Full mechanism, in text

Two electron-carrying parts

1 · Riboflavin → FMN / FAD

Vitamin B2 (riboflavin) exists in food as free riboflavin + protein-bound FMN/FAD — milk, yogurt, eggs, liver, lean meat, almonds, and leafy greens are the main sources.After absorption it enters cells and is enzymatically phosphorylated in two steps:

· Riboflavin + → FMN (flavin mononucleotide) — catalysed by RFK (riboflavin kinase)
· FMN + ATP → FAD (flavin adenine dinucleotide) — catalysed by FADS (FAD synthetase)

Neither FMN nor FAD is an 'electron shuttle' — they're 'electron handles' that stay tightly bound to enzymes, usually through covalent bonds (FAD to enzyme His or Cys) or very tight non-covalent binding, and rarely dissociate. This differs from /NADH (NAD⁺ is a 'separable co-substrate').

Chemical magic: the FMN/FAD center is an isoalloxazine ring — same position:

· Fully oxidised FAD (yellow — this is why riboflavin = yellow in the name)
· Take 1 e⁻ → FADH• (semiquinone, blue/red)
· Take 2 e⁻ → FADH₂ (colourless, fully reduced)

Key: FAD can accept one electron at a time or two — this makes it more flexible than NAD⁺, allowing it to serve as a bridge in 'one-electron transfer reactions' (radical chemistry, interfacing with iron-sulfur clusters / heme).

The human genome encodes roughly 80+ flavoprotein enzymes.

2 · Complex II · succinate → fumarate → Q

In Complex II (succinate dehydrogenase), FAD takes two electrons from succinate and passes them to coenzyme Q without pumping H⁺, so each FADH₂ yields less than an .FAD has its own entry point into the mitochondrial electron transport chain: Complex II = succinate dehydrogenase (SDH) — the only cycle enzyme that's simultaneously membrane-bound + participates in the ETC.

Reaction: succinate → fumarate, FAD takes two e⁻ → FADH₂; then FADH₂ in situ hands electrons to membrane coenzyme Q → downstream Complex III, IV.

Difference vs the entry:
· NADH enters via Complex I, pumps 4 H⁺ along the way
· FADH₂ enters via Complex II, doesn't pump H⁺ (Complex II has no proton pump function)
· Therefore 1 NADH ≈ 2.5 , while 1 FADH₂ ≈ 1.5 ATP — missing the Complex I proton contribution

SDH = a 'tumour suppressor gene': SDH mutations (SDHA/B/C/D) → succinate accumulation → inhibits prolyl hydroxylase → HIF-1α stabilisation → pseudo-hypoxia response → pheochromocytoma / paraganglioma (familial). This is a classic metabolic enzyme = tumour suppressor case.

FAD has a second ETC role: ETF-QO (electron transfer flavoprotein-Q oxidoreductase) — pools electrons from fatty acid β-oxidation + branched-chain amino acid metabolism into the Q pool. The acyl-CoA → trans-Δ²-enoyl-CoA step of each β-oxidation cycle is where FAD picks up the electrons.

3 · FAD also at dozens of other posts

Beyond Complex II, FAD sits on many enzymes as an electron switch that never leaves, working in energy metabolism, antioxidant defence, neurotransmitter breakdown and the circadian clock.Complex II is just one of FAD's posts — its range is far broader. Important flavoproteins:

Mitochondrial / energy metabolism:
· — reduces 5,10-CH₂-THF → 5-CH₃-THF (the bottleneck enzyme covered in the folate story; uses FAD to transfer electrons from to substrate)
· DLD (dihydrolipoyl dehydrogenase) — the E3 subunit of the PDH/α-KGDH complex; uses FAD to transfer electrons from the lipoyl arm to
· GR (glutathione reductase) — uses FAD to transfer electrons from NADPH to GSSG → 2 GSH (covered in the selenium / antioxidant story)
· TR (thioredoxin reductase) — maintains the thioredoxin system in its reduced state

Neurotransmitter / detoxification:
· MAO-A / MAO-B (monoamine oxidase) — deaminates and degrades dopamine, serotonin, norepinephrine — MAO inhibitors are classic antidepressants
· DAAO / DDO — degrade D-amino acids / D-aspartate
· FMO (flavin monooxygenase) — Phase I drug metabolism, processes nitrogen- / sulfur-containing drugs

Special sensory mechanism:
· Cryptochrome — core component of avian magnetoreception, insect UV vision, and the human circadian clock; its FAD generates a radical pair upon light absorption → couples to the geomagnetic field → chemical basis of bird navigation (active research area)

So FAD and FMN are 'electron switches that don't escape from the enzyme' — unlike NADH which roams the cytosol, they stay on the enzyme forever as a precisely tunable dial: position 1 (semiquinone) lets the enzyme do one-electron chemistry, position 2 (FADH₂) lets it do classical two-electron chemistry.

4 · Deficiency & migraine prophylaxis

Low B2 causes mild signs across many systems, and in a trial 400 mg of riboflavin a day beat placebo at cutting migraine headache days by 50% or more.B2 deficiency (ariboflavinosis) clinical features — because FAD spans energy, antioxidant, detox, and neurotransmitter lines, the presentation is 'multi-system mild symptoms':

· Angular cheilitis — red, cracked, crusted corners of the mouth — classic sign
· Glossitis — smooth-red tongue, pain, taste changes
· Cheilosis — red, swollen, cracked lips
· Ocular discomfort — photophobia, tearing, conjunctival congestion, increased cataract risk
· Seborrhoeic dermatitis — nasolabial fold, facial seborrhea
· Normochromic normocytic anaemia (FAD is involved in red cell metabolism)

Migraine prophylaxis — a clinical surprise:

Schoenen 1998 Neurology (N=55): riboflavin 400 mg/day × 3 months vs placebo —
· Proportion with ≥ 50% fewer headache days: riboflavin 59% vs placebo 15% ( 2.3)
· Multiple subsequent RCTs + Cochrane review support it as a second-line prophylactic
· Mechanism hypothesis: migraine patients have mitochondrial energy metabolism dysfunction; high-dose B2 improves ETC efficiency
· Very few side effects (yellow urine is a FAD metabolite, not a problem)

: men 1.3 mg / women 1.1 mg / pregnancy 1.4 mg / lactation 1.6 mg
No set: water-soluble, virtually non-toxic

Practical:
· Migraine patients: 400 mg/day is the RCT-supported dose; the typical 100-200 mg in a B complex is not enough
· General population doesn't need to supplement alone — a cup of milk has ~ 0.4 mg, breakfast cereals are commonly fortified
· UV light destroys riboflavin (this is why milk in clear bottles loses B2 to light) — choose opaque containers

Full mechanism, in text

1 · Riboflavin → FMN / FAD

2 · Complex II · succinate → fumarate → Q

3 · FAD also at dozens of other posts

4 · Deficiency & migraine prophylaxis

Riboflavin · FAD/FMN electron switch

Full mechanism, in text

1 · Riboflavin → FMN / FAD

2 · Complex II · succinate → fumarate → Q

3 · FAD also at dozens of other posts

4 · Deficiency & migraine prophylaxis

Riboflavin · FAD/FMN electron switch