page_title: Dioxygenase
section_title: Rieske dioxygenases
hierarchical_section_title: Dioxygenase / Iron-containing enzymes / Rieske dioxygenases
caption_attribution_description: English: Active site of naphthalene 1,2-dioxygenase from Rhodococcus sp.(PDB 2B1X)
context_page_description: Dioxygenases are oxidoreductase enzymes. Aerobic life, from simple single-celled bacteria species to complex eukaryotic organisms, has evolved to depend on the oxidizing power of dioxygen in various metabolic pathways. From energetic adenosine triphosphate generation to xenobiotic degradation, the use of dioxygen as a biological oxidant is widespread and varied in the exact mechanism of its use. Enzymes employ many different schemes to use dioxygen, and this largely depends on the substrate and reaction at hand.
context_section_description: The Rieske dioxygenases catalyze the cis-dihydroxylation of arenes to cis-dihydro-diol products. These enzymes are prominently found in soil bacteria such as Pseudomonas, and their reactions constitute the initial step in aromatic hydrocarbon biodegradation. Rieske dioxygenases are structurally more complex than other dioxygenases due to the need for an efficient electron transfer pathway (figure 2) to mediate the additional, simultaneous two-electron reduction of the aromatic substrate. 
  A catalytically-competent Rieske dioxygenase has three components: an NADH-dependent FAD reductase, a ferredoxin with two [2Fe-2S] Rieske clusters, and an α3β3 oxygenase with each α-subunit containing a mononuclear iron center and a [2Fe-2S] Rieske cluster. Within each α-subunit, the iron-sulfur cluster and mononuclear iron center are separated by a distance of some ~43 Å, much too far for efficient electron transfer to occur. Instead, it is proposed electron transfer is mediated through these two centers in adjacent subunits, that the iron-sulfur cluster of one subunit transfers electrons to the mononuclear iron center of the adjacent subunit which is conveniently separated by ~12 Å. While this distance would appear optimal for efficient electron transfer, replacement of the bridging aspartate residue causes a loss of enzyme function, suggesting that electron transfer instead proceeds through the hydrogen-bonding network held in place by this aspartate residue. 
  The mechanistic picture for this class of dioxygenases is not yet clear, but there is evidence supporting an iron(III) hydroperoxy intermediate in the reaction pathway. This species could represent the active oxidant, or it could undergo hemolytic O-O bond cleavage to yield an iron(V)-oxo intermediate as the working oxidizing agent. The Rieske dioxygenase are a powerful class of redox-active enzymes, and reactions such as sulfoxidation, desaturation, and benzylic oxidation have been reported in addition to dioxygenation.
caption: Active site of Rieske dioxygenase (naphthalene 1,2-dioxygenase from Rhodococcus sp.) (PDB 2B1X)
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