Structural and functional models of the dimanganese catalase enzymes. 3. Kinetics and mechanism of hydrogen peroxide dismutation

The mechanism of peroxide dismutation by synthetic mimics of the dimanganese catalase enzymes has been investigated by steady-state kinetic methods. These compounds, [LMn₂ ^II,II(μ-X)](ClO₄)₂, X^- = CH₃CO₂ ^- and ClCH₂CO₂ ^-, were found to share structural, redox, and spectroscopic properties analogous to the catalase enzymes (Pessiki et al. J. Am. Chem. Soc. preceding paper in this issue). The dismutation mechanism proceeds by two consecutive two-electron steps: H₂O₂ + 2e^- + 2H⁺ → 2H₂O and H₂O₂ → O₂ + 2e^- + 2H⁺ which are coupled to redox transformation of the catalyst: Mn₂ ^III,III ↔ Mn₂ ^II,II. The μ-carboxylate derivatives are inactive, but in the presence of water they autocatalytically dismutate H₂O₂ after an initial hydration reaction in which the μ-carboxylate ligand appears to dissociate, as judged by inhibition with acetate. The observed steady-state rate expression, v(O₂) = k_obs[H₂O₂]¹-[(LMn ₂(CH₃CO₂)(ClO₄)₂], ¹ k_obs = 0.23 M^-1 s^-1, exhibits the same molecularities with respect to peroxide and catalyst as observed for the enzyme from T. thermophilus, for which k_obs is 10⁷ faster. In contrast, the rate law for the μ-Cl^- derivative, LMn₂Cl₃, is second order in [H₂O₂] (Mathur et al. J. Am. Chem. Soc. 1987, 109, 5227). EPR and optical studies support a mechanism involving oxidation to a Mn₂ ^III,III intermediate and against formation of the mixed valence states, Mn₂ ^II,III and Mn₂ ^III,IV. The rate-limiting step for the model complexes is ascribed to either the inner-sphere two-electron intramolecular oxidation of the peroxide complex, [LMn₂ ^II,II(H₂O₂)]³⁺ → [LMn₂ ^III,III(OH)₂]³⁺, or a proton dissociation reaction coupled to this oxidation. Subsequent two-electron reduction to the Mn₂ ^II,II oxidation state via a second H₂O₂ molecule occurs 7-9-fold faster and completes the catalytic cycle. The 10⁷ faster rate for the enzyme is proposed to reflect either a substantially lower reduction potential for the MnCat^III,III oxidation state, the availability of active site residues which function as proton donors and acceptors, or both.