Crystal structure of the 100 kDa arsenite oxidase from Alcaligenes faecalis in two crystal forms at 1.64 Å and 2.03 Å

Background: Arsenite oxidase from Alcaligenes faecalis NCIB 8687 is a molybdenum/iron protein involved in the detoxification of arsenic. It is induced by the presence of AsO₂^- (arsenite) and functions to oxidize As^IIIO₂^-, which binds to essential sulfhydryl groups of proteins and dithiols, to the relatively less toxic As^VO₄^3- (arsenate) prior to methylation. Results: Using a combination of multiple isomorphous replacement with anomalous scattering (MIRAS) and multiple-wavelength anomalous dispersion (MAD) methods, the crystal structure of arsenite oxidase was determined to 2.03 Å in a P2₁ crystal form with two molecules in the asymmetric unit and to 1.64 Å in a P1 crystal form with four molecules in the asymmetric unit. 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 small subunit contains a Rieske-type [2Fe-2S] site. Conclusions: 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 bc₁ and cytochrome b₆f complexes and to the Rieske domain of naphthalene 1,2-dioxygenase.