In molecular biology, the ars operon is an
operon found in several
bacterial taxon. It is required 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
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. 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
Substrate may refer to:
Physical layers
*Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached
** Substrate (locomotion), the surface over which an organism lo ...
specificity to allow for arsenate pumping and resistance. ArsC is an approximately 150-residue arsenate
reductase
A reductase is an enzyme that catalyzes a reduction reaction.
Examples
* 5α-Reductase
* 5β-Reductase
* Dihydrofolate reductase
* HMG-CoA reductase
* Methemoglobin reductase
* Ribonucleotide reductase
* Thioredoxin reductase
* ''E. coli'' ...
that uses
reduced glutathione (GSH) to convert arsenate to arsenite with a
redox active
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
residue in the
active site
In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...
. ArsC forms an active quaternary
complex with GSH, arsenate, and
glutaredoxin
Glutaredoxins (also known as Thioltransferase) are small redox enzymes of approximately one hundred amino-acid residues that use glutathione as a cofactor. In humans this oxidation repair enzyme is also known to participate in many cellular func ...
1 (Grx1). The three
ligands
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...
must be present simultaneously for
reduction to occur.
ArsA and ArsB
ArsA and ArsB form an anion-translocating ATPase.
The ArsB
protein is distinguished by its overall
hydrophobic character, in keeping with its role as a membrane-associated channel. Sequence analysis reveals the presence of 13 putative transmembrane (TM) regions.
ArsC
The arsC protein
structure
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
has been solved.
It belongs to the
thioredoxin superfamily
fold
Fold, folding or foldable may refer to:
Arts, entertainment, and media
* ''Fold'' (album), the debut release by Australian rock band Epicure
*Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot
*Above ...
which is defined by a
beta-sheet core surrounded by
alpha-helices. The active cysteine
residue of ArsC is located in the loop between the first beta-strand and the first helix, which is also
conserved in the Spx protein and its
homologues.
The arsC family also comprises the Spx
proteins which are
Gram-positive bacterial transcription factors that
regulate
Regulate may refer to:
* Regulation
* '' Regulate...G Funk Era'', an album from rapper Warren G
** Regulate (song), title song from the album
See also
*
*
* Regulator (disambiguation)
Regulator may refer to:
Technology
* Regulator (automati ...
the
transcription of multiple
genes in response to disulphide
stress.
ArsD and ArsR
ArsD is a trans-acting repressor of the arsRDABC operon that confers resistance to arsenicals and antimonials in ''Escherichia coli''. It possesses two-pairs of
vicinal cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
residues, Cys(12)-Cys(13) and Cys(112)-Cys(113), that potentially form separate
binding sites for the metalloids that trigger
dissociation of ArsD from the operon. However, as a
homodimer it has four vicinal cysteine
pairs.
The ArsD family consists of several
bacterial arsenical resistance
operon trans-acting repressor
In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. A DNA-binding repressor blocks the attachment of RNA polymerase to the ...
ArsD
proteins.
ArsR is a trans-acting regulatory protein. It acts as a repressor on the arsRDABC operon when no arsenic is present in the cell. When arsenic is present in the cell ArsR will lose affinity for the operator and RNA polymerase can transcribe the arsDCAB genes.
ArsD and ArsR work together to regulate the ars operon.
arsenic chaperone, ArsD, encoded by the arsRDABC operon of Escherichia coli. ArsD transfers trivalent metalloids to ArsA, the catalytic subunit of an As(III)/Sb(III) efflux pump. Interaction with ArsD increases the affinity of ArsA for arsenite, thus increasing its ATPase activity at lower concentrations of arsenite and enhancing the rate of arsenite extrusion.
References
{{InterPro content, IPR010712
Protein families
Operons