Structure of the ArsI C–As Lyase: Insights into the Mechanism of Degradation of Organoarsenical Herbicides and Growth Promoters

• Arsenic is the most pervasive environmental toxin.
• ArsI is a microbial non-heme, ferrous-dependent dioxygenase that detoxifies organoarsenical herbicides and growth promoters.
• The structure of ArsI shows a novel organoarsenical binding site.
• A loop-gating mechanism is proposed for substrate access to the catalytic site.

Arsenic is a ubiquitous and carcinogenic environmental element that enters the biosphere primarily from geochemical sources, but also through anthropogenic activities. Microorganisms play an important role in the arsenic biogeochemical cycle by biotransformation of inorganic arsenic into organic arsenicals and vice versa. ArsI is a microbial non-heme, ferrous-dependent dioxygenase that transforms toxic methylarsenite [MAs(III)] to less toxic and carcinogenic inorganic arsenite [As(III)] by C–As bond cleavage. An ArsI ortholog, TcArsI, from the thermophilic bacterium Thermomonospora curvata was expressed, purified, and crystallized. The structure was solved in both the apo form and with Ni(II), Co(II), or Fe(III). The MAs(III) binding site is a vicinal cysteine pair in a flexible loop. A structure with the loop occupied with β-mercaptoethanol mimics binding of MAs(III). 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. In the ligand-free open state, the loop is exposed to solvent, where it can bind MAs(III). The loop moves toward the active site, where it forms a closed state that orients the C–As bond for dioxygen addition and cleavage. Elucidation of the enzymatic mechanism of this unprecedented C–As lyase reaction will enhance our understanding of recycling of environmental organoarsenicals.

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