An ab initio model system investigation of the proposed mechanism for activation of peroxidases: Cooperative catalytic contributions from the ion and microsolvent water

In the enzymatic cycle of the peroxidase family of heme proteins, hydrogen peroxide is transformed into the catalytically active species, an Fe=O heme species known as compound I. The postulated mechanism involves the formation of a transient HOOH-Fe heme complex that is transformed via proton transfer to the oxy water isomer H₂OO-Fe heme species, which is followed by facile cleavage of the O-O bond to yield compound I and water. The proton transfer process is thought to be aided by a highly conserved distal histidine that serves as a catalyst. The plausibility of this step has been assessed in this work by characterizing the isomerization of HOOH to H₂OO in complexes with Fe⁺, Fe²⁺, Fe³⁺, Na⁺, Mg²⁺, and Al³⁺ using second-order perturbation theory and the coupled-cluster method in conjunction with various basis sets. The putative catalytic role of a proton acceptor was investigated by determining the influence of one microsolvent water on the Mg²⁺ system. The results demonstrate that although the gas-phase isomerization is highly endothermic and possesses a large activation energy, the metal ion significantly stabilizes the oxywater isomer. The Na⁺ - Mg²⁺ -Al³⁺ sequence of complexes reveals that the stabilization effect increases sharply with the charge on the ion. Fe⁺ and Fe²⁺ calculations found a small amount of additional stabilization with respect to the corresponding Na⁺ and Mg²⁺ systems. While the barrier to isomerization was not significantly reduced by binding peroxide to a cation alone, it was reduced dramatically when a single water was added to the Mg²⁺ system. The ion contributes to this effect by increasing the protonicity of the H that is being transferred, allowing it to interact more strongly with the water. The proton transfer is thus strongly enhanced by cooperative contributions by the metal ion and microsolvent water.