Organoarsenicals are introduced into the environment both from microbial biotransformation of inorganic arsenic or anthropogenically. Methylarsenicals such as monosodium methylarsonic acid (MSMA) have been extensively utilized as herbicides, and aromatic arsenicals such as roxarsone (Rox) are used as antimicrobial growth promoters for poultry and swine. The active and toxic forms are the trivalent arsenic-containing species. Microorganisms with an <i>arsI </i>gene degrade these toxic organoarsenicals to less inorganic arsenic. ArsI is a non-heme ferrous-dependent dioxygenase that catalyzes C-As bond cleavage. The crystal structure of TcArsI from the thermophilic bacterium <i>Thermomonospora curvata</i> was solved in the apo form, with metals Ni(II), Co(II) or Fe(III) or with substrates phenylarsenite (PhAs(III)) or trivalent Rox(III). Many dioygenases bind substrates directly to the active site. In contrast, the ArsI organoarsenical binding site is a vicinal cysteine pair in a flexible loop. The structure of a mutant protein (Y100H/V102F) was solved in two different crystal forms with two other orientations of the flexible loop. These results suggest that a loop-gating mechanism controls the catalytic reaction (Fig. 1). In the ligand-free open state, the loop is exposed to solvent, where it can bind trivalent organoarsenical. The loop moves toward the active site, where forms a closed state that orients the C-As bond for dioxygen addition and cleavage. Elucidation of the enzymatic mechanism of this novel C-As lyase reaction will enhance our understanding of recycling of environmental organoarsenicals.

ArsI, a C-As lyase for degradation of environmental organoarsenicals

256th National Meeting (2018)

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