Quantum Mechanical and Quantum Mechanical/Molecular Mechanical Studies of the Iron-Dioxygen Intermediates and Proton Transfer in Superoxide Reductase.

Classical and quantum-chemical computations are employed to probe the reaction intermediates and proton-transfer processes in superoxide reductase (SOR) from Desulfoarculus baarsii. Ab initio studies of the SOR active site, as well as classical and QM/MM MD simulations on the overall enzymatic reaction, are performed. We explore the use of a Hubbard U correction to standard density functional theory (DFT) in order to obtain a better description of the strongly correlated d electrons in the transition-metal center. The results obtained from the standard and Hubbard-U-corrected DFT approaches are compared with those obtained using different hybrid-DFT functionals. We show that the Hubbard U correction gives a significant improvement in the description of the structural, energetic, and electronic properties of SOR. We establish that adopting the Hubbard U correction in the QM/MM approach leads to increased accuracy with essentially no additional computational cost. Our results suggest that Lys(48) is one of the likely sources of the first proton donation to the superoxide, either directly or through an interstitial water molecule. Our QM/MM calculations highlight the important role of the interactions and hydrogen-bond network created by the imidazole rings of the His ligands and the internal water molecules. Whereas the hydrogen-bonding pattern due to internal waters can facilitate the protonation event, the interactions with the His ligands and the hydrogen bonds with water can stabilize the dioxygen ligand in a side-on conformation, which, in turn, prevents the immediate proton transfer from Lys(48), as indicated by recent experimental studies.