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The structures of riboflavin,flavin mononucleotide(FMN),and flavin adenine dinucleotide(FAD).Flavin coenzymes bind tightly to the enzymes that use them,with typical dissociation constants in the range of 10-8to10-11M,so that only very low levels of free flavin coenzymes occur in most cells.Even in organisms that rely on the nicotinamide coenzymes(NADH and NADPH)for many of their oxidation-reduction cycles,the flavin coenzymes fill essential roles.Flavins are stronger oxidizing agents than NAD+ and tron and two-electron pathways and can be reoxidized easily by molecular oxygen.Enzymes that use flavins to carry out their reactions-flavoenzymes-are involved in many kinds of oxidation-reduction reactions.