The role of Arginine 38 in HRP enzyme revisited: a computational investigation

Molecular dynamics simulations on hydrogen peroxide complex with wild-type (WT) and Arg38Leu mutated (R38L) Horseradish Peroxidase (HRP) were carried out over nanoseconds timescale in water solution at 300 K. Comparison of the results provides interesting insights about the role of highly conserved Arg38 and His42 residues in the chemical features of HRP, underlying its biological activity which initiates with Compound0 (Cpd0). In the WT-HRP enzyme current molecular dynamics simulations show, for the first time, that Arg38 residue: i) prevents the entrance of water inside the reaction cavity, hence providing a hydrophobic reactive scenario, ii) it maintains the distance between His42 and heme-H2O2 complex suitable for the occurrence of proton transfer reaction leading, thereafter, to heme-H2O2 disruption according to Poulos-Kraut mechanism. On the other hand, R38L mutant can be considered as a "wet enzyme" where the presence of water solvent molecules in the heme reaction pocket, unfavoring the initial heme-H2O2 complex formation, decreases the catalytic efficiency in agreement with experimental kinetics measurements. Furthermore, we note that Arg38Leu mutation pushes the His42 residue far from the heme-H2O2 complex, making unlikely a direct proton transfer and suggesting that, in the mutant, a solvent water molecule could be involved in the first step of the Poulos-Kraut mechanism.