Step by Step
FAD
Riboflavin forms FAD and FMN
Riboflavin (B2) is the precursor for two essential coenzymes: FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide). Both are electron carriers — they accept and donate two electrons (and two protons) in redox reactions. FAD → FADH₂ (reduced form), which donates electrons to the ETC at Complex II → CoQ.
Memory trick: Riboflavin = 'Ribbed and Flavored' electron carrier. FAD = the workhorse.
Role
Key roles in metabolism
FAD/FMN are required for: Complex I (FMN) and Complex II (FAD) of the ETC. Fatty acid oxidation (acyl-CoA dehydrogenase — first step of beta-oxidation, uses FAD). Pyruvate dehydrogenase complex (dihydrolipoamide dehydrogenase subunit uses FAD). Succinate dehydrogenase (Krebs cycle, Complex II). Amino acid oxidation.
Def
Deficiency syndrome — the 3 Cs
Riboflavin deficiency (ariboflavinosis) causes: Angular cheilitis (cracking at mouth corners), Cheilosis (lip inflammation), Corneal vascularization (blood vessel growth into cornea). Also: glossitis (red, swollen tongue — 'magenta tongue'), seborrheic dermatitis, normocytic anemia. Common in alcoholism and malnutrition.
Memory trick: B2 deficiency = '2 lips and 2 eyes' — angular cheilitis + corneal changes.
Src
Sources and stability
Sources: dairy products, eggs, lean meats, fortified cereals, green vegetables. Riboflavin is light-sensitive — milk stored in opaque containers to prevent degradation. Relatively heat-stable (unlike vitamin C). Excreted in urine — gives urine a bright yellow color (harmless), especially with supplements.
Applied Walkthrough
1
An alcoholic with poor nutrition develops cracking at the corners of the mouth (angular cheilitis), a smooth red tongue (glossitis), and corneal vascularization — classic riboflavin deficiency.
2
FAD is depleted → Complex II of the ETC and acyl-CoA dehydrogenase (beta-oxidation) are impaired → reduced energy production from fats.
3
Supplementation with riboflavin resolves the mucosal and corneal symptoms within weeks.
4
In the lab: measuring erythrocyte glutathione reductase activity (an FAD-dependent enzyme) — increased activity after FAD addition = functional riboflavin deficiency.
Exam Application
Exams test riboflavin's coenzyme forms (FAD and FMN), their role in the ETC (Complex I uses FMN, Complex II uses FAD), the clinical features of deficiency (angular cheilitis, glossitis, corneal vascularization), and that riboflavin is light-sensitive. The connection between FAD and fatty acid oxidation (acyl-CoA dehydrogenase) is also testable.
⚠ Common Trap
Students confuse FAD (from riboflavin/B2) with NAD⁺ (from niacin/B3) — both are electron carriers in the ETC but they enter at different points (FADH₂ → Complex II → CoQ; NADH → Complex I). FADH₂ produces less ATP (~1.5) than NADH (~2.5) because it bypasses Complex I's proton pumping.
✓ Quick Self-Check
1. What two coenzymes does riboflavin (B2) form?
FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide).
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2. Name two metabolic pathways that require FAD/FMN.
Any two of: ETC (Complex I uses FMN, Complex II uses FAD), fatty acid beta-oxidation (acyl-CoA dehydrogenase), Krebs cycle (succinate dehydrogenase), pyruvate dehydrogenase complex.
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3. What are the classic signs of riboflavin deficiency?
Angular cheilitis (mouth corner cracking), cheilosis, corneal vascularization, glossitis (magenta tongue), seborrheic dermatitis.
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4. Why does riboflavin deficiency cause less ATP production from fat?
Acyl-CoA dehydrogenase (first step of beta-oxidation) requires FAD — without it, fatty acids cannot enter the beta-oxidation cycle efficiently.
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5. Why should milk be stored in opaque containers?
Riboflavin is light-sensitive — UV light degrades it, reducing the vitamin content of milk.
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