Metabolic functions of the flavin coenzymes
The metabolic function of the flavin coenzymes is as electron carriers in a wide variety of oxidation and reduction reactions central to all metabolic processes, including the mitochondrial electron transport chain, and key enzymes in fatty acid and amino acid oxidation, and the citric acid cycle. The flavin coenzymes remain bound to the enzyme throughout the catalytic cycle. The majority of flavoproteins have FAD as the prosthetic group rather than riboflavin phosphate; some have both flavin coenzymes and some have other prosthetic groups as well.
Flavins can undergo a one-electron reduction to the semiquinone radical or a two-electron reduction to dihydroflavin. In some enzymes formation of dihy-droflavin occurs by two single-electron steps, with intermediate formation of the semiquinone radical. Dihydroflavin can be oxidized by reaction with a substrate, NAD(P)+, or cytochromes in a variety of dehydrogenases, or can react with molecular oxygen in oxygenases and mixed function oxidases (hydroxylases).
Reoxidation of the reduced flavin in oxygenases and mixed function oxidases proceeds by way of formation of the flavin radical and flavin hydroperoxide, with the intermediate generation of superoxide and perhy-droxyl radicals and hydrogen peroxide. Because of this, flavin oxidases make a significant contribution to the total oxidant stress of the body. Overall, some 3–5% of the daily consumption of about 30 mol of oxygen by an adult is converted to singlet oxygen, hydrogen peroxide, and superoxide, perhydroxyl, and hydroxyl radicals, rather than undergoing complete reduction to water in the electron transport chain. There is thus a total production of some 1.5 mol of reactive oxygen species daily, potentially capable of causing damage to membrane lipids, proteins, and nucleic acids.