Theoretical Study of the Catalytic Mechanism of the Cu-Only Superoxide Dismutase

• The catalytic mechanisms for the wild-type and the mutated Cu-only superoxide dismutase were studied using the hybrid density functional B3LYP and a quantum chemical cluster approach. Optimal protonation states of the active site were examined for each stage of the catalytic cycle. For both the reductive and the oxidative half-reactions, the arrival of the substrate O-2(center dot-) was found to be accompanied by a chargecompensating H+ with exergonicities of -15.4 kcal center dot mol and -4.7 kcal center dot mol, respectively. The second-sphere Glu-110 and first-sphere His-93 were suggested to be the transient protonation site for the reductive and the oxidative half-reactions, respectively, which collaborates with the hydrogen bonding water chain to position the substrate near the redox-active copper center. For the reductive half-reaction, the rate-limiting step was found to be the inner-sphere electron transfer from the partially coordinated O-2(center dot-) to Cu-II with a barrier of 8.1 kcal center dot mol. The formed O-2 is released from the active site with an exergonicity of -14.9 kcal center dot mol. For the oxidative half-reaction, the inner-sphere electron transfer from CuI to the partially coordinated O-2(center dot-) was found to be accompanied by the proton transfer from the protonated His-93 and barrierless. The rate-limiting step was found to be the second proton transfer from the protonated Glu-110 to HO2 with a barrier of 7.3 kcal center dot mol. The barriers are reasonably consistent with experimental activities, and a proton-transfer rate-limiting step in the oxidative half-reaction could explain the experimentally observed pH-dependence. For the E110Q CuSOD, Asp-113 was suggested to be likely to serve as the transient protonation site in the reductive half-reaction. The rate-limiting barriers were found to be 8.0 and 8.6 kcal center dot mol, respectively, which could explain the slightly lower performance of E110X mutants. The results were found to be stable, with respect to the percentage of exact exchange in B3LYP.

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NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

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