Riboflavin (vitamin B2) !

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

Updated 2026-11-01

Loading the 3D model…

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.

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.

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.

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.

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.

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) (PubChem, NCBI, 2026) · Public domain; Riboflavin: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Riboflavin transporter deficiency neuronopathy: MedlinePlus Genetics (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).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.