Thermodynamics of Oxidation-Reduction Reactions in Mammalian Nitric-oxide Synthase Isoforms

Ying Tong Gao, Susan M.E. Smith, J. Brice Weinberg, Heather J. Montgomery, Elena Newman, J. Guy Guillemette, Dipak K. Ghosh, Linda J. Roman, Pavel Martasek, John C. Salerno

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53 Scopus citations


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., FMNH2, FAD, FADH., and FADH2. The FMN/FMNH. couple is high potential (∼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 ∼270 mV in the presence of saturating arginine. Ca2+/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.

Original languageEnglish (US)
Pages (from-to)18759-18766
Number of pages8
JournalJournal of Biological Chemistry
Issue number18
StatePublished - Apr 30 2004

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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