Tri-Esterase and Promiscuous Di-Esterase Activities of a di-Co(II)containing Organo-Phosphate Degrading Enzyme Reaction Mechanisms

The reaction mechanism for the hydrolysis of trimethylphosphate and of the obtained phosphodiester by the di-Co(II) derivative of Organo-Phosphate Degrading Enzyme from Agrobacterium radiobacter P230(OpdA), have been investigated at density functional level of theory in the framework of the cluster model approach. Both mechanisms proceed by a multistep sequence and each catalytic cycle begins with the nucleophilic attack by a metal-bound hydroxide on the phosphorus atom of the substrate, leading to the cleavage of the phosphate-ester bond. Four exchange-correlation functionals have been used to derive the potential energy profiles in protein environments. Despite the enzyme is confirmed to work better as tri-esterase, as revealed by the barrier heights in the rate limiting steps of catalytic processes, its promiscuous ability to hydrolyze also the product of the reaction has been confirmed. The important role played by water molecules and some residues in the outer coordination sphere has been elucidated, while the binuclear Co(II) centre accomplishes both structural and catalytic functions. To correctly describe the electronic configuration of the d shell of the metal ions, high- and low- spin arrangement jointly with the occurrence of antiferromagnetic (AFM) coupling, have been herein considered.