Aromatic-ring-hydroxylating dioxygenases
Aromatic-ring-hydroxylating dioxygenases (ARHD) incorporate two atoms of dioxygen (O2) into their substrates in the dihydroxylation reaction. The product is (substituted) cis-1,2-dihydroxycyclohexadiene, which is subsequently converted to (substituted) benzene glycol by a cis-diol dehydrogenase.
A large family of multicomponent mononuclear (non-heme) 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 (αβ)n or (α)n oligomers. Two prosthetic groups, a Rieske-type [Fe2S2] center and a mononuclear iron, are associated with the α-subunit in the (αβ)n-type enzymes. Electron transfer components are composed of flavoprotein (NADH:ferredoxin oxidoreductase) and Rieske-type [Fe2S2] 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 (PDR) 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:
EC 1.14.12.3 benzene 1,2-dioxygenase
EC 1.14.12.7 phthalate 4,5-dioxygenase
EC 1.14.12.8 4-sulfobenzoate 3,4-dioxygenase
EC 1.14.12.9 4-chlorophenylacetate 3,4-dioxygenase
EC 1.14.12.10 benzoate 1,2-dioxygenase
EC 1.14.12.11 toluene dioxygenase
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