does with a fermentable substrate.

Inorganic cobalt is taken up in part through the divalent metal transporter DMT1, the same route copper, iron, manganese and zinc use. High intake of one divalent mineral can reduce uptake of another when they arrive together. This applies to inorganic cobalt salts, not to cobalt already locked inside cobalamin, which is absorbed by an entirely separate intrinsic-factor pathway.

Zinc and inorganic cobalt compete for the same divalent transporter at the enterocyte brush border. The interaction concerns inorganic cobalt salts used in fortification and animal feed rather than the cobalt in a B12 supplement. Once cobalt is inside a corrin ring it no longer travels this route.

Manganese also moves through DMT1 and so belongs on the list of divalent minerals that share the route with inorganic cobalt. The practical size of the competition at supplement intakes is not well characterised. Early is the honest label.

Histidine's imidazole nitrogen coordinates transition metals strongly, and histidine is one of the ligands used to characterise cobalt binding in solution. Amino acid chelates are used commercially to present trace minerals in a bound rather than a free ionic form. The chelate changes how the mineral behaves in the gut lumen rather than what it does once absorbed.

Glycinate chelates are the common commercial way to supply trace minerals in a form that resists precipitation by phytate and other lumenal binders. Cobalt glycinate belongs to that family, used chiefly in feed and fortification. The chelate is a delivery decision, not a change to the mineral itself.

Reviews of trace mineral status in cattle group copper, selenium, zinc, cobalt and iron together because they are supplied and assessed as a package. That grouping is a supply convention, not a mechanistic interaction between selenium and cobalt. The work is in livestock and does not carry across to human dosing.

Fermentable fibre changes which bacteria dominate the colon and how much corrinoid-producing activity is present there. Cobalt is the metal those organisms need to build corrinoids. The connection is plausible from microbial biochemistry and is not demonstrated as a human nutritional effect.