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Aromatic-ring-hydroxylating dioxygenases incorporate two atoms of dioxygen into their substrates in the dihydroxylation reaction. The product is cis-1,2-dihydroxycyclohexadiene, which is subsequently converted to benzene glycol by a cis-diol dehydrogenase.
A large family of multicomponent mononuclear iron oxygenases has been identified. Components of bacterial aromatic-ring dioxygenases constitute two different functional classes: hydroxylase components and electron transfer components. Hydroxylase components are either n or n oligomers. Two prosthetic groups, a Rieske-type center and a mononuclear iron, are associated with the α-subunit in the n-type enzymes. Electron transfer components are composed of flavoprotein and Rieske-type ferredoxin. In benzoate and toluate 1,2-dioxygenase systems, a single protein containing reductase and Rieske-type ferredoxin domains transfers the electrons from NADH to the hydroxylase component. In the phthalate 4,5-dioxygenase system, phthalate dioxygenase reductase has the same function. PDR is a single protein comprising FMN-binding reductase and plant-type ferredoxin domains. Thus, the electron transfer in ARHD systems can be summarised as: