Health, Safety & Environment

Arsenite Oxidase

Electron transfer through arsenite oxidase: Insights into Rieske interaction with cytochrome c

Arsenic is a widely distributed environmental toxin whose presence in drinking water poses a threat to > 140 million people worldwide. The respiratory enzyme arsenite oxidase from various bacteria catalyses the oxidation of arsenite to arsenate and is being developed as a biosensor for arsenite. The arsenite oxidase from Rhizobium sp. str. NT-26 (a member of the Alphaproteobacteria) is a heterotetramer consisting of a large catalytic subunit (AioA), which contains a molybdenum centre and a 3Fe-4S cluster, and a small subunit (AioB) containing a Rieske 2Fe-2S cluster. Stopped-flow spectroscopy and isothermal titration calorimetry (ITC) have been used to better understand electron transfer through the redox-active centres of the enzyme, which is essential for biosensor development. Results show that oxidation of arsenite at the active site is extremely fast with a rate of > 4000 s-1 and reduction of the electron acceptor is rate-limiting. An AioB-F108A mutation results in increased activity with the artificial electron acceptor DCPIP and decreased activity with cytochrome c, which in the latter as demonstrated by ITC is not due to an effect on the protein-protein interaction but instead to an effect on electron transfer. These results provide further support that the AioB F108 is important in electron transfer between the Rieske subunit and cytochrome c and its absence in the arsenite oxidases from the Betaproteobacteria may explain the inability of these enzymes to use this electron acceptor.

Watson, C., Niks, D., Hille, R., Vieira, M., Schoepp-Cothenet, B., Marques, A. T., Romao, M. J., Santos-Silva, T., and Santini, J. M.,Electron transfer through arsenite oxidase: Insights into Rieske interaction with cytochrome c, Biochimica Et Biophysica Acta-Bioenergetics, 2017, 1858, 865-872.

Arsenite oxidase from Alcaligenes faecalis NCIB 8687 is a molybdenum/iron protein involved in the detoxification of arsenic. It oxidizes arsenite [(AsO 2-)-O-III], which binds to essential sulfhydryl groups of proteins and dithiols, to the relatively less toxic arsenate [(AsO4 3-)-O-V]. Arsenite oxidase consists of a large subunit of 825 residues and a small subunit of approximately 134 residues. The large subunit contains a Mo site, consisting of a Mo atom bound to two pterin cofactors, and a [3Fe-4S] cluster. The large subunit of arsenite oxidase is similar to other members of the dimethylsulfoxide (DMSO) reductase family of molybdenum enzymes, particularly the dissimilatory periplasmic nitrate reductase from Desulfovibrio desulfuricans , but is unique in having no covalent bond between the polypeptide and the Mo atom. The small subunit has no counterpart among known Mo protein structures but is homologous to the Rieske [2Fe-2S] protein domain of the cytochrome be, and cytochrome b(6)f complexes and to the Rieske domain of naphthalene 1,2-dioxygenase

Ellis, P.J., Conrads, T., Hille, R., and Kuhn, P., Crystal structure of the 100 kDa arsenite oxidase from Alcaligenes faecalis in two crystal forms at 1.64 angstrom and 2.03 angstrom, Structure, 2001, 9, 125-132.

Lebrun, E., Brugna, M., Baymann, F., Muller, D., Lievremont, D., Lett, M. C., and Nitschke, W., Arsenite oxidase, an ancient bioenergetic enzyme, Molecular Biology and Evolution, 2003, 20, 686-693.