flavoprotein

Meaning: A flavoprotein is a protein with a tightly bound flavin prosthetic group, either FAD (flavin adenine dinucleotide) or FMN (flavin mononucleotide), both made from riboflavin (vitamin B2). The flavin ring accepts electrons, so flavoproteins catalyze redox reactions. It can take two electrons at once or one at a time through a semiquinone intermediate, so it links two-electron donors to one-electron carriers such as iron-sulfur centers. Examples: succinate dehydrogenase (FAD), NADH dehydrogenase (FMN), acyl-CoA dehydrogenase.

Equation: FAD + 2H⁺ + 2e⁻ ⇌ FADH₂, where FAD is the oxidized flavin, FADH₂ is the reduced flavin, H⁺ is a proton and e⁻ is an electron (charge −1.60 × 10⁻¹⁹ C). Amounts are in mol.

On the MCAT: Electron transport passages ask where electrons enter the chain. Succinate dehydrogenase (complex II) passes electrons from FADH₂ to ubiquinone, bypassing complex I and pumping no protons, so FADH₂ yields less ATP than NADH. Questions may also ask which vitamin supplies the cofactor.

Common confusion: FAD is not a freely diffusing shuttle like NAD⁺. It generally stays bound to its enzyme, and its electrons are passed on within the enzyme complex rather than carried to complex I. Also, complex I uses FMN, not FAD.