Evidence for a bidomain structure of constitutive cerebellar nitric oxide synthase.
Nitric oxide synthase (NOS) catalyzes the NADPH-dependent, Ca2+/calmodulin-dependent formation of NO and citrulline from L-arginine and molecular oxygen. The localization of the heme-binding consensus sequence in the NH2-terminal half of NOS and of the binding sequences for nucleotides (FMN and FAD) in the COOH-terminal half suggests a bidomain structure. In addition, the presence of a putative calmodulin-binding sequence between the heme- and flavin-binding domains of the enzyme suggests a role for calmodulin in modulating a spatial orientation of these domains that is required for catalytic activity. First, to determine the effects of calmodulin and the functionality of the separated domains, Ca2+/calmodulin binding-induced conformational changes in NOS were measured by fluorescence quenching, from which a binding constant of approximately 1 nM for calmodulin was calculated. Second, electron transport to various artificial acceptors was measured. The addition of Ca2+/calmodulin increased cytochrome c reduction from 10-15-fold while stimulating the rate of 2,6-dichlorophenolindophenol and ferricyanide reduction only slightly, if at all. Calmodulin stimulation of NOS results in NADPH-mediated cytochrome c reduction, which is sensitive to superoxide dismutase, and the reduction of acetylated cytochrome c, which is only weakly reducible by unstimulated NOS. Thus, this stimulated activity is presumably superoxide anion-mediated. Third, limited proteolysis of NOS in the absence of calmodulin resulted in a time-dependent increase in cytochrome c reductase activity, which was not inhibitable by superoxide dismutase, and a decrease in catalysis of NO formation. SDS-polyacrylamide gel electrophoresis analysis of the tryptic digest demonstrated the formation of approximately 89- and approximately 79-kDa fragments. Sequence analysis of the peptides confirmed that trypsin cleaves the enzyme in the putative calmodulin-binding region beginning with Ala728. This region was protected from proteolysis by the addition of Ca2+/calmodulin. The separated NH2-terminal domain exhibited the characteristic spectrum of bound heme, while the COOH-terminal domain showed the characteristic spectrum of bound flavins. Other cleavage patterns were obtained in the presence of calmodulin. The data demonstrate that the heme- and flavin-binding domains of NOS can be isolated in functionally intact forms.
Sheta
E A
EA
Department of Biochemistry, University of Texas Health Science Center, San Antonio 78284-7760.
McMillan
K
K
Masters
B S
BS
D000594:Amino Acid Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D001921:Brain / Q000201:enzymology*; D002147:Calmodulin / Q000378:metabolism; D002478:Cells, Cultured; D006418:Heme / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D008969:Molecular Sequence Data; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D019001:Nitric Oxide Synthase; D013482:Superoxide Dismutase / Q000378:metabolism
D002147:Calmodulin; D006418:Heme; D019001:Nitric Oxide Synthase; D013482:Superoxide Dismutase; D000594:Amino Acid Oxidoreductases; D009251:NADPH-Ferrihemoprotein Reductase