Protein coordination to manganese determines the high catalytic rate of dimanganese catalases. Comparison to functional catalase mimics

Catalysis of hydrogen peroxide dismutation by the dimanganese catalase from Thermus thermophilus has been measured and found to obey Michaelis-Menton kinetics with no evidence for substrate inhibition at concentrations up to 0.45 M H₂O₂. Comparison among three dimanganese catalases (Thermus thermophilus. Thermoleophilium album, and Lactobacillus plantarum) reveals that their apparent second-order rate constants, k_cat/K_m, differ by at most a factor of 5, even though the individual kinetic constants differ by as much as a factor of 20. This similarity suggests that all three enzymes may have the same rate-determining step. For T. thermophilus catalase we find that k_cat/K_m ∼ k_bi, the bimolecular rate constant at limiting substrate concentrations. Thus, the rate of the rate-determining step is unaltered over the entire range of substrate concentrations, unlike T. album and L. plantarum catalases where substrate inhibition has been reported. Comparison to structurally characterized dimanganese complexes and dimetalloproteins (arginase, hemerythrin), which are functional, albeit kinetically slow, catalase mimics, reveals that high catalase activity correlates with a greater number of stronger σ-ligand donors like anionic carboxylatos vs neutral histidines that stabilize the oxidized Mn₂(III,III) state over reduced Mn₂(II,II). A critical feature for enzymatic functionality in vivo is suppression of one-electron chemistry leading to formation of the mixed-valence forms, Mn₂(III,IV) and Mn₂(II,III), which are kinetically inactive or precursors to inactive species, respectively. Evidence is presented from model compounds suggesting that the μ-carboxylato bridge between Mn ions in catalase may play the key role in suppressing formation of these detrimental oxidation states through destabilization of these one-electron redox processes.