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

# FAD (flavin adenine dinucleotide)

FAD is the riboflavin-derived coenzyme that collects electrons in the Krebs cycle and β-oxidation and hands them to ubiquinone.

Updated 2026-11-01

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## Key facts

- It accepts 2 electrons and 2 protons (FAD → FADH₂); unlike NAD⁺, it usually stays bound to its enzyme as a prosthetic group.
- It is made from riboflavin (vitamin B2), as is the FMN of complex I.
- The FADH₂ of succinate dehydrogenase passes its electrons on in complex II and yields about 1.5 ATP.
- PDH (E3), α-ketoglutarate dehydrogenase, acyl-CoA dehydrogenase and glutathione reductase also use it.

**Also called**
FADH₂ (reduced form)

**Formula**
C₂₇H₃₃N₉O₁₅P₂

**PubChem**
[CID 643975](https://pubchem.ncbi.nlm.nih.gov/compound/643975)

## Structure

- An isoalloxazine ring (riboflavin, the working part) joined through ribitol and two phosphates to an AMP.

## Clinical notes

- Riboflavin deficiency causes angular cheilitis, glossitis and seborrheic dermatitis.
- Red-cell glutathione reductase activity is used to assess riboflavin status.
- Flavins are light-sensitive: newborn phototherapy can lower riboflavin.

## Pathways

- [Electron transport chain and oxidative phosphorylation](https://durgical.com/en/biochemistry/pathways/electron-transport-chain-and-oxidative-phosphorylation/)
- [Krebs cycle (citric acid cycle)](https://durgical.com/en/biochemistry/pathways/krebs-cycle/)

## Related conditions

- [Hereditary paraganglioma and pheochromocytoma (SDH)](https://durgical.com/en/conditions/hereditary-paraganglioma-and-pheochromocytoma/)

## Related

- [Succinate dehydrogenase (complex II)](https://durgical.com/en/biochemistry/enzymes/succinate-dehydrogenase/)
- [NAD⁺ (nicotinamide adenine dinucleotide)](https://durgical.com/en/biochemistry/molecules/nad/)
- [Ubiquinone (coenzyme Q₁₀)](https://durgical.com/en/biochemistry/molecules/ubiquinone/)
- [Pyruvate dehydrogenase complex (PDH)](https://durgical.com/en/biochemistry/enzymes/pyruvate-dehydrogenase-complex/)
- [Complex I (NADH dehydrogenase)](https://durgical.com/en/biochemistry/enzymes/complex-i/)
- [Glutathione (GSH)](https://durgical.com/en/biochemistry/molecules/glutathione/)

## Test yourself

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

## Which vitamin is FAD made from?

Riboflavin (vitamin B2).

## Why does FADH₂ yield less ATP than NADH?

Its electrons reach ubiquinone through complex II and skip complex I's pumping: about 1.5 ATP versus 2.5.

## Which Krebs cycle enzyme uses bound FAD?

Succinate dehydrogenase (complex II).

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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: [Flavin Adenine Dinucleotide (CID 643975)](https://pubchem.ncbi.nlm.nih.gov/compound/643975) (PubChem, NCBI, 2026) · Public domain; [Riboflavin: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002411.htm) (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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