Arsenic, a toxic metalloid is naturally found in the environment but can exist as a harmful pollutant generated from industrial waste waters and gold mines. This poses a great threat to human health by contaminating the ground water system. Arsenic can exist in both organic and inorganic forms and in four oxidation states, arsines and methyl arsines (As3-), elemental arsenic (As0), arsenite (AsO33-) and arsenate (AsO43-). Although arsenic is toxic and hazardous to human health, some prokaryotes have developed unique mechanisms that utilize inorganic forms of arsenic, such as arsenite (AsO33-) and arsenate (AsO43-) for respiration.
The organism Psuedorhizobium sp. str. NT-26 respires with arsenite and employs the arsenite oxidase (AioAB) for its crucial respiratory activity, which catalyzes the oxidation of arsenite to arsenate. The AioAB enzyme consists of two subunits: AioA (contains a molybdenum center and 3Fe-4S cluster) and AioB (contains a Rieske [2Fe-2S] cluster). Arsenite is oxidized to arsenate at the Mo site, concomitantly reducing Mo(VI) to Mo(IV). The electrons are then passed to the 3Fe-4S cluster, the Rieske cluster in AioB and to an electron acceptor, which is cytochrome c552 (cytc552). The crystal structure of the AioAB/cytc552 electron transfer complex reveals two A2B2/(cytc552)2 assemblies per asymmetric unit. Three of the four cytc552 molecules in the asymmetric unit dock with AioAB in a cleft at the interface between the AioA and AioB subunits, with an edge-to-edge distance of 7.5 Å between the heme of cytc552 and the [2Fe-2S] Rieske cluster in the AioB subunit. The interface between the AioAB and cytc552 proteins features electrostatic and non-polar interactions and is stabilized by two salt-bridges. This presentation will discuss the transient and catalytically efficient nature of the AioAB/cytc552 complex that underpins the ability of this organism to respire using the arsenite present in contaminated environments.