The structure of the complex between the arsenite oxidase from Pseudorhizobium banfieldiae sp. strain NT-26 and its native electron acceptor cytochrome c.

The arsenite oxidase (AioAB) from Pseudorhizobium banfieldiae sp. strain NT-26 catalyzes the oxidation of arsenite to arsenate and transfers electrons to its cognate electron acceptor cytochrome c (cytc). This activity underpins the ability of this organism to respire using arsenite present in contaminated environments. The crystal structure of the AioAB/cytc electron transfer complex reveals two AB/(cytc) assemblies per asymmetric unit. Three of the four cytc molecules in the asymmetric unit dock to 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 cytc and the [2Fe-2S] Rieske cluster in the AioB subunit. The interface between the AioAB and cytc proteins features electrostatic and nonpolar interactions and is stabilized by two salt bridges. A modest number of hydrogen bonds, salt bridges and relatively small, buried surface areas between protein partners are typical features of transient electron transfer complexes. Interestingly, the fourth cytc molecule is positioned differently between two AioAB heterodimers, with distances between its heme and the AioAB redox active cofactors that are outside the acceptable range for fast electron transfer. This unique cytc molecule appears to be positioned to facilitate crystal packing rather than reflecting a functional complex.