Structure of the ArsI C-As lyase with bound substrate roxarsone and mutational studies of active site residues: Elucidating the catalytic mechanism of degradation of organoarsenicals

Organoarsenicals such as monosodium methyl arsenate (MSMA or MAs(V)) and roxarsone (4–hydroxyl-3-nitro-benzenenarsenate or Rox(V)) have been extensively used as an herbicide and growth enhancers for poultry, respectively. The degradation of organoarsenicals to inorganic acid As(III) contaminates crops and drinking water. The bacterial enzyme ArsI is responsible for aerobic degradation of methylarsenite (MAs(III)) and trivalent roxarsone (Rox(III)) by C-As bond cleavage. The previous crystal structure of ArsI suggests that a loop gating mechanism controls the catalytic reaction. However, the full catalytic mechanism of ArsI such as the mode of substrate binding and activation of dioxygen was unclear because of lack of a substrate binding structure. Here we report the crystal structure of ArsI with bound Rox(III) and studies of site directed mutagenesis of active site residues. These studies help to elucidate the catalytic mechanism of ArsI and will enhance understanding of the recycling of environmental organoarsenicals.