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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Riboflavin (vitamin B2)

Riboflavin forms FAD and FMN, the electron carriers of many oxidation reactions in the cell.

Updated 2026-11-01

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## Key facts

- Active forms: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD); each accepts two hydrogens.
- FAD: succinate dehydrogenase (complex II), acyl-CoA dehydrogenases, dihydrolipoyl dehydrogenase (E3 of PDH and α-ketoglutarate dehydrogenase), glutathione reductase.
- FMN: complex I of the respiratory chain.
- Deficiency (ariboflavinosis): angular cheilitis, magenta glossitis, seborrhoeic dermatitis and corneal vascularisation.

## In detail

- Sources: milk and dairy, eggs, meat, organ meats, leafy greens, fortified cereals. Light destroys it (which is why milk is packed in opaque containers).
- Activation: flavokinase makes FMN and FAD synthetase adds AMP to make FAD; thyroid hormone stimulates these enzymes.
- Function: the isoalloxazine ring takes two electrons and two protons (FADH₂, FMNH₂) and hands them to the respiratory chain or another acceptor.
- Flavins are also needed to activate pyridoxine to PLP, to convert tryptophan to niacin, and by methylenetetrahydrofolate reductase (MTHFR).
- Deficiency: rarely isolated; it accompanies other B-complex deficits, alcoholism and some drugs (chlorpromazine, tricyclic antidepressants).
- Excess: not toxic; the surplus turns urine bright yellow.
- Lab: red-cell glutathione reductase activation coefficient (rises when FAD is lacking) or urinary riboflavin.

**Also called**
Vitamin B2, FAD, FMN

**Formula**
C₁₇H₂₀N₄O₆

**PubChem**
[CID 493570](https://pubchem.ncbi.nlm.nih.gov/compound/493570)

## Structure

- A tricyclic isoalloxazine ring (the part that carries electrons and gives the yellow colour) joined to a ribitol chain.
- FMN = riboflavin + phosphate; FAD = FMN + AMP.

## Clinical notes

- Cracks at the corners of the mouth, a red tongue and dermatitis around the nose point to B2 deficiency, though iron and B6 deficits cause them too.
- Riboflavin transporter deficiency (SLC52A2 and SLC52A3 genes) causes a severe neuropathy that responds to high-dose riboflavin.
- Glutaric acidaemia type II (an electron-transfer flavoprotein defect) can respond to riboflavin.

## Pathways

- [Krebs cycle (citric acid cycle)](https://durgical.com/en/biochemistry/pathways/krebs-cycle/)

## Related

- [Succinate dehydrogenase (complex II)](https://durgical.com/en/biochemistry/enzymes/succinate-dehydrogenase/)
- [Pyruvate dehydrogenase complex (PDH)](https://durgical.com/en/biochemistry/enzymes/pyruvate-dehydrogenase-complex/)
- [α-Ketoglutarate dehydrogenase](https://durgical.com/en/biochemistry/enzymes/ketoglutarate-dehydrogenase/)
- [Vitamin B6 (pyridoxal phosphate)](https://durgical.com/en/biochemistry/molecules/vitamin-b6/)
- [Niacin (vitamin B3)](https://durgical.com/en/biochemistry/molecules/niacin/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which coenzymes are made from riboflavin?

FMN (riboflavin phosphate) and FAD (FMN joined to AMP).

## Which Krebs cycle enzyme has covalently bound FAD?

Succinate dehydrogenase, which is also complex II of the respiratory chain.

## Which clinical signs suggest riboflavin deficiency?

Angular cheilitis, stomatitis, magenta glossitis, seborrhoeic dermatitis and corneal vascularisation.

## Which functional test assesses riboflavin?

Activation of red-cell glutathione reductase by added FAD: a large rise means the vitamin was lacking.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Riboflavin (CID 493570)](https://pubchem.ncbi.nlm.nih.gov/compound/493570) (PubChem, NCBI, 2026) · Public domain; [Riboflavin: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002411.htm) (MedlinePlus, US National Library of Medicine, 2026); [Riboflavin transporter deficiency neuronopathy: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/riboflavin-transporter-deficiency-neuronopathy/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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