Vitamins themselves do no chemistry — enzymes convert them into coenzymes with a reactive chemical "handle" that the protein's active site can't build from a plain amino acid. The enzyme holds substrate and coenzyme close together in the active site; the coenzyme's reactive ring picks up a group from the substrate, carries it across the pocket, and hands it to the acceptor, cycling between oxidized and reduced (or loaded/unloaded) forms.
B1 thiamine → TPP : thiazole ylide carbanion carries an aldehyde (–CHO)
B2 riboflavin→ FAD : isoalloxazine ring accepts 2 e⁻ + 2 H⁺ → FADH2
B3 niacin → NAD⁺ : nicotinamide C4 accepts a hydride (H⁻) → NADH
- TPP — in pyruvate decarboxylation the thiazolium ring's C2 carbanion attacks the carbonyl of pyruvate, releases CO₂, and carries the remaining acetaldehyde group until it is handed off.
- FAD — the flavin ring's two nitrogens accept a hydride/electron pair from a substrate C–H bond during oxidation steps of the respiratory chain, becoming FADH2, then reoxidizing at the electron-transport chain.
- NAD⁺/NADH — the nicotinamide ring's para carbon (C4) accepts a hydride ion directly; this single ring flip is the most common hydride-transfer step in all of metabolism.
- Enzyme + coenzyme toggle — turn it off to see the same bond-breaking step attempted without a coenzyme: the energy diagram's barrier rises sharply because there is no reactive ring to stabilise the transition state.
Real-world relevance: this is why B-vitamin deficiency is a metabolic disease, not just a nutrition footnote — without thiamine, pyruvate cannot be decarboxylated and central carbon metabolism stalls at exactly this step.