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 diman ganese catalase enzymes has been investigated by steady-state kinetic methods. These compounds, [LMn(2)(II,II)(mu-X)](ClO4)(2), X(-) = CH3CO 2- and ClCH2CO2-, were found to share structural, redox, and spectrosc opic properties analogous to the catalase enzymes (Pessiki et al. J. A m. Chem. Sec. preceding paper in this issue). The dismutation mechanis m proceeds by two consecutive two-electron steps: H2O2 + 2e(-) + 2H(+) --> 2H(2)O and H2O2 --> O-2 + 2e(-) + 2H(+) which are coupled to redo x transformation of the catalyst: Mn-2(III),(III) <----> Mn-2(II),(II) . The mu-carboxylate derivatives are inactive, but in the presence of water they autocatalytically dismutate H2O2 after an initial hydration reaction in which the mu-carboxylate ligand appears to dissociate, as judged by inhibition with acetate. The observed steady-state rate exp ression, nu(O-2) = k(obs)[H2O2](1)- [(LMn(2)(CH3CO2)(ClO4)(2)],(1) k(o bs) = 0.23 M(-1) s(-1), exhibits the same molecularities with respect to peroxide and catalyst as observed for the enzyme from T. thermophil us, for which k(obs) is 10(7) faster. In contrast, the rate law for th e mu-Cl-derivative, LMn(2)Cl(3), is second order in [H2O2] (Mathur et al. J. Am. Chem. Soc. 1987, 109, 5227). EPR and optical studies suppor t a mechanism involving oxidation to a Mn-2(III,III) intermediate and against formation of the mixed valence states, Mn-2(II,III) and Mn-2(I II,IV). The rate-limiting step for the model complexes is ascribed to either the inner-sphere two-electron intramolecular oxidation of the p eroxide complex, [LMn(2)(II,II)(H2O2)](3+) --> [LMn(2)(III,III)(OH)(2) ](3+), or a proton dissociation reaction coupled to this oxidation. Su bsequent two-electron reduction to the Mn-2(II,II) oxidation state via a second H2O2 molecule occurs 7-9-fold faster and completes the catal ytic cycle. The 10(7) faster rate for the enzyme is proposed to reflec t either a substantially lower reduction potential for the MnCat(III,I II) oxidation state, the availability of active site residues which fu nction as proton donors and acceptors, or both.