Nickel superoxide dismutase reaction mechanism studied by hybrid density functional methods

The reaction mechanism for the disproportionation of the toxic superoxide radical to molecular oxygen and hydrogen peroxide by the nickel-dependent superoxide dismutase (NiSOD) has been studied using the B3LYP hybrid DFT method. Based on the recent X-ray structures of the enzyme in the resting oxidized Ni(III) and X-ray-reduced Ni(II) states, the model investigated includes the backbone spacer of six residues (sequence numbers 1-6) as a structural framework. The side chains of residues His1, Cys2, and Cys6, which are essential for nickel binding and catalysis, were modeled explicitly. The catalytic cycle consists of two half-reactions, each initiated by the successive substrate approach to the metal center. The two protons necessary for the dismutation are postulated to be delivered concertedly with the superoxide radical anions. The first (reductive) phase involves Ni(III) reduction to Ni(II), and the second (oxidative) phase involves the metal reoxidation back to its resting state. The Cys2 thiolate sulfur serves as a transient protonation site in the interim between the two half-reactions, allowing for the dioxygen and hydrogen peroxide molecules to be released in the reductive and oxidative phases, respectively. The His1 side chain nitrogen and backbone amides of the active site channel are shown to be less favorable transient proton locations, as compared to the Cys2 sulfur. Comparisons are made to the Cu- and Zn-dependent SOD, studied previously using similar models.