The C termini of constitutive nitric-oxide synthases control electron flow through the flavin and heme domains and affect modulation by calmodulin

Linda J. Roman, Pavel Martásek, R. Timothy Miller, Dawn E. Harris, Melissa A. De La Garza, Thomas M. Shea, Jung Ja P. Kim, Bettie Sue Siler Masters

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Abstract

The sequences of nitric-oxide synthase flavin domains closely resemble that of NADPH-cytochrome P450 reductase (CPR). However, all nitric-oxide synthase (NOS) isoforms are 20-40 residues longer in the C terminus, forming a "tail" that is absent in CPR. To investigate its function, we removed the 33 and 42 residue C termini from neuronal NOS (nNOS) and endothelial NOS (eNOS), respectively. Both truncated enzymes exhibited cytochrome c reductase activities without calmodulin that were 7-21-fold higher than the nontruncated forms. With calmodulin, the truncated and wild-type enzymes reduced cytochrome c at approximately equal rates. Therefore, calmodulin functioned as a nonessential activator of the wild-type enzymes and a partial noncompetitive inhibitor of the truncated mutants. Truncated nNOS and eNOS plus calmodulin catalyzed NO formation at rates that were 45 and 33%, respectively, those of their intact forms. Without calmodulin, truncated nNOS and eNOS synthesized NO at rates 14 and 20%, respectively, those with calmodulin. By using stopped-flow spectrophotometry, we demonstrated that electron transfer into and between the two flavins is faster in the absence of the C terminus. Although both CPR and intact NOS can exist in a stable, one-electron-reduced semiquinone form, neither of the truncated enzymes do so. We propose negative modulation of FAD-FMN interaction by the C termini of both constitutive NOSs.

Original languageEnglish (US)
Pages (from-to)29225-29232
Number of pages8
JournalJournal of Biological Chemistry
Volume275
Issue number38
DOIs
StatePublished - Sep 22 2000

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ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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