Reorientation of the Methyl Group in MAs(III) is the Rate-Limiting Step in the ArsM As(III) <i>S</i>‑Adenosylmethionine Methyltransferase Reaction

The most common biotransformation of trivalent inorganic arsenic (As­(III)) is methylation to mono-, di-, and trimethylated species. Methylation is catalyzed by As­(III) <i>S</i>-adenosylmethionine (SAM) methyltransferase (termed ArsM in microbes and AS3MT in animals). Methylarsenite (MAs­(III)) is both the product of the first methylation step and the substrate of the second methylation step. When the rate of the overall methylation reaction was determined with As­(III) as the substrate, the first methylation step was rapid, whereas the second methylation step was slow. In contrast, when MAs­(III) was used as the substrate, the rate of methylation was as fast as the first methylation step when As­(III) was used as the substrate. These results indicate that there is a slow conformational change between the first and second methylation steps. The structure of CmArsM from the thermophilic alga Cyanidioschyzon merolae sp. 5508 was determined with bound MAs­(III) at 2.27 Å resolution. The methyl group is facing the solvent, as would be expected when MAs­(III) is bound as the substrate rather than facing the SAM-binding site, as would be expected for MAs­(III) as a product. We propose that the rate-limiting step in arsenic methylation is slow reorientation of the methyl group from the SAM-binding site to the solvent, which is linked to the conformation of the side chain of a conserved residue Tyr70