title: Effects of Ca(2+)-activated calmodulin on neuronal nitric oxide synthase reductase activity and binding of substrates: pH dependence of kinetic parameters.
abstract: The pH dependence of basal and calmodulin- (CaM-) stimulated neuronal nitric oxide synthase (nNOS) reduction of 2,6-dichloroindophenol (DCIP) and cytochrome c(3+) was investigated. The wave-shaped log V versus pH profile revealed that optimal DCIP reduction occurred when a group, pK(a) of 7.6-7.8, was ionized. The (V/K)(NADPH) and (V/K)(DCIP) versus pH profiles increased with the protonation of a group with a pK(a) of 6.5 or 5.9 and the ionization of two groups with the same pK(a) of 7.5 or 7.0, respectively. (V/K)(DCIP) decreased with the ionization of a group, pK(a) of 9.0. Similar V, (V/K)(NADPH), and (V/K)(DCIP) versus pH profiles for DCIP reduction were obtained with and without CaM, indicating that CaM does not influence ionizable groups involved in catalysis or substrate binding. In contrast, CaM affected the pH dependence of cytochrome c(3+) reduction. The wave-shaped log V versus pH profile for basal cytochrome c(3+) reduction revealed that ionization of a group, pK(a) of 8.6, increased catalysis. Log V for CaM-stimulated cytochrome c(3+) reduction displayed a bell-shaped pH dependence with the protonation of a group with a pK(a) of 6.4 and the ionization of a group with a pK(a) of 9.3, resulting in a loss of activity. The log(V/K)(cytc) versus pH profiles with and without CaM were bell-shaped with the ionization of a group at pK(a) of 7.1 or 7.6 (CaM) or pK(a) of 9.4 or 9.6 (CaM), increasing and decreasing (V/K)(cytc). These results suggest that CaM may change the nature of the rate-limiting catalytic steps or ionizable groups involved in cytochrome c(3+) reduction.
authors.affiliation: Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331, USA.
authors.forename: Kirsten R
authors.initials: KR
authors.lastname: Wolthers
authors.forename: Michael I
authors.initials: MI
authors.lastname: Schimerlik
mesh_terms: D015086:2,6-Dichloroindophenol / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000494:pharmacology*; D002147:Calmodulin / Q000378:metabolism*; D002384:Catalysis; D003574:Cytochrome c Group / Q000037:antagonists & inhibitors / Q000378:metabolism; D004579:Electron Transport; D004926:Escherichia coli / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D009249:NADP / Q000737:chemistry / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000378:metabolism*; D052248:Nitric Oxide Synthase Type I; D010084:Oxidation-Reduction; D011485:Protein Binding; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification; D013379:Substrate Specificity
chemical_list: D002147:Calmodulin; D003574:Cytochrome c Group; D011994:Recombinant Proteins; D009249:NADP; C017165:cytochrome c(3); D015086:2,6-Dichloroindophenol; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; C496322:Nos1 protein, rat; D002118:Calcium