title: Restricting the conformational freedom of the neuronal nitric-oxide synthase flavoprotein domain reveals impact on electron transfer and catalysis.
abstract: The signaling molecule nitric oxide (NO) is synthesized in animals by structurally related NO synthases (NOSs), which contain NADPH/FAD- and FMN-binding domains. During catalysis, NADPH-derived electrons transfer into FAD and then distribute into the FMN domain for further transfer to internal or external heme groups. Conformational freedom of the FMN domain is thought to be essential for the electron transfer (ET) reactions in NOSs. To directly examine this concept, we utilized a "Cys-lite" neuronal NOS flavoprotein domain and substituted Cys for two residues (Glu-816 and Arg-1229) forming a salt bridge between the NADPH/FAD and FMN domains in the conformationally closed structure to allow cross-domain disulfide bond formation or cross-linking by bismaleimides of various lengths. The disulfide bond cross-link caused a ≥95% loss of cytochromecreductase activity that was reversible with DTT treatment, whereas graded cross-link lengthening gradually increased activity, thus defining the conformational constraints in the catalytic process. We used spectroscopic and stopped-flow techniques to further investigate how the changes in FMN domain conformational freedom impact the following: (i) the NADPH interaction; (ii) kinetics of electron loading (flavin reduction); (iii) stabilization of openversusclosed conformational forms in two different flavin redox states; (iv) reactivity of the reduced FMN domain toward cytochromec; (v) response to calmodulin binding; and (vi) the rates of interflavin ET and the FMN domain conformational dynamics. Together, our findings help explain how the spatial and temporal behaviors of the FMN domain impact catalysis by the NOS flavoprotein domain and how these behaviors are governed to enable electron flow through the enzyme.
authors.affiliation: From the Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio 44195 and.|the Department of Chemistry, Cleveland State University, Cleveland, Ohio 44115.
authors.forename: Yue
authors.initials: Y
authors.lastname: Dai
authors.affiliation: From the Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio 44195 and.
authors.forename: Mohammad Mahfuzul
authors.initials: MM
authors.lastname: Haque
authors.affiliation: From the Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio 44195 and stuehrd@ccf.org.
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Stuehr
mesh_terms: D000818:Animals; D002147:Calmodulin / Q000737:chemistry; D002384:Catalysis; D003432:Cross-Linking Reagents / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D045304:Cytochromes c / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D004583:Electrons; D005415:Flavins / Q000737:chemistry; D005420:Flavoproteins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008301:Maleimides / Q000737:chemistry; D009154:Mutation; D009249:NADP / Q000737:chemistry; D009569:Nitric Oxide / Q000737:chemistry; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism*; D010084:Oxidation-Reduction; D000072417:Protein Domains; D051381:Rats; D013056:Spectrophotometry, Ultraviolet; D013438:Sulfhydryl Compounds / Q000737:chemistry
chemical_list: D002147:Calmodulin; D003432:Cross-Linking Reagents; D004220:Disulfides; D005415:Flavins; D005420:Flavoproteins; D008301:Maleimides; D013438:Sulfhydryl Compounds; D009569:Nitric Oxide; D009249:NADP; D045304:Cytochromes c; D052248:Nitric Oxide Synthase Type I; C496322:Nos1 protein, rat; D003545:Cysteine
keywords: cysteine-mediated cross-linking; domain motion; electron transfer; flavin; kinetic model; nitric oxide; pre-steady-state kinetics; redox enzyme