A Computational and Conceptual DFT Study on the Michaelis Complex of pI258 Arsenate Reductase. Structural Aspects and Activation of the Electrophile and Nucleophile

Abstract

The first step in the reduction of arsenate to arsenite catalyzed by the enzyme arsenate reductase (ArsC) from Staphylococcus aureus plasmid pI258 involves the nucleophilic attack of a cysteine thiolate (Cys10) on the arsenic atom, leading to a covalent sulfur−arseno intermediate. We present a quantum chemical study on the onset of the nucleophilic displacement reaction. To optimize the reactant state geometry, a density functional study was performed on Cys10, on dianionic arsenate, and on the catalytic site sequence motif: X-X-Asn13-X-X-Arg16-Ser17. Both the hydrogen bond from Arg16 to the leaving hydroxyl group of arsenate and the hydrogen bonds from various backbone amide nitrogens of the catalytic site to the other oxygen atoms of arsenate are responsible for the increased electrophilicity of the central arsenic atom. In particular, Arg16 is identified as a residue that destabilizes the groundstate of the complex. Furthermore, the binding of dianionic arsenate to the enzyme induces negative charge t...