Arsenite oxidase

Some compounds of this type may have a high affinity for proteins that is not due to their binding to two thiol groups (35). In particular, arsenite also reacts with the molybdenum-pterin cofactor of many enzymes (35a-d). This usually inhibits the enzyme, but in particular cases (35e) the arsenite may be oxidized indeed the enzyme arsenite oxidase contains such a center (35f). 

As stated in Section I, arsenate and phosphate are very similar. Hence, organisms have difficulty in assimilating phosphate without taking up arsenate, and this will uncouple their metabolism (Section II). They make use of an important difference between arsenate and phosphate to avoid this, i.e., the vastly greater ease of reduction of arsenate to arsenite than of phosphate to phosphite. By itself such a reduction would be no help, since arsenite is intensely toxic, but further processes can follow, such as alkylation to produce organoarsenic compounds (6), or extrusion of arsenite from the organism (e.g., 36, 37), driven by hydrolysis of ATP (38). Extruded arsenite may then be rendered less toxic by oxidation to arsenate by the arsenite oxidase mentioned in section III,A. 

Other members of this family that have been structurally determined by X-ray diffraction include formate dehydrogenase (FDH), trimethylamine oxidase (TMAO), dissimilatory nitrate reductase(NAP), and most recently, arsenite oxidase (AsO). Only the distinctive points of their structures will be briefly described here. 

Arsenite oxidase was solved at higher resolution (1.64 A) offering a more reliable view of the active site (40). Two dithiolene chelates are symmetrically bound at normal distances (2.4 A) and a single oxo ligand is observed at 1.6 A. The absence of any other covalent link from the protein leaves the Mo as five coordinate (alanine replaces the aminoacid position normally occupied by the coordination of serine, cysteine or selenocysteine residues). This Mo environment was interpreted as indicating a reduced Mo site, possibly from photoreduction in the X-ray beam. 

Arsenite oxidase A.faecalis a/3, 85 MoO(OH)(MGD)2 [Fe3S4]or[Fe2S2] AsOs " — As04 " 1G8K... 

FIGURE 72.2. Arsenic detoxification mechanisms (reduction, oxidation, methylation, and resistance) in prokaryotes. (A) Respiratory arsenate reductase (Arr) is involved in the reduction of As(V) by the dissimilatory arsenate respiring organisms. (B) Arsenite oxidase (Aox/Aso) is responsible for oxidation of As(III) by chemoautotrophic or heterotrophic arsenite oxidizers. 

Alcaligenes faecalis and five members of the p Proteobacteria are heterotrophic arsenite oxidizers, whereas Pseudomonas arsenitoxidans and NT-26 grew anaerobically through chemoautotrophic oxidation (Oremland and Stolz, 2005 Santini et al, 2000). However, six members of a Proteobacteria (Ben-5, NT-3, NT-4, NT-2, NT-26, and NT-25) and one member of y Proteobacteria (MLHE-1) were known chemohthoautotrophic arsenite oxidizers (Oremland et al, 2002). The best characterized and probably most studied of aU arsenite oxidizers is Alcaligenes faecalis, a heterotrophic arsenite oxidizer (Osborne and Enrlich, 1976). The arsenite oxidase from Alcaligenes faecalis has been purified and structurally characterized (Ellis et al, 2001). A similar enzyme has also been purified from the heterotrophic arsenite oxidizers Hydrogenophaga sp. strain NT-14 (Vanden Hoven and Santini, 2004) and the chemolithoautotrophic Rhizobium sp. strain NT-26 (Santini and Vanden Hoven, 2004), which indicate that the arsenite oxidase enzyme is also a member of the DMSO reductase family of molybdenum enzymes, similar to the respiratory arsenate reductases (Arr). The arsenite oxidase heterodimer comprises an 88 kDa catalytic subunit encoded by the aoxB gene that contains a [3Fe-4S] cluster and molybdenum bound to the pyranopterin cofactor and a 14 kDa subunit... 

Ellis, P.J., Comads, T., Hille, R., Kuhn, P. (2001). Crystal structme of the lOOkDa arsenite oxidase from Alcaligenes faecalis in two crystal forms at 1.64 A and 2.03 A. Structure 9 125-32. 

S. A., Santini, J.M. (2007). Detection, diversity and expression of aerobic bacterial arsenite oxidase genes. Environ. Microbiol. 9 934-43. 

Muller, D., Lievremont, D., Simeonova, D.D., Hubert, J.C., Lett, M.C. (2003). Arsenite oxidase aox genes from a metal-resistant hetSL-proteobacterium. J. Bacterial. 185 135 1. 

Rhine, E.D., Ni Chadhain, S.M., Zylstra, G.J., Young, L.Y. (2007). The arsenite oxidase genes (aroAB) in novel chemoautotrophic arsenite oxidizers. Biochem. Biophys. Res. Commun. 354 662-1. 

Santini, J.M., Vanden Hoven, R.N. (2004). Molybdenum-containing arsenite oxidase of the chemolithoautotrophic arsenite oxidizer NT-26. J. Bacteriol. 186 1614-19. 

Lebrun, E. et al. (2003). Arsenite oxidase, an ancient bioenergetic enzyme. Molecular Biology and Evolution, 20, 686-93. 

Andeeson GL, Williams J and Hille R (1992) The purification and characterization of arsenite oxidase from Alcaligenesfaecalis, a molybdenum-containing hydroxylase. J Biol Chem 267 23674-23682. 

Arsenite oxidase of Alicaligenes faecalis is comprised of a large subunit that contains a molybdenum center that is very similar to that of the nitrate reductase, a Fe3S4 cluster, plus a smaller unit containing a Reiske-type Fc2S2 cluster. 

Chemolithoautotroph NT-26 (novel species belonging to Agrobacterium/Rhizobium branch) Periplasmic arsenite oxidase 1.8 82... 

Detoxification 5 mM As, log growth Alcaligenes faecalis Cytoplasmic arsenite oxidase 83... 

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