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.

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

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.

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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).

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) (PubChem, NCBI, 2026) · Public domain; Riboflavin: MedlinePlus Medical Encyclopedia (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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