Arsenate reduction: thiol cascade chemistry with convergent evolution.

The frequent abundance of arsenic in the environment has guided the evolution of enzymes for the reduction of arsenate. The arsenate reductases (ArsC) from different sources have unrelated sequences and structural folds, and can be divided into different classes on the basis of their structures, reduction mechanisms and the locations of catalytic cysteine residues. The thioredoxin-coupled arsenate reductase class is represented by Staphylococcus aureus pI258 ArsC and Bacillus subtilis ArsC. The ArsC from Escherichia coli plasmid R773 and the eukaryotic ACR2p reductase from Saccharomyces cerevisiae represent two distinct glutaredoxin-linked ArsC classes. All are small cytoplasmic redox enzymes that reduce arsenate to arsenite by the sequential involvement of three different thiolate nucleophiles that function as a redox cascade. In contrast, the ArrAB complex is a bacterial heterodimeric periplasmic or a surface-anchored arsenate reductase that functions as a terminal electron acceptor and transfers electrons from the membrane respiratory chain to arsenate. Finally, the less well documented arsenate reductase activity of the monomeric arsenic(III) methylase, which is an S-adenosylmethionine (AdoMet)-dependent methyltransferase. After each oxidative methylation cycle and before the next methylation step, As(V) is reduced to As(III). Methylation by this enzyme is also considered an arsenic-resistance mechanism for bacteria, fungi and mammals.