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 ~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 ~89- and ~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.