Arsenic is a toxic metalloid whose trivalent and pentavalent ions inhibita variety of biochemical processes. Operons that encode arsenic resistancehave been found in multicopy plasmids from both Gram-positive andGram-negative bacteria []. The resistance mechanism is encoded from a singleoperon, which houses an anion pump. The pump has two polypeptide components:a catalytic subunit (the ArsA protein), which functions as anoxyanion-stimulated ATPase; and an arsenite export component (the ArsB protein),which is associated with the inner membrane []. The ArsA and ArsB proteinsare thought to form a membrane complex that functions as ananion-translocating ATPase.The ArsB protein is distinguished by its overall hydrophobic character,in keeping with its role as a membrane-associated channel. Sequenceanalysis reveals the presence of 13 putative transmembrane (TM) regions.
This entry describes a distinct clade, including ArsC itself, of the broader family of ArsC and related proteins. This clade is almost completely restricted to the proteobacteria. An anion-translocating ATPase has been identified as the product of the arsenical resistance operon of resistance plasmid R773 []. When expressed in Escherichia coli this ATP-driven oxyanion pump catalyses extrusion of the oxyanions arsenite, antimonite and arsenate. The pump is composed of two polypeptides, the products of the arsA and arsB genes. The pump alone produces resistance to arsenite and antimonite. This protein, ArsC, catalyzes the reduction of arsenate to arsenite, and thus extends resistance to include arsenate. ArsC contains a single catalytic cysteine, within a thioredoxin fold, that forms a covalent thiolate-As(V) intermediate, which is reduced by GRX through a mixed GSH-arsenate intermediate []. This family of predominantly bacterial enzymes is unrelated to two other families of arsenate reductases which show similarity to low-molecular-weight acid phosphatases and phosphotyrosyl phosphatases [, ].
Several bacterial taxon have a chromosomal resistance system, encoded by the ars operon, for the detoxification of arsenate, arsenite, and antimonite []. This system transports arsenite and antimonite out of the cell. The pump is composed of two polypeptides, the products of the arsA and arsB genes. This two-subunit enzyme produces resistance to arsenite and antimonite. Arsenate, however, must first be reduced to arsenite before it is extruded. A third gene, arsC, expands the substrate specificity to allow for arsenate pumping and resistance. ArsC is an approximately 150-residue arsenate reductase that uses reduced glutathione (GSH) to convert arsenate to arsenite with a redox active cysteine residue in the active site. ArsC forms an active quaternary complex with GSH, arsenate, and glutaredoxin 1 (Grx1). The three ligands must be present simultaneously for reduction to occur [].The arsC family also comprises the Spx proteins which are GRAM-positive bacterial transcription factors that regulate the transcription of multiple genes in response to disulphide stress [, ].The arsC protein structure has been solved []. It belongs to the thioredoxin superfamily fold which is defined by a β-sheet core surrounded by α-helices. The active cysteine residue of ArsC is located in the loop between the first β-strand and the first helix, which is also conserved in the Spx protein and its homologues.
