Arsenite In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions. ...

resistance (Ars) efflux pumps of bacteria may consist of two proteins, ArsB
TC# 2.A.45.1.1
the integral membrane constituent with twelve transmembrane spanners) and ArsA
TC# 3.A.4.1.1
the ATP-hydrolyzing, transport energizing subunit, as for the chromosomally-encoded ''

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

'' system), or of one protein (just the ArsB integral membrane protein of the plasmid-encoded ''

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

'' system). ArsA proteins have two ATP binding domains and probably arose by a tandem gene duplication event. ArsB proteins all possess twelve transmembrane spanners and may also have arisen by a tandem gene duplication event. Structurally, the Ars pumps resemble ABC-type efflux pumps, but there is no significant sequence similarity between the Ars and ABC pumps. When only ArsB is present, the system operates by a pmf-dependent mechanism, and consequently belongs in TC subclass 2.A (i.e
TC# 2.A.45
. When ArsA is also present, ATP hydrolysis drives efflux, and consequently the system belongs in TC subclass 3.A (i.e.
TC# 3.A.4
. ArsB therefore appears twice in the TC system (ArsB and ArsAB) but ArsA appears only once. These pumps actively expel both

arsenite In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions. ...

and

antimonite In chemistry, antimonite refers to a salt of antimony(III), such as NaSb(OH)4 and NaSbO2 (meta-antimonite), which can be prepared by reacting alkali with antimony trioxide, Sb2O3.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemist ...

Homology

Homologues of ArsB are found in

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

and

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

as well as

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

. Homologues are also found in archaea and eukarya. Several paralogues may sometimes be found in a single organism. Among the distant homologues found in eukaryotes are members of the DASS family
TC# 2.A.47
including the rat renal Na⁺:sulfate cotransporter
Q07782
and the human renal Na⁺:dicarboxylate cotransporter
gbU26209
. ArsB proteins are therefore members of a superfamily (called the IT (ion transporter) superfamily). However, ArsB has uniquely gained the ability to function in conjunction with ArsA in order to couple ATP hydrolysis to anion efflux. ArsAB belongs to the ArsA ATPase Superfamily. Debatably, a unique member of the ArsB family is the rice silicon (

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

) efflux pump, Lsi2
TC# 2.A.45.2.4
. The silicon uptake systems, Lsi1
TC# 1.A.8.12.2
, and Lsi2 are expressed in roots, on the plasma membranes of cells in both the

exodermis The exodermis is a physiological barrier that has a role in root function and protection. The exodermis is a membrane of variable permeability responsible for the radial flow of water, ions, and nutrients. It is the outer layer of a plant's co ...

and the

endodermis The endodermis is the central, innermost layer of cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances (Casparian strip) to restrict apoplastic flow of water to the ...

. In contrast to Lsi1, which is localized on the distal side, Lsi2 is localized on the proximal side of the same cells. Thus these cells have an influx transporter on one side and an efflux transporter on the other side of the cell to permit the effective transcellular transport of the nutrient. ArsA proteins are homologous to nitrogenase iron (NifH) proteins 2 of bacteria and to

protochlorophyllide reductase In enzymology, protochlorophyllide reductases (POR) are enzymes that catalyze the conversion from protochlorophyllide to chlorophyllide ''a''. They are oxidoreductases participating in the biosynthetic pathway to chlorophylls. There are t ...

iron sulfur ATP-binding proteins of cyanobacteria, algae and plants.

Mechanism

ArsA homologues are found in bacteria, archaea and eukarya (both animals and plants), but there are far fewer of them in the databases than ArsB proteins, suggesting that many ArsB homologues function by a pmf-dependent mechanism, probably an arsenite:H⁺ antiport mechanism. In the ''E. coli'' ArsAB transporter, both ArsA and ArsB recognize and bind their anionic substrates. A model has been proposed in which ArsA alternates between two virtually exclusive conformations. In one, (ArsA¹) the A1 site is closed but the A2 site is open, but in the other (ArsA²) the opposite is true. Antimonite b(III)sequesters ArsA in the ArsA¹ conformation which catalyzes ATP hydrolysis at A2 to drive ArsA between conformations that have high (nucleotide-bound ArsA) and low (nucleotide-free ArsA) affinity for antimonite. It is proposed that ArsA uses this process to sequester Sb(III) and eject it into the ArsB channel. In the case of ArsAB, at the interface of these two halves are two nucleotide-binding domains and a metalloid-binding domain. Cys-113 and Cys-422 have been shown to form a high-affinity metalloid binding site. The crystal structure of ArsA shows two other bound metalloid atoms, one liganded to Cys-172 and His-453, and the other liganded to His-148 and Ser-420. There is only a single high-affinity metalloid binding site in ArsA. Cys-172 controls the affinity of this site for metalloid and hence the efficiency of metalloactivation of the ArsAB efflux pump.

Transport Reaction

The overall reaction catalyzed by ArsB (presumably by uniport) is:

Arsenite or Antimonite (in) → Arsenite or Antimonite (out).

The overall reaction catalyzed by ArsB-ArsA is:

Arsenite or Antimonite (in) + ATP ⇌ Arsenite or Antimonite (out) + ADP + P_i.

Homology

Mechanism

Transport Reaction

See also

References

Further reading