We identified the first enzymes that use mycothiol and mycoredoxin in a thiol/disulfide redox cascade. The enzymes are two arsenate reductases from Corynebacterium glutamicum (Cg_ArsC1 and Cg_ArsC2), which play a key role in the defense against arsenate. In vivo knockouts showed that the genes for Cg_ArsC1 and Cg_ArsC2 and those of the enzymes of the mycothiol biosynthesis pathway confer arsenate resistance. With steady-state kinetics, arsenite analysis, and theoretical reactivity analysis, we unraveled the catalytic mechanism for the reduction of arsenate to arsenite in C. glutamicum. The active site thiolate in Cg_ArsCs facilitates adduct formation between arsenate and mycothiol. Mycoredoxin, a redox enzyme for which the function was never shown before, reduces the thiol-arseno bond and forms arsenite and a mycothiol-mycoredoxin mixed disulfide. A second molecule of mycothiol recycles mycoredoxin and forms mycothione that, in its turn, is reduced by the NADPHdependent mycothione reductase. Cg_ArsCs show a low specificity constant of ϳ5 M ؊1 s ؊1 , typically for a thiol/disulfide cascade with nucleophiles on three different molecules. With the in vitro reconstitution of this novel electron transfer pathway, we have paved the way for the study of redox mechanisms in actinobacteria.

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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.

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Total and toxic (sum of As(III), As(V), monomethylarsenic (MMA), and dimethylarsenic (DMA)) As concentrations were assessed in 19 respectively 4 different fish and shellfish species from the North Sea. Following results were obtained: (i) for fish an average total As concentration of 12.8 microg/g ww and a P90 value of 30.6 microg/g ww; (ii) for shellfish an average total As concentration of 21.6 microg/g ww and a P90 value of 40.0 microg/g ww; (iii) for fish an average toxic As concentration of 0.132 microg/g ww and a P90 value of 0.232 microg/g ww; (iv) for shellfish an average toxic As concentration of 0.198 microg/g ww and a P90 value of 0.263 microg/g ww. For the Belgian consumer the average daily intake of total arsenic from fish, shellfish, fruit, and soft drinks (the main food carriers of As in Belgium) amounts to 285 microg/day with more than 95% coming from fish and shellfish, while for a high level consumer it amounts to 649 microg/day, more than twice the average value. Using the same daily consumption pattern for the selected food products as for total As, we find that the average daily intake of toxic As amounts to 5.8 microg/day, with a 50% contribution of fish and shellfish and the high level intake to 9.5 microg/day. When considering the FOA/WHO Expert Committee's recommendation for inorganic As intake of 2 microg/kg bw/day or 140 microg/day for a 70 kg person, the toxic dose in Belgium is thus an order of magnitude lower.