Although arsenic is a trace element in terms of its natural abundance, it nonetheless
has a common presence within the earth's crust. Because it is classified as a
group VB element in the periodic table, it shares many chemical and biochemical
properties in common with its neighbors phosphorus and nitrogen. Indeed, in the
case of this element's most oxidized (+5) oxidation state, arsenate [HAsO_4^(2-) or
As (V)], its toxicity is based on its action as an analog of phosphate. Hence,
arsenate ions uncouple the oxidative phosphorylation normally associated with
the enzyme glyceraldehyde 3-phosphate dehydrogenase, thereby preventing the
formation ofphosphoglyceroyl phosphate, a key high-energy intermediate in glycolysis.
To guard against this, a number of bacteria possess a detoxifying arsenate
reductase pathway (the arsC system) whereby cytoplasmic enzymes remove internal
pools of arsenate by achieving its reduction to arsenite [H_2AsO_3- or As
(III)]. However, because the arsenite product binds with internal sulfhydryl
groups that render it even more toxic than the original arsenate, efficient arsenite
efflux from the cell is also required and is achieved by an active ion ''pumping'' system (1). The details of this bacterial arsenic detoxification phenomenon have
been well established in the literature, and Chapter 10 in this volume provided
a thorough review. Here, we discuss bacterial respiration of arsenate and its significance
in the environment. As a biological phenomenon, respiratory growth
on arsenate is quite remarkable, given the toxicity of the element. Moreover, the
consequences of microbial arsenate respiration may, at times, have a significant
impact on environmental chemistry
