Electrostatic Effect on Electron Transfer at the Active Site of Heme Peroxidases:  A Comparative Molecular Orbital Study on Cytochrome C Peroxidase and Ascorbate Peroxidase

Ab initio minimal basis set molecular orbital and discretized Poisson−Boltzmann electrostatic potential calculations were carried out on large models of the active site of heme peroxidases (cytochrome C peroxidase and cytosolic ascorbate peroxidase) to rationalize the location of the radical site in Compound I, an intermediate of the enzymatic reaction. Both molecular orbital and electrostatic potential calculations suggest that the spin distribution depends on the protonation state of the proximal His...Asp...Trp triad. If the transferable proton is shifted from the Trp side chain to Asp, the radical localizes on the indole group, while if it remains on the Trp the unpaired electron transfers to the heme group. Therefore, in cytochrome C peroxidase, Trp is deprotonated in Compound I, while it is protonated and neutral in cytosolic ascorbate peroxidase. Protonation state of the proximal residues is influenced by the electrostatic effect of the protein environment that differs in these enzymes, especially in the immediate vicinity of the Asp side chain. In contrast to earlier propositions we did not find evidence for the effect of the neighboring potassium-binding loop on the localization of the free radical. The heme peroxidases studied provide examples for the electrostatic, protonation-mediated modulation of electron transfer.