SummaryArsenate reductases (ArsCs) evolved independently as a defence mechanism against toxic arsenate. In the genome of Corynebacterium glutamicum, there are two arsenic resistance operons (ars1 and ars2) and four potential genes coding for arsenate reductases (Cg_ArsC1, Cg_ArsC2, Cg_ArsC1' and Cg_ArsC4). Using knockout mutants, in vitro reconstitution of redox pathways, arsenic measurements and enzyme kinetics, we show that a single organism has two different classes of arsenate reductases. Cg_ArsC1 and Cg_ArsC2 are single-cysteine monomeric enzymes coupled to the mycothiol/mycoredoxin redox pathway using a mycothiol transferase mechanism. In contrast, Cg_ArsC1' is a three-cysteine containing homodimer that uses a reduction mechanism linked to the thioredoxin pathway with a k cat/KM value which is 10 3 times higher than the one of Cg_ArsC1 or Cg_ArsC2. Cg_ArsC1' is constitutively expressed at low levels using its own promoter site. It reduces arsenate to arsenite that can then induce the expression of Cg_ArsC1 and Cg_ArsC2. We also solved the X-ray structures of Cg_ArsC1' and Cg_ArsC2. Both enzymes have a typical low-molecular-weight protein tyrosine phosphatases-I fold with a conserved oxyanion binding site. Moreover, Cg_ArsC1' is unique in bearing an N-terminal three-helical bundle that interacts with the active site of the other chain in the dimeric interface.
Hand-pump subsurface arsenic removal (SAR) has been investigated in rural Bangladesh with different groundwater conditions and intermittent operation modes. Multiple injection-abstraction cycles were performed after injection of 1 m3 of aerated water. From these experiments it can be concluded that hand-pump SAR, in the traditional injection-abstraction design, does not provide drinking water below the WHO arsenic guideline of 10 μg/L. Results show that arsenic removal was not enhanced by: (i) injection of O2-rich water, (ii) higher Fe:As ratios in the groundwater, or by (iii) multiple injection-abstraction cycles, i.e. at location 1, the breakthrough occurred at abstraction-injection ratios of Va/Vi = 2, for cycle 23. It is proposed that dissolved organic carbon (DOC), bicarbonate and phosphate have a significant effect on the arsenic adsorption process. However, iron removal was very efficient and abstraction-injection ratios increased within successive cycles, with Va/Vi > 8 for cycle 23. Furthermore, intermittent operation reduced arsenic concentrations after stop and restart, suggesting insufficient contact time between soluble arsenic and oxidized iron surfaces around the tube well.
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