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

Abstract

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.

Identification of Resistance Genes and Response to Arsenic in Rhodococcus aetherivorans BCP1

“…MD1, Thermomonospora curvata DSM 43183, and Nostoc sp. PCC 7120, depicts the conservation of the cysteine pair Cys98-99 which is predicted to be the binding sites of MAs(III) and other trivalent organoarsenicals (Supplementary Figure S2) (Yoshinaga and Rosen, 2014; Yan et al, 2015; Nadar et al, 2016). In addition, the associated ArsR (named as ArsR ∗ ) possesses two conserved cysteines corresponding to the cysteines 101 and 102 which represent the binding sites for the MAs(III) in the MAs(III)-responsive repressor ArsRs (Chen et al, 2017).…”

Socio-Technical Changes for Sustainable Rice Production: Rice Husk Amendment, Conservation Irrigation, and System Changes

“…Because iAs is methylated to dimethylarsinic acid (DMA) under reducing soil conditions, more oxic conditions may lead to less DMA production and therefore more iAs uptake relative to DMA Jia et al, 2013;Chen et al, 2019a;Dykes et al, 2021). These findings underscore the need for further biogeochemical research on the effects of redox conditions on microbial As methylation-demethylation reactions in the soil solution, and whether oxic conditions may enhance As demethylation via oxygen-dependent cleavage of As-C bonds by ArsI enzymes (Yoshinaga and Rosen, 2014;Nadar et al, 2016;Pawitwar et al, 2017).…”

Biochemical Characterization of ArsI: A Novel C–As Lyase for Degradation of Environmental Organoarsenicals

“…31 The substrate-binding loop acts as a gate that opens as the loop moves to solvent, where substrate is bound, in turn, the loop moves to the active site, where catalysis occurs, closing the gate. The flexible substrate-binding loop allows substrate to bind independently of the metal.…”