Only certain bacteria and archaea can make vitamin B12, the most complex vitamin
Vitamin B12 is the most chemically intricate of all vitamins, built around a cobalt atom, and no plant or animal can manufacture it: only certain bacteria and archaea do. Animals need it for DNA synthesis, myelin and red blood cells, while plants get by with other enzyme pathways.
Also called cobalamin, B12 is water-soluble and one of eight B vitamins. Its cobalt sits at the centre of a ring-shaped corrin ligand, with further bonds to a benzimidazole group and an adenosyl group. Swap that adenosyl group and you get relatives: cyanide gives cyanocobalamin, hydroxide gives hydroxocobalamin, and a methyl group gives methylcobalamin. Inside cells, the working forms are methylcobalamin in the cytosol and adenosylcobalamin in the mitochondria.
Cyanocobalamin is a manufactured form. Bacterial fermentation yields the natural versions, which are then treated with potassium cyanide, sodium nitrite and heat; the cyanide stops the molecule degrading, which is why this version dominates supplements and fortified cereals. The body converts it back into the active forms. Hydroxocobalamin has a neat second job: given intravenously, it treats cyanide poisoning by trading its hydroxyl group for cyanide, forming harmless cyanocobalamin that leaves in urine.
Not everything that resembles B12 works. Pseudovitamin B12, structurally similar but inactive, is the main corrinoid in spirulina, an algal supplement sometimes wrongly promoted as a source. Antivitamin compounds go further and block the real thing; they are designed by replacing cobalt with rhodium, nickel or zinc, and could help researchers probe B12 pathways or attack pathogens that depend on it.
Natural sources are animal foods such as meat, fish, eggs and dairy. Healthy adults rarely run short because the body stores plenty and uses it slowly, but deficiency among older people is a significant concern linked to dementia, most often because the stomach loses intrinsic factor, which absorption requires. Shortage can cause megaloblastic and pernicious anaemia, neuropathy and, left untreated, irreversible nerve damage.
Source: Vitamin B12
