Kinetic and spectroscopic characterization of the native and inhibitor bound forms of the Lactobacillus plantarum manganese catalase and related proteins.

Abstract

Manganese catalase is a dinuclear Mn enzyme which cycles between the Mn(II)/Mn(II) and Mn(III)/Mn(III) redox states while performing hydrogen peroxide disproportionation. X-ray absorption (XAS) data for the Mn(II)/Mn(II) catalase were compared to data for inorganic Mn models and for the proteins arginase, Mn substituted ribonucleotide reductase and hemerythrin, in order to characterize the role of metal-ligand structure in catalase activity. All four proteins have similar oxygen and nitrogen based Mn-nearest neighbor ligand environments, although the distribution of ligands is such that there are two resolvable shells (Mn-O and Mn-N) in the catalase and hemerythrin. Outer shell carbon XAS scattering places lower limits on the number of bound imidazoles at ca. 4 for hemerythrin, 2 for catalase and 1 for arginase and ribonucleotide reductase, in good agreement with crystallographic data reported by others. Manganese-manganese distances of 3.53 A and 3.39 A were determined for the fluoride inhibited and ornithine + peroxide forms of catalase and arginase, respectively. Fluoride inhibition is pH dependent and can be successfully modeled using a non competitive slow-binding inhibition model ($K\sb{i}$ = 0.4 mM at pH = 7.0). Inhibition is independent of Mn oxidation state. Regardless of starting conditions, the final inhibited Mn(II)/Mn(II)-F enzyme has a structure that is nearly identical to that of the uninhibited Mn(II)/Mn(II) enzyme, except that Mn$\cdots$Mn scattering intensity increases for the halide bound sample, suggesting an increase in the rigidity of the enzyme's Mn$\sb2$ core. Azide and cyanide bind to the Mn(III)/Mn(IV) catalase in a pH dependent fashion (azide $K\sb{d}$ = 15 mM at pH = 7.0). XAS and magnetic circular dichroism analyses suggest that the Mn$\rm\sb2O\sb2$ core goes from a puckered to a more planar structure upon ion binding. Electron spin echo envelope modulation spectra collected on isotopically labeled forms of azide and cyanide show these ions bind at an outer sphere position and induce this conformational change. Based on the combined spectroscopic and kinetic studies, a mechanism has been proposed for the ornithine and peroxide inhibition of arginase and a revised mechanism has been suggested for the Mn catalase. The revised mechanism for the Mn catalase suggests that the enzyme, in it's active states, alternate between Mn$\sb2$ core structures which are bis-($\mu$-CO$\sb2\sp-$),($\mu$-OH$\sb2$) and bis-($\mu$-CO$\sb2\sp-$),($\mu$-OH) bridged in the Mn(II)/Mn(II) and Mn(III)/Mn(III) oxidation states, respectively. In addition, a distal amino acid base assists in catalase by acting as a proton donator and acceptor, as well as stabilizing solvent binding.