Soft metal thiol chemistry is not involved in the transport of arsenite by the Ars pump

The Arsenic Detoxification System in Corynebacteria: Basis and Application for Bioremediation and Redox Control

Arsenic (As) is widespread in the environment and highly toxic. It has been released by volcanic and anthropogenic activities and causes serious health problems worldwide. To survive arsenic-rich environments, soil and saprophytic microorganisms have developed molecular detoxification mechanisms to survive arsenic-rich environments, mainly by the enzymatic conversion of inorganic arsenate (As^V) to arsenite (As^III) by arsenate reductases, which is then extruded by arsenite permeases. One of these Gram-positive bacteria, Corynebacterium glutamicum, the workhorse of biotechnological research, is also resistant to arsenic. To sanitize contaminated soils and waters, C. glutamicum strains were modified to work as arsenic "biocontainers." Two chromosomally encoded ars operons (ars1 and ars2) are responsible for As resistance. The genes within these operons encode for metalloregulatory proteins (ArsR1/R2), arsenite permeases (Acr3-1/-2), and arsenate reductases (ArsC1/C2/C1'). ArsC1/C2 arsenate reductases are coupled to the low molecular weight thiol mycothiol (MSH) and to the recently discovered mycoredoxin-1 (Mrx-1) present in most Actinobacteria. This MSH/Mrx-1 redox system protects cells against different forms of stress, including reactive oxygen species (ROS), metals, and antibiotics. ROS can modify functional sulfur cysteines by oxidizing the thiol (-SH) to a sulfenic acid (-SOH). These oxidation-sensitive protein cysteine thiols are redox regulated by the MSH/Mrx-1 couple in Corynebacterium and Mycobacterium. In summary, the molecular mechanisms involved in arsenic resistance system in C. glutamicum have paved the way for understanding the cellular response against oxidative stress in Actinobacteria.

As(III) S-adenosylmethionine methyltransferases and other arsenic binding proteins

Efflux is by far the most common means of arsenic detoxification is by methylation catalyzed by a family of As(III) S-adenosylmethionine (SAM) methyltransferases (MTs) enzymes designated ArsM in microbes or AS3MT in higher eukaryotes. The protein sequence of more than 5000 AS3MT/ArsM orthologues have been deposited in the NCBI database, mostly in prokaryotic and eukaryotic microbes. As(III) SAM MTs are members of a large superfamily of MTs involved in numerous physiological functions. ArsMs detoxify arsenic by conversion of inorganic trivalent arsenic (As(III)) into mono-, di- and trimethylated species that may be more toxic and carcinogenic than inorganic arsenic. The pathway of methylation remains controversial. Several hypotheses will be examined in this review.

Pathways of arsenic uptake and efflux

Arsenic is the most prevalent environmental toxic substance and ranks first on the U.S. Environmental Protection Agency's Superfund List. Arsenic is a carcinogen and a causative agent of numerous human diseases. Paradoxically arsenic is used as a chemotherapeutic agent for treatment of acute promyelocytic leukemia. Inorganic arsenic has two biological important oxidation states: As(V) (arsenate) and As(III) (arsenite). Arsenic uptake is adventitious because the arsenate and arsenite are chemically similar to required nutrients. Arsenate resembles phosphate and is a competitive inhibitor of many phosphate-utilizing enzymes. Arsenate is taken up by phosphate transport systems. In contrast, at physiological pH, the form of arsenite is As(OH)(3), which resembles organic molecules such as glycerol. Consequently, arsenite is taken into cells by aquaglyceroporin channels. Arsenic efflux systems are found in nearly every organism and evolved to rid cells of this toxic metalloid. These efflux systems include members of the multidrug resistance protein family and the bacterial exchangers Acr3 and ArsB. ArsB can also be a subunit of the ArsAB As(III)-translocating ATPase, an ATP-driven efflux pump. The ArsD metallochaperone binds cytosolic As(III) and transfers it to the ArsA subunit of the efflux pump. Knowledge of the pathways and transporters for arsenic uptake and efflux is essential for understanding its toxicity and carcinogenicity and for rational design of cancer chemotherapeutic drugs.

Properties of arsenite efflux permeases (Acr3) from Alkaliphilus metalliredigens and Corynebacterium glutamicum

Members of the Acr3 family of arsenite permeases confer resistance to trivalent arsenic by extrusion from cells, with members in every phylogenetic domain. In this study bacterial Acr3 homologues from Alkaliphilus metalliredigens and Corynebacterium glutamicum were cloned and expressed in Escherichia coli. Modification of a single cysteine residue that is conserved in all analyzed Acr3 homologues resulted in loss of transport activity, indicating that it plays a role in Acr3 function. The results of treatment with thiol reagents suggested that the conserved cysteine is located in a hydrophobic region of the permease. A scanning cysteine accessibility method was used to show that Acr3 has 10 transmembrane segments, and the conserved cysteine would be predicted to be in the fourth transmembrane segment.

Arsenic and antimony: comparative approach on mechanistic toxicology

A chemico-toxicological similarity between arsenic and antimony exists and their toxicology is often seen. Indeed, both elements possess several common properties, e.g. they are clastogenic but not mutagenic in the trivalent state and they have a carcinogenic potential: trivalent arsenicals are known to be human carcinogens and antimony(III) oxide (by inhalation) has been shown to cause lung cancer in female rats. For years, arsenic has been known to be environmentally toxic. Elevated human exposure to this element, mostly caused by the intake of contaminated tap water, is associated with increased incidences of cancer at various sites. It is still not clear how arsenic compounds exert their genotoxic effect. It may be connected with an inhibition of DNA repair or the induction of oxidative stress. Little work has been done on the toxicology of antimony as it is less widely present in the environment. There is evidence that in mammals antimony, unlike arsenic, is not detoxified via methylation but it still remains unclear what mechanism is responsible for antimony's genotoxicity. In general, there is little information known about this element to accurately determine its impact on human health. Thus, the aim of this paper is to review current knowledge for future risk assessment and further scientific work.

Alternate energy coupling of ArsB, the membrane subunit of the Ars anion-translocating ATPase

The arsenical resistance (ars) operon of the conjugative R-factor R773 confers resistance to arsenical and antimonial compounds in Escherichia coli, where resistance results from active extrusion of arsenite catalyzed by the products of the arsA and arsB genes. Previous in vivo studies on the energetics of arsenite extrusion showed that expression of both genes produced an ATP-coupled arsenite extrusion system that was independent of the electrochemical proton gradient. In contrast, in cells expressing only the arsB gene, arsenite extrusion was coupled to electrochemical energy and independent of ATP, suggesting that the Ars transport system exhibits a dual mode of energy coupling depending on the subunit composition. In vitro the ArsA-ArsB complex has been shown to catalyze ATP-coupled uptake of 73AsO2(-1) in everted membrane vesicles. However, transport catalyzed by ArsB alone has not previously been observed in vitro. In this study we demonstrate everted membrane vesicles prepared from cells expressing only arsB exhibit uptake of 73AsO2(-1) coupled to electrochemical energy.

The role of arsenic-thiol interactions in metalloregulation of the ars operon

The ars operon of the Escherichia coli plasmid R773 that confers arsenical and antimonial resistance is negatively regulated by the ArsR repressor. ArsR residues Cys-32 and Cys-34 were previously identified as involved in induction by arsenite and antimonite, suggesting coordination between As(III) and the two cysteine thiolates. However, in small molecule thiolate-As(III) complexes, arsenic is frequently three-coordinate. A site-directed mutagenic approach was employed in a search for a third arsenic ligand. ArsR proteins with C32G, C34G, and C32G/C34G substitutions were active repressors, but were not inducible in vivo. In vitro, the altered repressor-ars DNA complexes could not be dissociated by inducers. Alteration of Cys-37 and Ser-43, residues located in or near the putative helix-turn-helix DNA-binding region of the protein, had no effect on the inducibility of the operon. While these results indicated that neither the thiolate of Cys-37 nor the hydroxyl oxygen of Ser-43 is required for induction, they did not eliminate either atom as a potential arsenic ligand. Another approach involved reaction with an alternative inducer, phenylarsine oxide, which can form only two coordinations. Phenylarsine oxide was shown to be as effective as or more effective than arsenite or antimonite in induction in vivo. In vitro, the organic arsenical was more effective than either arsenite or antimonite in dissociating the repressor-promoter complex. Thus, two ArsR-arsenic bonds are sufficient for induction. The interaction of ArsR proteins with As(III) was examined using a phenylarsine oxide affinity resin. ArsR proteins containing any two of the three cysteine residues Cys-32, Cys-34, and Cys-37 bound to the resin. Alteration of any two of the three resulted in loss of binding. Arsenic X-ray absorption spectroscopy of ArsR treated stoichiometrically with arsenite confirmed the average arsenic coordination as AsS3 These results suggest that all three cysteine thiolates are arsenic ligands, but binding to only two, the Cys-32 and Cys-34 thiolates, is required to produce the conformational change that results in release of the repressor from the DNA and induction.