Mechanism and Catalytic Site Atlas

Introduction

In the following mechanism, the main chain residues that are activating and stabilising the arsenite and Cys10 thiolate are omitted for clarity.

Ser 17 hydrogen bonds to the thiol hydrogen of Cys 10, lowering the pKa and increasing the nucleophilicity of the thiol S atom. Arg 16 polarises the substrate, making the As atom more electrophilic. S atom of Cys 10 nucleophilically attacks the As of the arsenate substrate. The transition state is stabilised by Asp 105 via a water molecule. A hydroxyl group of arsenate is protonated by an acidic water molecule, and leaves, forming a Cys 10-HAsO3- intermediate. The Cys 82 S atom nucleophilically attacks Cys 10 S atom, forming a Cys10-Cys82 disulphide intermediate. Electrons from the As-S bond shuttle to the arsenic, giving the release of arsenite. The Cys 89 S atom attacks Cys 82 S atom, forming a Cys 82 - Cys 89 disulphide intermediate, and breaking the Cys 10 - Cys 82 disulphide and regenerating Cys 10. The Cys 89 S atom is nucleophilically attacked by an S atom of thioredoxin at the surface of ArsC. This breaks the Cys 82 - Cys 89 disulphide, and forms one between Cys 89 and thioredoxin. The second S atom of thioredoxin nucleophilically attacks the first S atom of thioredoxin, breaking the Cys 89 - thioredoxin disulphide, and forming reduced thioredoxin.