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Flavin Adenine Dinucleotide (FAD) Figure 28.25 • Bioactivation of riboflavin to FMN and FAD.

Figure 28.25. FMN is actually named incorrectly because it is not a nucleotide. The metabolic functions of this vitamin involve these two enzyme cofactors, which participate in numerous, vital oxidation-reduction processes. FMN (ri-boflavin 5'-phosphate) is produced from the vitamin and ATP

by riboflavin kinase (EC 2.7.1.26). Amitriptyline, imipramine, and chlorpromazine can inhibit riboflavin kinase resulting in a functional riboflavin deficiency. FAD originates from an FMN and ATP reaction that involves reversible din-ucleotide formation catalyzed by FAD synthetase (EC 2.7.7.2). FMN and FAD function in combination with many enzymes, often characterized as flavoproteins, as the prosthetic groups of their coenzymes and rarely as the actual coen-zyme. In some instances, the FAD or FMN is found cova-lently bound to the enzyme.

Flavoproteins function under aerobic or anaerobic conditions as oxidases and dehydrogenases. Examples include glucose oxidase, xanthine oxidase, cytochrome reductase, and acyl-CoA dehydrogenase. The riboflavin moiety of the complex is considered a hydrogen-transporting agent functioning as a hydrogen acceptor. The hydrogen donors may be NADH, NADPH, or other suitable substrate. The isoal-loxazine rings accept two hydrides stepwise to form the di-hydroriboflavin derivative (Fig. 28.26).

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