Arsenate

Arsenate

[ahr-suh-neyt, -nit]
The arsenate ion is AsO43−. An arsenate (compound) is any compound that contains this ion.

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.

Ions

  • In acidic conditions we have arsenic acid, H3AsO4;
  • in weakly acid conditions we have the dihydrogen arsenate ion, H2AsO4;
  • in weakly basic conditions we have hydrogen arsenate ion HAsO42−;
  • and finally, in basic conditions, the arsenate ion AsO43−.

Arsenate poisoning

Arsenate can replace inorganic phosphate in the step of glycolysis that produces 1,3-bisphosphoglycerate, yielding 1-arseno-3-phosphoglycerate instead. This molecule 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.

Bacteria using and generating arsenate

Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenates to form arsenites. The enzymes involved are known as arsenate reductases.

In 2008, bacteria were discovered that employ a version of photosynthesis with arsenites as electron donors, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). The researchers conjectured that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive.

References

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