title: Thermodynamics of oxidation-reduction reactions in mammalian nitric-oxide synthase isoforms.
abstract: The three mammalian nitric-oxide synthases produce NO from arginine in a reaction requiring 3 electrons per NO, which are supplied to the catalytic center from NADPH through reductase domains incorporating FAD and FMN cofactors. The isoforms share a common reaction mechanism and requirements for reducing equivalents but differ in regulation; the endothelial and neuronal isoforms are controlled by calcium/calmodulin modulation of the electron transfer system, while the inducible isoform binds calmodulin at all physiological Ca(2+) concentrations and is always on. The thermodynamics of electron transfer through the flavin domains in all three isoforms are basically similar. The major flavin states are FMN, FMNH., FMNH(2), FAD, FADH., and FADH(2). The FMN/FMNH. couple is high potential ( approximately 100 mV) in all three isoforms and is unlikely to be catalytically competent; the other three flavin couples form a nearly isopotential group clustered around -250 mV. Reduction of the flavins by the pyridine nucleotide couple at -325 mV is thus moderately thermodynamically favorable. The ferri/ferroheme couple in all three isoforms is approximately -270 mV in the presence of saturating arginine. Ca(2+)/calmodulin has no effect on the potentials of any of the couples in endothelial nitric-oxide synthase (eNOS) or neuronal nitric-oxide synthase (nNOS). The pH dependence of the flavin couples suggests the presence of ionizable groups coupled to the flavin redox/protonation states.
authors.affiliation: Biology Department, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
authors.forename: Ying Tong
authors.initials: YT
authors.lastname: Gao
authors.forename: Susan M E
authors.initials: SM
authors.lastname: Smith
authors.forename: J Brice
authors.initials: JB
authors.lastname: Weinberg
authors.forename: Heather J
authors.initials: HJ
authors.lastname: Montgomery
authors.forename: Elena
authors.initials: E
authors.lastname: Newman
authors.forename: J Guy
authors.initials: JG
authors.lastname: Guillemette
authors.forename: Dipak K
authors.initials: DK
authors.lastname: Ghosh
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Roman
authors.forename: Pavel
authors.initials: P
authors.lastname: Martasek
authors.forename: John C
authors.initials: JC
authors.lastname: Salerno
mesh_terms: D000818:Animals; D002147:Calmodulin / Q000494:pharmacology; D002417:Cattle; D004563:Electrochemistry; D005415:Flavins / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007527:Isoenzymes / Q000737:chemistry; D019001:Nitric Oxide Synthase / Q000737:chemistry*; D052247:Nitric Oxide Synthase Type II; D052250:Nitric Oxide Synthase Type III; D010084:Oxidation-Reduction; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D013816:Thermodynamics; D017186:Titrimetry
chemical_list: D002147:Calmodulin; D005415:Flavins; D007527:Isoenzymes; D011994:Recombinant Proteins; D006418:Heme; C496319:NOS2 protein, human; C496314:NOS3 protein, human; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; D052250:Nitric Oxide Synthase Type III; C496318:Nos2 protein, rat; C496316:Nos3 protein, rat