NiII Complex Formation and Protonation States at the Active Site of a Nickel Superoxide Dismutase-Derived Metallopeptide: Implications for the Mechanism of Superoxide Degradation.

A small, catalytically active metallopeptide (Nim6 SOD, m6 SOD=ACDLAC), which was derived from the nickel superoxide dismutase (NiSOD) active site was employed to study the mechanism of superoxide degradation, especially focusing on the protonation states of the NiII donor atoms, the proton source, and the role of the N-terminal proton(s). Therefore, the NiII -metallopeptide was studied at various pHs and temperatures using UV/Vis and NMR spectroscopy. These studies indicate a strong reduction of the pKa of the NiII -ligating donor atoms, resulting in a fully deprotonated NiII active-site environment. Furthermore, no titratable proton could be observed within a pH ranging from 6.5 to 10.5. This rules out a recently discussed adiabatic proton tunneling-like hydrogen-atom transfer process for the metallopeptides, not found in the native enzyme. Furthermore, variable-temperature 1 H NMR measurements uncovered an extended hydrogen-bond network within the NiII active site of the metallopeptide similar to the enzyme. With respect to the deprotonated NiII active site, the residual N-terminal proton, which is a prerequisite for catalytic activity, cannot act as proton source. Most likely, it stabilizes the NiII -coordinated substrate in an end-on fashion, thus allowing for an inner-sphere electron transfer. Lastly, and unlike the enzyme, the catalytic rate constant of superoxide degradation by the metallopeptides was determined to be strongly pH dependent, suggesting bulk water to be directly involved in proton donation, which in turn strongly suggests the N-terminal histidine to be the respective proton donor in the enzyme.