The first observable intermediate, C(4a)-hydroperoxy flavin spectrum with λmax of ~370-390 nm (15, 16) while the second intermediate, C(4a)-hydroxy flavin typically absorbs at slightly shorter wavelength. These intermediate were also observed in the reaction of other enzymes in this class such as phenol hydroxylase, 2-methyl-3-hydroxypyridine-5-carboxylic acid monooxygenase (17), salicylate hydroxylase (11), anthranilate hydroxylase (12), melilolate hydroxylase (18). C(4a)-hydroperoxy flavin is an hydroxylating reaction in which its terminal oxygen is incorporated into the substrate. C(4a)-hydroxy flavin, the intermediate resulted from the hydroxylation step, dehydrates at the final step to return to the oxidized flavin species to complete the catalytic cycle (7, 8). However, the uncoupled pathway, where the C(4a)-hydroperoxy flavin eliminates hydrogen peroxide without hydroxylating substrate, also as a minor path (7, …show more content…
Flavin reductases catalyze the reduction of flavin by NAD(P)H and have been thought to be the main supplier providing free reduced flavin for the oxygenase component (19, 20, 21). The flavin reductases have been classified into two classes (22) (Scheme 4): class I includes the reductases containing a tightly bound flavin as a cofactor and class II includes the reductases having no cofactor bound to the enzyme. In the reaction of class I enzymes, the enzyme-bound flavin is initially reduced by NAD(P)H and later transfer electrons to a free flavin substrate. In the reaction of class II reductase, the reduction of flavin requires that both substrates form a ternary complexe since there is no redox cofactor on the enzyme. The reduced flavin products from both types of reductases were found involved in oxygenation of aromatic compounds (20, 23, 24) in light-emitting reaction of bacterial luciferase (25, 26), in reduction of ribonucleotide reductase (27), in the release of iron from ferrisiderophore (28), and in the formation of superoxide radical