The Effect of a Water Molecule on the Mechanism of Formation of Compound 0 in Horseradish Peroxidase

We demonstrate by QM/MM calculations that the formation of Compound 0, an intermediate in the catalytic cycle of horseradish peroxidase, is catalyzed by a single water molecule, which relays proton transfer between the substrate (H 2 O 2 ) and the acceptor (His42). Thus, a single water molecule enhances the thermodynamic and kinetic acidities of H 2 O 2 by many orders of magnitude. The behavior of the water molecule during the activation of H 2 O 2 may account for the experimental reports of extensive entropic effects during the formation of the principal active species of the enzyme Compound I.