The C Termini of Constitutive Nitric-oxide Synthases Control Electron Flow through the Flavin and Heme Domains and Affect Modulation by Calmodulin

• Published in: The Journal of biological chemistry 2000-09, Vol.275 (38), p.29225-29232
• Main Authors: Roman, Linda J., Martásek, Pavel, Miller, R. Timothy, Harris, Dawn E., de la Garza, Melissa A., Shea, Thomas M., Kim, Jung-Ja P., Masters, Bettie Sue Siler
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Calmodulin - chemistry; Calmodulin - metabolism; Electron Transport; Escherichia coli; flavins; Flavins - chemistry; Flavins - metabolism; Heme - chemistry; Heme - metabolism; Molecular Sequence Data; Nitric Oxide Synthase - chemistry; Nitric Oxide Synthase - metabolism; Nitric Oxide Synthase Type III; Rats; Sequence Alignment
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M004766200

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.