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Structure of an As(III) S-Adenosylmethionine Methyltransferase: Insights into the Mechanism of Arsenic Biotransformation

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journal contribution
posted on 10.07.2012, 00:00 by A. Abdul Ajees, Kavitha Marapakala, Charles Packianathan, Banumathi Sankaran, Barry P. Rosen
Enzymatic methylation of arsenic is a detoxification process in microorganisms but in humans may activate the metalloid to more carcinogenic species. We describe the first structure of an As­(III) S-adenosylmethionine methyltransferase by X-ray crystallography that reveals a novel As­(III) binding domain. The structure of the methyltransferase from the thermophilic eukaryotic alga Cyanidioschyzon merolae reveals the relationship between the arsenic and S-adenosylmethionine binding sites to a final resolution of ∼1.6 Å. As­(III) binding causes little change in conformation, but binding of SAM reorients helix α4 and a loop (residues 49–80) toward the As­(III) binding domain, positioning the methyl group for transfer to the metalloid. There is no evidence of a reductase domain. These results are consistent with previous suggestions that arsenic remains trivalent during the catalytic cycle. A homology model of human As­(III) S-adenosylmethionine methyltransferase with the location of known polymorphisms was constructed. The structure provides insights into the mechanism of substrate binding and catalysis.