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

Background: Arsenite oxidase from Alcaligenes faecalis NCIB 8687 is a molyb denum/iron protein involved in the detoxification of arsenic. It is induced by the presence of AsO2- (arsenite) and functions to oxidize (AsO2-)-O-III , which binds to essential sulfhydryl groups of proteins and dithiols, to t he relatively less toxic (AsO43-)-O-V (arsenate) prior to methylation. Results: Using a combination of multiple isomorphous replacement with anoma lous scattering (MIRAS) and multiple-wavelength anomalous dispersion (MAD) methods, the crystal structure of arsenite oxidase was determined to 2.03 A ngstrom in a P2(1) crystal form with two molecules in the asymmetric unit a nd to 1.64 Angstrom in a P1 crystal form with four molecules in the asymmet ric 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 memb ers of the dimethylsulfoxide (DMSO) reductase family of molybdenum enzymes, particularly the dissimilatory periplasmic nitrate reductase from Desulfov ibrio desulfuricans, but is unique in having no covalent bond between the p olypeptide and the Mo atom. The small subunit has no counterpart among know n Mo protein structures but is homologous to the Rieske [2Fe-2S] protein do main of the cytochrome be, and cytochrome b(6)f complexes and to the Rieske domain of naphthalene 1,2-dioxygenase.