title: Chemical mechanism and rate-limiting steps in the reaction catalyzed by Streptococcus faecalis NADH peroxidase.
abstract: The pH dependence of the kinetic parameters V, V/KNADH, and V/KH2O2 has been determined for the flavoenzyme NADH peroxidase. Both V/KNADH and V/KH2O2 decrease as groups exhibiting pK's of 9.2 and 9.9, respectively, are deprotonated. The V profile decreases by a factor of 5 as a group exhibiting a pK of 7.2 is deprotonated. Primary deuterium kinetic isotope effects on NADH oxidation are observed on V only, and the magnitude of DV is independent of H2O2 concentration at pH 7.5. DV/KNADH is pH independent and equal to 1.0 between pH 6 and pH 9.5, but DV is pH dependent, decreasing from a value of 7.2 at pH 5.5 to 1.9 at pH 9.5. The shape of the DV versus pH profile parallels that observed in the V profile and yields a similar pK of 6.6 for the group whose deprotonation decreases DV. Solvent kinetic isotope effects obtained with NADH or reduced nicotinamide hypoxanthine dinucleotide as the variable substrate are observed on V only, while equivalent solvent kinetic isotope effects on V and V/K are observed when H2O2 is used as the variable substrate. In all cases linear proton inventories are observed. Primary deuterium kinetic isotope effects on V for NADH oxidation decrease as the solvent isotopic composition is changed from H2O to D2O. These data are consistent with a change in the rate-limiting step from a step in the reductive half-reaction at low pH to a step in the oxidative half-reaction at high pH. Analysis of the multiple kinetic isotope effect data suggests that at high D2O concentrations the rate of a single proton transfer step in the oxidative half-reaction is slowed. These data are used to propose a chemical mechanism involving the pH-dependent protonation of a flavin hydroxide anion, following flavin peroxide bond cleavage.
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461.
authors.forename: V S
authors.initials: VS
authors.lastname: Stoll
authors.forename: J S
authors.initials: JS
authors.lastname: Blanchard
mesh_terms: D003903:Deuterium; D013293:Enterococcus faecalis / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009243:NAD / Q000378:metabolism; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000378:metabolism*; D012997:Solvents; D013379:Substrate Specificity
chemical_list: D012997:Solvents; D009243:NAD; D003903:Deuterium; D010544:Peroxidases; C021845:NAD+ peroxidase