After reading this article you will learn about:- 1. Occurrence of Arsenic 2. Biochemical Effects of Arsenic 3. Transformation.
Occurrence of Arsenic:
The element arsenic is widely distributed in nature, being highly concentrated in coals. It occurs mostly in the form of arsenide’s of true metals or as arsenopyrites. The average content of arsenic in freshwater is about 1.5 to 2 ppb and in sea water it is 0.15 to 6 ppb.
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The inorganic forms of arsenic, as arsenate (5+ oxidation state) and the more toxic arsenate (3+ oxidation state), have been used extensively as herbicides and insecticides.
Tonnes of arsenate, for example, have been added to freshwater lakes to kill aquatic plants. An analysis of environmental samples has shown the presence of dimethyl arsenic acid and methyl arsenic acid in natural water, seashells, bird eggshells, human urine and shellfish.
Biochemical Effects of Arsenic:
Arsenic commonly occurs in insecticides, fungicides and herbicides. Among its compounds, those of As (III) are the most toxic.
As (III) exerts its toxic action by attacking -SH groups of an enzyme, thereby inhibiting enzyme action. 
The enzymes which generate cellular energy in the citric acid cycle are adversely affected. The inhibitory action is based on inactivation of pyruvate dehydrogenase by complexation with As (III), whereby the generation of ATP is prevented. 
By virtue of its chemical similarity to P (phosphorus), As interferes with some biochemical processes involving P This is observed in the biochemical generation of the key energy-yielding substance, ATP (adenosine triphosphate).
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An important step in ATP generation is the enzymatic synthesis of 1 3-diphosphoglycerate from glyceraldehyde 3-phosphate. Arsenate interferes by producing 1 -arseno-3-phosphoglycerate instead of 1, 3-diphosphoglycerate. Phosphorylation is replaced by arsenolysis which consists of spontaneous hydrolysis to 3-phosphoglycerate and arsenate.
Arsenic (III) compounds at high concentrations coagulate proteins, possibly by attacking the sulphur bonds maintaining the secondary and tertiary structures of proteins.
The three major biochemical actions of As are coagulation of proteins, complexation with coenzymes, and uncoupling of phosphorylation.
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The general antidotes for As poisoning are chemicals having -SH groups capable of bonding to As (III), e.g. 2, 3-dimercaptopropanol (BAL)
Transformation of Arsenic:
Toxic arsenate (III) has been oxidised to arsenate by a number of microorganisms like species of Achromobacter, Pseudomonas arthrobacter. In contrast, a yeast, Candida, an algae, Chlorella, and some marine bacteria are capable of reducing arsenate to the toxic arsenate.