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The arsenite oxidase (Aio) from the facultative autotrophic  Alphaproteobacterium Rhizobium  sp. NT-26 is a bioenergetic enzyme involved in the oxidation of arsenite to arsenate. The enzyme from the distantly related heterotroph,  Alcaligenes faecalis , which is thought to oxidise arsenite for detoxification, consists of a large α subunit (AioA) with  bis -molybdopterin guanine dinucleotide at its active site and a 3Fe-4S cluster, and a small β subunit (AioB) which contains a Rieske 2Fe-2S cluster. The successful heterologous expression of the NT-26 Aio in  Escherichia coli  has resulted in the solution of its crystal structure. The NT-26 Aio, a heterotetramer, shares high overall similarity to the heterodimeric arsenite oxidase from  A. faecalis  but there are striking differences in the structure surrounding the Rieske 2Fe-2S cluster which we demonstrate explains the difference in the observed redox potentials (+225 mV  vs.  +130/160 mV, respectively). A combination of site-directed mutagenesis and electron paramagnetic resonance was used to explore the differences observed in the structure and redox properties of the Rieske cluster. In the NT-26 AioB the substitution of a serine (S126 in NT-26) for a threonine as in the  A. faecalis  AioB explains a −20 mV decrease in redox potential. The disulphide bridge in the  A. faecalis  AioB which is conserved in other betaproteobacterial AioB subunits and the Rieske subunit of the cytochrome  bc   1  complex is absent in the NT-26 AioB subunit. The introduction of a disulphide bridge had no effect on Aio activity or protein stability but resulted in a decrease in the redox potential of the cluster. These results are in conflict with previous data on the betaproteobacterial AioB subunit and the Rieske of the  bc   1  complex where removal of the disulphide bridge had no effect on the redox potential of the former but a decrease in cluster stability was observed in the latter.

The Respiratory Arsenite Oxidase: Structure and the Role of Residues Surrounding the Rieske Cluster