Arsenate

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Arsenate
Arsenate ion.svg
Arsenate-anion-3D-spacefill.png
Names
IUPAC name
arsorate
Identifiers
  • 12523-21-6 Yes check.svgY
3D model (JSmol)
ChemSpider
PubChem CID
  • InChI=1S/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5)/p-3 X mark.svgN
    Key: DJHGAFSJWGLOIV-UHFFFAOYSA-K X mark.svgN
  • InChI=1/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5)/p-3
    Key: DJHGAFSJWGLOIV-DFZHHIFOAQ
  • [O-][As+]([O-])([O-])[O-]
Properties
AsO3−
4
Molar mass 138.919
Conjugate acid Arsenic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Infobox references

The arsenate ion is As O 3−
4
. An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate resembles phosphate in many respects, since arsenic and phosphorus occur in the same group (column) of the periodic table. Arsenates are moderate oxidizers, with an electrode potential of +0.56  V for reduction to arsenites.

Contents

Occurrence

Arsenates occur naturally in a variety of minerals. Those minerals may contain hydrated or anhydrous arsenates. Unlike phosphates, arsenates are not lost from a mineral during weathering. Examples of arsenate-containing minerals include adamite, alarsite, annabergite, erythrite and legrandite. [1] Where two arsenate ions are required to balance the charge in a formula, it is called diarsenate for example trizinc diarsenate, Zn3(AsO4)2.

Ions

The word arsenate is derived from arsenic acid, H3AsO4. This moderately strong acid converts to dihydrogen arsenate (H
2
AsO
4
), hydrogen arsenate (HAsO2−
4
), and arsenate (AsO3−
4
), depending on pH. The quantitative relationship between these species is defined by the acid dissociation constants:

H3AsO4 + H2O H
2
AsO
4
+ H3O+ (log K1 = −2.19)
H
2
AsO
4
+ H2O HAsO2−
4
+ H3O+ (log K2 = −6.94)
HAsO2−
4
+ H2O AsO3−
4
+ H3O+ (log K3 = −11.5)

These values are similar to those of the hydrogen phosphates. Hydrogen arsenate and dihydrogen arsenate predominate in aqueous solution near neutral pH.

Arsenate poisoning

Arsenate can replace inorganic phosphate in the step of glycolysis that produces 1,3-bisphosphoglycerate from glyceraldehyde 3-phosphate. This yields 1-arseno-3-phosphoglycerate instead, which is unstable and quickly hydrolyzes, forming the next intermediate in the pathway, 3-phosphoglycerate. Therefore, glycolysis proceeds, but the ATP molecule that would be generated from 1,3-bisphosphoglycerate is lost – arsenate is an uncoupler of glycolysis, explaining its toxicity. [2]

As with other arsenic compounds, arsenite binds to lipoic acid, [3] inhibiting the conversion of pyruvate into acetyl-CoA, blocking the Krebs cycle and therefore resulting in further loss of ATP. [4]

See also

Related Research Articles

Arsenic Chemical element, symbol As and atomic number 33

Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but only the gray form, which has a metallic appearance, is important to industry.

Glycolysis Metabolic pathway

Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvic acid, CH3COCOOH. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

Phosphate Chemical compound

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid H
3
PO
4
.

Entner–Doudoroff pathway

The Entner-Doudoroff Pathway is a metabolic pathway that is most notably in Gram-negative bacteria, certain Gram-positive bacteria and archaea. Glucose is the starting product in the ED pathway and through a series of enzyme assisted chemical reactions it is catabolized into pyruvate. Entner and Doudoroff (1952) and MacGee and Doudoroff (1954) first reported the ED pathway in the bacterium Pseudomonas saccharophila. While originally thought to be just an alternative to glycolysis (EMP) and the pentose phosphate pathway (PPP), some studies now suggest that the original role of the EMP may have originally been about anabolism and repurposed over time to catabolism, meaning the ED pathway may be the older pathway. Recent studies have also shown the prevalence of the ED pathway may be more widespread than first predicted with evidence supporting the presence of the pathway in cyanobacteria, ferns, algae, mosses, and plants. Specifically, there is direct evidence that Hordeum vulgare uses the Entner–Doudoroff pathway.

Glyceraldehyde 3-phosphate Chemical compound

Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is the metabolite that occurs as an intermediate in several central pathways of all organisms. With the chemical formula H(O)CCH(OH)CH2OPO32-, this anion is a monophosphate ester of glyceraldehyde.

Arsenic acid Chemical compound

Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but is only found in solution, where it is largely ionized. Its hemihydrate form (H3AsO4·1/2H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.

In chemistry, an arsenite is a chemical compound containing an arsenic oxoanion 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. IUPAC have recommended that arsenite compounds are to be named as arsenate(III), for example ortho-arsenite is called trioxidoarsenate(III). Ortho-arsenite contrasts to the corresponding anions of the lighter members of group 15, phosphite which has the structure HPO2−
3
and nitrite, NO
2
which is bent.

3-Phosphoglyceric acid Chemical compound

3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P). This glycerate is a biochemically significant metabolic intermediate in both glycolysis and the Calvin-Benson cycle. The anion is often termed as PGA when referring to the Calvin-Benson cycle. In the Calvin-Benson cycle, 3-phosphoglycerate is typically the product of the spontaneous scission of an unstable 6-carbon intermediate formed upon CO2 fixation. Thus, two equivalents of 3-phosphoglycerate are produced for each molecule of CO2 that is fixed. In glycolysis, 3-phosphoglycerate is an intermediate following the dephosphorylation (reduction) of 1,3-bisphosphoglycerate.

1,3-Bisphosphoglyceric acid Chemical compound

1,3-Bisphosphoglyceric acid (1,3-Bisphosphoglycerate or 1,3BPG) is a 3-carbon organic molecule present in most, if not all, living organisms. It primarily exists as a metabolic intermediate in both glycolysis during respiration and the Calvin cycle during photosynthesis. 1,3BPG is a transitional stage between glycerate 3-phosphate and glyceraldehyde 3-phosphate during the fixation/reduction of CO2. 1,3BPG is also a precursor to 2,3-bisphosphoglycerate which in turn is a reaction intermediate in the glycolytic pathway.

Arsenic pentoxide Chemical compound

Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is arsenic(III) oxide (As2O3). All arsenic compounds are highly toxic and thus find only limited commercial applications.

Scheeles Green highly toxic arsenic-based pigment

Scheele's Green, also called Schloss Green, is chemically a cupric hydrogen arsenite, CuHAsO
3
. It is chemically related to Paris Green. It is a yellowish-green pigment which in the past was used in some paints, but has since fallen out of use because of its toxicity and the instability of its color in the presence of sulfides and various chemical pollutants. Scheele's Green was invented in 1775 by Carl Wilhelm Scheele. By the end of the 19th century, it had virtually replaced the older green pigments based on copper carbonate.

Phosphoglycerate kinase

Phosphoglycerate kinase is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate (1,3-BPG) to ADP producing 3-phosphoglycerate (3-PG) and ATP :

Phosphoglycerate mutase

Phosphoglycerate mutase (PGM) is any enzyme that catalyzes step 8 of glycolysis. They catalyze the internal transfer of a phosphate group from C-3 to C-2 which results in the conversion of 3-phosphoglycerate (3PG) to 2-phosphoglycerate (2PG) through a 2,3-bisphosphoglycerate intermediate. These enzymes are categorized into the two distinct classes of either cofactor-dependent (dPGM) or cofactor-independent (iPGM). The dPGM enzyme is composed of approximately 250 amino acids and is found in all vertebrates as well as in some invertebrates, fungi, and bacteria. The iPGM class is found in all plants and algae as well as in some invertebrate, fungi, and Gram-positive bacteria. This class of PGM enzyme shares the same superfamily as alkaline phosphatase.

Fructose-bisphosphate aldolase

Fructose-bisphosphate aldolase, often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Aldolase can also produce DHAP from other (3S,4R)-ketose 1-phosphates such as fructose 1-phosphate and sedoheptulose 1,7-bisphosphate. Gluconeogenesis and the Calvin cycle, which are anabolic pathways, use the reverse reaction. Glycolysis, a catabolic pathway, uses the forward reaction. Aldolase is divided into two classes by mechanism.

Sodium arsenate Chemical compound

Sodium arsenate is the inorganic compound with the formula Na3AsO4. Related salts are also called sodium arsenate, including Na2HAsO4 (disodium hydrogen arsenate) and NaH2AsO4 (sodium dihydrogen arsenate). The trisodium salt is a white or colourless solid that is highly toxic. It is usually handled as the dodecahydrate Na3AsO4.12H2O.

Disodium methyl arsonate Chemical compound

Disodium methyl arsonate (DSMA) is the organoarsenic compound with the formula CH3AsO3Na2. It is a colorless, water-soluble solid derived from methanearsonic acid. It is used as a herbicide. Tradenames include Metharsinat, Arrhenal, Disomear, Metharsan, Stenosine, Tonarsan, Tonarsin, Arsinyl, Arsynal, and Diarsen.

Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element in Earth's crust, and although many arsenic compounds are often considered highly toxic to most life, a wide variety of organoarsenic compounds are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organisms. This pattern is general for other related elements, including selenium, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many toxic arsenic compounds are found in some aquifers, potentially affecting many millions of people via biochemical processes.

1-Arseno-3-phosphoglycerate

1-Arseno-3-phosphoglycerate is a compound produced by the enzyme glyceraldehyde 3-phosphate dehydrogenase, present in high concentrations in many organisms, from glyceraldehyde 3-phosphate and arsenate in the glycolysis pathway. The compound is unstable and hydrolyzes spontaneously to 3-phosphoglycerate, bypassing the energy producing step of glycolysis.

Arsenate-reducing bacteria are bacteria which reduce arsenates. Arsenate-reducing bacteria are ubiquitous in arsenic-contaminated groundwater (aqueous environment). Arsenates are salts or esters of arsenic acid (H3AsO4), consisting of the ion AsO43−. They are moderate oxidizers that can be reduced to arsenites and to arsine. Arsenate can serve as a respiratory electron acceptor for oxidation of organic substrates and H2S or H2. Arsenates occur naturally in minerals such as adamite, alarsite, legrandite, and erythrite, and as hydrated or anhydrous arsenates. Arsenates are similar to phosphates since arsenic (As) and phosphorus (P) occur in group 15 (or VA) of the periodic table. Unlike phosphates, arsenates are not readily lost from minerals due to weathering. They are the predominant form of inorganic arsenic in aqueous aerobic environments. On the other hand, arsenite is more common in anaerobic environments, more mobile, and more toxic than arsenate. Arsenite is 25–60 times more toxic and more mobile than arsenate under most environmental conditions. Arsenate can lead to poisoning, since it can replace inorganic phosphate in the glyceraldehyde-3-phosphate --> 1,3-biphosphoglycerate step of glycolysis, producing 1-arseno-3-phosphoglycerate instead. Although glycolysis continues, 1 ATP molecule is lost. Thus, arsenate is toxic due to its ability to uncouple glycolysis. Arsenate can also inhibit pyruvate conversion into acetyl-CoA, thereby blocking the TCA cycle, resulting in additional loss of ATP.

Sodium dihydrogen arsenate is the inorganic compound with the formula NaH2AsO4. Related salts are also called sodium arsenate, including Na2HAsO4 (disodium hydrogen arsenate) and NaH2AsO4 (sodium dihydrogen arsenate). Sodium dihydrogen arsenate is a colorless solid that is highly toxic.

References

  1. Mineralienatlas - Mineralklasse Phosphate, Arsenate, Vanadate. (in German)
  2. Hughes, Michael F. (2002). "Arsenic toxicity and potential mechanisms of action" (PDF). Toxicology Letters (133): 4.[ permanent dead link ]
  3. "Archived copy". Archived from the original on 20 March 2018. Retrieved 4 February 2018.CS1 maint: archived copy as title (link)
  4. Kim Gehle; Selene Chou; William S. Beckett (1 October 2009), Arsenic Toxicity Case Study, Agency for Toxic Substances and Disease Registry