 | Acta Crystallographica Section D Acta Crystallographica Section D BIOLOGICAL CRYSTALLOGRAPHY |
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![[Figure 7]](cb5072fig7mag.jpg)
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Figure 7
Proposed reaction scheme for AsMTs. The proposed reaction pathway is a general one for AsMTs, but the numbering of the cysteine residues follows the sequence of CmArsM. (1) In the first round of methylation AsMT binds arsenic(III) in a series of three thiol-transfer reactions. (2) The methyl group of SAM is attacked by the arsenic lone pair. (3) A positively charged pentavalent MAs(V) intermediate is formed and (4) reduced to an enzyme-bound MAs(III) intermediate by Cys44, with the formation of a Cys44–Cys72 disulfide. (5) The disulfide is reduced by Trx, regenerating the enzyme for the second round of methylation. (6) MAs(III), which is tightly bound to Cys174–Cys224, is methylated, producing an enzyme-bound positively charged pentavalent DMAs(V) intermediate (7) which is reduced to DMAs(III) by Cys72 with the formation of a Cys72–Cys174 disulfide. (8) DMAs(III), which is weakly bound to the single Cys224, dissociates from the enzyme and is oxidized nonenzymatically to DMAs(V). Finally, the Cys72–Cys174 disulfide is reduced by Trx, allowing the cycle to begin over. |
![[Figure 7]](cb5072fig7.jpg)
| BIOLOGICAL CRYSTALLOGRAPHY |
ISSN: 1399-0047
