Characterization of C415 mutants of neuronal nitric oxide synthase

Site and mechanism of uncoupling of nitric-oxide synthase: Uncoupling by monomerization and other misconceptions

Nitric oxide synthase (NOS) catalyzes the transformation of l-arginine, molecular oxygen (O2), and NADPH-derived electrons to nitric oxide (NO) and l-citrulline. Under some conditions, however, NOS catalyzes the reduction of O2 to superoxide (O2-) instead, a phenomenon that is generally referred to as uncoupling. In principle, both the heme in the oxygenase domain and the flavins in the reductase domain could catalyze O2- formation. In the former case the oxyferrous (Fe(II)O2) complex that is formed as an intermediate during catalysis would dissociate to heme and O2-; in the latter case the reduced flavins would reduce O2 to O2-. The NOS cofactor tetrahydrobiopterin (BH4) is indispensable for coupled catalysis. In the case of uncoupling at the heme this is explained by the essential role of BH4 as an electron donor to the oxyferrous complex; in the case of uncoupling at the flavins it is assumed that the absence of BH4 results in NOS monomerization, with the monomers incapable to sustain NO synthesis but still able to support uncoupled catalysis. In spite of little supporting evidence, uncoupling at the reductase after NOS monomerization appears to be the predominant hypothesis at present. To set the record straight we extended prior studies by determining under which conditions uncoupling of the neuronal and endothelial isoforms (nNOS and eNOS) occurred and if a correlation exists between uncoupling and the monomer/dimer equilibrium. We determined the rates of coupled/uncoupled catalysis by measuring NADPH oxidation spectrophotometrically at 340 nm and citrulline synthesis as the formation of [3H]-citrulline from [3H]-Arg. The monomer/dimer equilibrium was determined by FPLC and, for comparison, by low-temperature polyacrylamide gel electrophoresis. Uncoupling occurred in the absence of Arg and/or BH4, but not in the absence of Ca2+ or calmodulin (CaM). Since omission of Ca2+/CaM will completely block heme reduction while still allowing substantial FMN reduction, this argues against uncoupling by the reductase domain. In the presence of heme-directed NOS inhibitors uncoupling occurred to the extent that these compound allowed heme reduction, again arguing in favor of uncoupling at the heme. The monomer/dimer equilibrium showed no correlation with uncoupling. We conclude that uncoupling by BH4 deficiency takes place exclusively at the heme, with virtually no contribution from the flavins and no role for NOS monomerization.

Generation and characterization of functional phosphoserine-incorporated neuronal nitric oxide synthase holoenzyme

Phosphorylation is an important pathway for the regulation of nitric oxide synthase (NOS) at the posttranslational level. However, the molecular underpinnings of NOS regulation by phosphorylations remain unclear to date, mainly because of the problems in making a good amount of active phospho-NOS proteins. Herein, we have established a system in which recombinant rat nNOS holoprotein can be produced with site-specific incorporation of phosphoserine (pSer) at residue 1412, using a specialized bacterial host strain for pSer incorporation. The pSer1412 nNOS protein demonstrates UV-Vis, far-UV CD and fluorescence spectral properties that are identical to those of nNOS overexpressed in other bacterial strains. The protein is also functional, possessing normal NO production and NADPH oxidation activities in the presence of abundant substrate L-Arg. Conversely, the rate of FMN-heme interdomain electron transfer (IET) in pSer1412 nNOS is considerably lower than that of wild-type (wt) nNOS, while the phosphomimetic S1142E mutant possesses similar electron transfer kinetics to that of wt. The successful incorporation and high yield of pSer1412 into rat nNOS and the significant change in the IET kinetics upon the phosphorylation demonstrate a highly useful method for incorporating native phosphorylation sites as a substantial improvement to commonly used phosphomimetics.

Role of an isoform-specific residue at the calmodulin-heme (NO synthase) interface in the FMN - heme electron transfer

The interface between calmodulin (CaM) and the NO synthase (NOS) heme domain is the least characterized interprotein interface that the NOS isoforms must traverse through during catalysis. Our previous molecular dynamics simulations predicted a salt bridge between K497 in human inducible NOS (iNOS) heme domain and D118(CaM). Herein, the FMN - heme interdomain electron transfer (IET) rate was found to be notably decreased by charge-reversal mutation, while the IET in the iNOS K497D mutant is significantly restored by the CaM D118K mutation. The results of wild-type protein with added synthetic peptides further demonstrate the critical nature of K497 relative to the rest of the peptide sequence in modulating the IET. These data provide definitive evidence supporting the regulatory role of the isoform-specific K497 residue.

Molecular architecture of mammalian nitric oxide synthases

NOSs are homodimeric multidomain enzymes responsible for producing NO. In mammals, NO acts as an intercellular messenger in a variety of signaling reactions, as well as a cytotoxin in the innate immune response. Mammals possess three NOS isoforms--inducible, endothelial, and neuronal NOS--that are composed of an N-terminal oxidase domain and a C-terminal reductase domain. Calmodulin (CaM) activates NO synthesis by binding to the helical region connecting these two domains. Although crystal structures of isolated domains have been reported, no structure is available for full-length NOS. We used high-throughput single-particle EM to obtain the structures and higher-order domain organization of all three NOS holoenzymes. The structures of inducible, endothelial, and neuronal NOS with and without CaM bound are similar, consisting of a dimerized oxidase domain flanked by two separated reductase domains. NOS isoforms adopt many conformations enabled by three flexible linkers. These conformations represent snapshots of the continuous electron transfer pathway from the reductase domain to the oxidase domain, which reveal that only a single reductase domain participates in electron transfer at a time, and that CaM activates NOS by constraining rotational motions and by directly binding to the oxidase domain. Direct visualization of these large conformational changes induced during electron transfer provides significant insight into the molecular underpinnings governing NO formation.

FRET conformational analysis of calmodulin binding to nitric oxide synthase peptides and enzymes

Calmodulin (CaM) is a ubiquitous Ca (2+)-sensor protein that binds and activates the nitric oxide synthase (NOS) enzymes. We have used fluorescence resonance energy transfer (FRET) to examine the conformational transitions of CaM induced by its binding to synthetic nitric oxide synthase (NOS) CaM-binding domain peptides and full length heme-free constitutive NOS (cNOS) enzymes over a range of physiologically relevant free Ca (2+) concentrations. We demonstrate for the first time that the domains of CaM collapse when associated with Ca (2+)-independent inducible NOS CaM-binding domain, similar to the previously solved crystal structures of CaM bound to the Ca (2+)-dependent cNOS peptides. We show that the association of CaM is not detectable with the cNOS peptides at low free Ca (2+) concentrations (<40 nM). In contrast, we demonstrate that CaM associates with the cNOS holo-enzymes in the absence of Ca (2+) and that the Ca (2+)-dependent transition occurs at a lower free Ca (2+) concentration with the cNOS holo-enzymes. Our results suggest that other regions outside of the CaM-binding domain in the cNOS enzymes are involved in the recruitment and binding of CaM. We also demonstrate that CaM binds to the cNOS enzymes in a sequential manner with the Ca (2+)-replete C-lobe binding first followed by the Ca (2+)-replete N-lobe. This novel FRET study helps to clarify some of the observed similarities and differences between the Ca (2+)-dependent/independent interaction between CaM and the NOS isozymes.

Nitric Oxide Synthase (NOS) Characterization in Leishmania amazonensis Axenic Amastigotes

BACKGROUND

Although Leishmania virulence may be modulated by environmental and genetic factors of their mammalian hosts and sand fly vectors, molecular determinants of Leishmania sp. are the key elements. This work evidences that Leishmania amazonensis axenic amastigotes produce comparatively more NO than infective promastigotes.

METHODS

A soluble NOS was purified from L. amazonensis axenic amastigotes by affinity chromatography (2',5'-ADP-agarose), and on SDS-PAGE the enzyme migrates as a single protein band.

RESULTS

The presence of a constitutive NOS was detected through immunofluorescence using antibody against neuronal NOS (nNOS) and in NADPH consumption assays.

CONCLUSIONS

The present data show that NOS is prominent in axenic amastigote preparations, suggesting an association with the infectivity and/or an escaping mechanism of the parasite. The relationship between the NO-generating systems in the parasite and in their host cell warrants further investigation.

Nitric oxide negatively regulates mammalian adult neurogenesis

Neural progenitor cells are widespread throughout the adult central nervous system but only give rise to neurons in specific loci. Negative regulators of neurogenesis have therefore been postulated, but none have yet been identified as subserving a significant role in the adult brain. Here we report that nitric oxide (NO) acts as an important negative regulator of cell proliferation in the adult mammalian brain. We used two independent approaches to examine the function of NO in adult neurogenesis. In a pharmacological approach, we suppressed NO production in the rat brain by intraventricular infusion of an NO synthase inhibitor. In a genetic approach, we generated a null mutant neuronal NO synthase knockout mouse line by targeting the exon encoding active center of the enzyme. In both models, the number of new cells generated in neurogenic areas of the adult brain, the olfactory subependyma and the dentate gyrus, was strongly augmented, which indicates that division of neural stem cells in the adult brain is controlled by NO and suggests a strategy for enhancing neurogenesis in the adult central nervous system.

Neuronal nitric oxide synthase generates superoxide from the oxygenase domain

When l-arginine is depleted, neuronal nitric oxide synthase (nNOS) has been reported to generate superoxide. A flavoprotein module construct of nNOS has been demonstrated to be sufficient for superoxide production. In contrast, nNOS was reported not to be involved in superoxide formation, because such formation occurred with a mixture of the boiled enzyme and redox-active cofactors. We aimed to resolve these controversial issues by examining superoxide generation, without the addition of redox-active cofactors, by recombinant wild-type nNOS and by C415A-nNOS, which has a mutation in the haem proximal site. In a superoxide-sensitive adrenochrome assay, the initial lag period of C415A-nNOS was increased 2-fold compared with that of native nNOS. With ESR using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide, prominent signals of the superoxide adduct were obtained with wild-type nNOS, whereas an enzyme preparation boiled for 5 min did not produce superoxide. Higher concentrations of NaCN (10 mM) decreased superoxide formation by 63%. Although the activity of the reductase domain was intact, superoxide generation from C415A-nNOS was decreased markedly, to only 10% of that of the wild-type enzyme. These results demonstrate that nNOS truly catalyses superoxide formation, that this involves the oxygenase domain, and that full-length nNOS hinders the reductase domain from producing superoxide.

Neuronal nitric oxide synthase: a substrate for SHP-1 involved in sst2 somatostatin receptor growth inhibitory signaling

Somatostatin receptor sst2 is an inhibitory G protein-coupled receptor, which inhibits normal and tumor cell growth by a mechanism involving the tyrosine phosphatase SHP-1. We reported previously that SHP-1 associates transiently with and is activated by sst2 and is a critical component for sst2 growth inhibitory signaling. Here, we demonstrate that in Chinese hamster ovary cells expressing sst2, SHP-1 is associated at the basal level with the neuronal nitric oxide synthase (nNOS). Following sst2 activation by the somatostatin analog RC-160, SHP-1 rapidly recruits nNOS tyrosine dephosphorylates and activates it. The resulting NO activates guanylate cyclase and inhibits cell proliferation. Coexpression of a catalytically inactive SHP-1 mutant with sst2 blocks RC-160-induced nNOS dephosphorylation and activation, as well as guanylate cyclase activation. In mouse pancreatic acini, RC-160 treatment reduces nNOS tyrosine phosphorylation accompanied by an increase of its activity. By opposition, in acini from viable motheaten (mev/mev) mice, which express a markedly inactive SHP-1, RC-160 has no effect on nNOS activity. Finally, expression of a dominant-negative form of nNOS prevents both RC-160-induced p27 up-regulation and cell proliferation inhibition. We therefore identified nNOS as a novel SHP-1 substrate critical for sst2-induced cell-growth arrest.

Neuronal nitric-oxide synthase mutant (Ser-1412 --> Asp) demonstrates surprising connections between heme reduction, NO complex formation, and catalysis

Rat neuronal NO synthase (nNOS) contains an Akt-dependent phosphorylation motif in its reductase domain. We mutated a target residue in that site (Ser-1412 to Asp) to mimic phosphorylation and then characterized the mutant using conventional and stopped-flow spectroscopies. Compared with wild-type, S1412D nNOS catalyzed faster cytochrome c and ferricyanide reduction but displayed slower steady-state NO synthesis with greater uncoupling of NADPH oxidation. Paradoxically, the mutant had faster heme reduction, faster heme-NO complex formation, and greater heme-NO complex accumulation at steady state. To understand how these behaviors related to flavin and heme reduction rates, we utilized three soybean calmodulins (CaMs) that supported a range of slower flavin and heme reduction rates in mutant and wild-type nNOS. Reductase activity and two catalytic parameters (speed and amount of heme-NO complex formation) related directly to the speed of flavin and heme reduction. In contrast, steady-state NO synthesis increased, reached a plateau, and then fell at the highest rate of heme reduction that was obtained with S1412D nNOS + CaM. Substituting with soybean CaM slowed heme reduction and increased steady-state NO synthesis by the mutant. We conclude the following. 1) The S1412D mutation speeds electron transfer out of the reductase domain. 2) Faster heme reduction speeds intrinsic NO synthesis but diminishes NO release in the steady state. 3) Heme reduction displays an optimum regarding NO release during steady state. The unique behavior of S1412D nNOS reveals the importance of heme reduction rate in controlling steady-state activity and suggests that nNOS already has a near-optimal rate of heme reduction.

Dexras1: a G protein specifically coupled to neuronal nitric oxide synthase via CAPON

Because nitric oxide (NO) is a highly reactive signaling molecule, chemical inactivation by reaction with oxygen, superoxide, and glutathione competes with specific interactions with target proteins. NO signaling may be enhanced by adaptor proteins that couple neuronal NO synthase (nNOS) to specific target proteins. Here we identify a selective interaction of the nNOS adaptor protein CAPON with Dexras1, a brain-enriched member of the Ras family of small monomeric G proteins. We find that Dexras1 is activated by NO donors as well as by NMDA receptor-stimulated NO synthesis in cortical neurons. The importance of Dexras1 as a physiologic target of nNOS is established by the selective decrease of Dexras1 activation, but not H-Ras or four other Ras family members, in the brains of mice harboring a targeted genomic deletion of nNOS (nNOS-/-). We also find that nNOS, CAPON, and Dexras1 form a ternary complex that enhances the ability of nNOS to activate Dexras1. These findings identify Dexras1 as a novel physiologic NO effector and suggest that anchoring of nNOS to specific targets is a mechanism by which NO signaling is enhanced.

Cu2+ and Zn2+ inhibit nitric-oxide synthase through an interaction with the reductase domain

Cu(2+) and Zn(2+) inhibit all of the NADPH-dependent reactions catalyzed by neuronal nitric-oxide synthase (nNOS) including ferricytochrome c reduction, NADPH oxidation, and citrulline formation. Cu(2+) and Zn(2+) also inhibit ferricytochrome c reduction by the independent reductase domain. Zn(2+) affects all activities of the full-length nNOS and the reductase domain to the same extent (estimated IC(50) values from 9 to 31 microm), suggesting Zn(2+) occupation of a single site in the reductase domain. Citrulline formation and NADPH oxidation by the full-length nNOS and ferricytochrome c reduction by the reductase domain are affected similarly by Cu(2+), with estimated IC(50) values ranging from 6 to 33 microm. However, Cu(2+) inhibits ferricytochrome c reduction by the full-length nNOS 2 orders of magnitude more potently, with an estimated IC(50) value of 0.12 microm. These data suggest the possibility that Cu(2+) may interact with nNOS at two sites, one composed exclusively of the reductase domain (which is perhaps also involved in Zn(2+)-mediated inhibition), and another that includes components of both domains. Occupation of the second (higher affinity) site could then promote the selective inhibition of ferricytochrome c reduction in full-length nNOS. Neither the inhibition by Cu(2+) nor that by Zn(2+) is dependent on calmodulin.

Calmodulin activates intramolecular electron transfer between the two flavins of neuronal nitric oxide synthase flavin domain

The neuronal NO synthase (nNOS) flavin domain, which has similar redox properties to those of NADPH-cytochrome P450 reductase (P450R), contains binding sites for calmodulin, FAD, FMN, and NADPH. The aim of this study is to elucidate the mechanism of activation of the flavin domain by calcium/calmodulin (Ca(2+)/CaM). In this study, we used the recombinant nNOS flavin domains, which include or delete the calmodulin (CaM)-binding site. The air-stable semiquinone of the nNOS flavin domains showed similar redox properties to the corresponding FAD-FMNH(&z.ccirf;) of P450R. In the absence or presence of Ca(2+)/CaM, the rates of reduction of an FAD-FMN pair by NADPH have been investigated at different wavelengths, 457, 504 and 590 nm by using a stopped-flow technique and a rapid scan spectrophotometry. The reduction of the oxidized enzyme (FAD-FMN) by NADPH proceeds by both one-electron equivalent and two-electron equivalent mechanisms, and the formation of semiquinone (increase of absorbance at 590 nm) was significantly increased in the presence of Ca(2+)/CaM. The air-stable semiquinone form of the enzyme was also rapidly reduced by NADPH. The results suggest that an intramolecular one-electron transfer between the two flavins is activated by the binding of Ca(2+)/CaM. The F(1)H(2), which is the fully reduced form of the air-stable semiquinone, can donate one electron to the electron acceptor, cytochrome c. The proposed mechanism of activation by Ca(2+)/CaM complex is discussed on the basis of that provided by P450R.

A point mutation in a plant calmodulin is responsible for its inhibition of nitric-oxide synthase

The calcium/calmodulin-dependent activation of nitric-oxide synthase (NOS) and its production of nitric oxide (NO) play a key regulatory role in plant and animal cell function. SCaM-1 is a plant calmodulin (CaM) isoform that is 91% identical to mammalian CaM (wild type CaM (wtCaM)) and a selective competitive antagonist of NOS (Cho, M. J., Vaghy, P. L., Kondo, R., Lee, S. H., Davis, J. P., Rehl, R., Heo, W. D., and Johnson, J. D. (1998) Biochemistry 37, 15593-15597). We have used site-directed mutagenesis to show that a point mutation, involving the substitution of valine for methionine at position 144, is responsible for SCaM-1's inhibition of mammalian NOS. An M144V mutation in wild type CaM produced a mutant (M144V) which exhibited nearly identical inhibition of NOS's NO production and NADPH oxidation, with a similar K(i) (approximately 15 nM) as SCaM-1. A V144M back mutation in SCaM-1 significantly restored its ability to activate NOS's catalytic functions. The length of the hydrophobic amino acid side chain at position 144 appears to be critical for NOS activation, since M144L and M144F activated NOS while M144V and M144C did not. Despite their competitive antagonism of NOS, M144V, like SCaM-1, exhibited a similar dose-dependent activation of phosphodiesterase and calcineurin as wtCaM. SCaM-1 and M144V produced greater inhibition of NOS's oxygenase domain function (NO production) than its reductase domain functions (NADPH oxidation and cytochrome c reduction). Thus, CaM's methionine 144 plays a critical role the activation of NOS, presumably by influencing the function of NOS's oxygenase domain.

The activation of neuronal NO synthase is mediated by G-protein betagamma subunit and the tyrosine phosphatase SHP-2

In CHO cells we had found that CCK positively regulated cell proliferation via the activation of a soluble guanylate cyclase. Here we demonstrate that CCK stimulated a nitric oxide synthase (NOS) activity. The production of NO was involved in the proliferative response elicited by CCK regarding the inhibitory effect of NOS inhibitors L-NAME and alpha-guanidinoglutaric acid. We identified the NOS activated by the peptide as the neuronal isoform: the expression of the C415A neuronal NOS mutant inhibited both CCK-induced stimulation of NOS activity and cell proliferation. These two effects were also inhibited after expression of the C459S tyrosine phosphatase SHP-2 mutant and the betaARK1 (495-689) sequestrant peptide, indicating the requirement of activated SHP-2 and G-betagamma subunit. Kinetic analysis (Western blot after coimmunoprecipitation and specific SHP-2 activity) revealed that in response to CCK-treatment, SHP-2 associated to G-beta1 subunit, became activated, and then dephosphorylated the neuronal NOS through a direct association. These data demonstrate that the neuronal NOS is implicated in proliferative effect evoked by CCK. A novel growth signaling pathway is described, involving the activation of neuronal NOS by dephosphorylation of tyrosyl residues.

Role of reductase domain cluster 1 acidic residues in neuronal nitric-oxide synthase. Characterization of the FMN-FREE enzyme

The nNOS reductase domain is homologous to cytochrome P450 reductase, which contains two conserved clusters of acidic residues in its FMN module that play varied roles in its electron transfer reactions. To study the role of nNOS reductase domain cluster 1 acidic residues, we mutated two conserved acidic (Asp(918) and Glu(919)) and one conserved aromatic residue (Phe(892)), and investigated the effect of each mutation on flavin binding, conformational change, electron transfer reactions, calmodulin regulation, and catalytic activities. Each mutation destabilized FMN binding without significantly affecting other aspects including substrate, cofactor or calmodulin binding, or catalytic activities upon FMN reconstitution, indicating the mutational effect was restricted to the FMN module. Characterization of the FMN-depleted mutants showed that bound FMN was essential for reduction of the nNOS heme or cytochrome c, but not for ferricyanide or dichlorophenolindolphenol, and established that the electron transfer path in nNOS is NADPH to FAD to FMN to heme. Steady-state and stopped-flow kinetic analysis revealed a novel role for bound FMN in suppressing FAD reduction by NADPH. The suppression could be relieved either by FMN removal or calmodulin binding. Calmodulin binding induced a conformational change that was restricted to the FMN module. This increased the rate of FMN reduction and triggered electron transfer to the heme. We propose that the FMN module of nNOS is the key positive or negative regulator of electron transfer at all points in nNOS. This distinguishes nNOS from other related flavoproteins, and helps explain the mechanism of calmodulin regulation.

Characterization of mouse nNOS2, a natural variant of neuronal nitric-oxide synthase produced in the central nervous system by selective alternative splicing

Mouse neuronal nitric-oxide synthase 2 (nNOS2) is a unique natural variant of constitutive neuronal nitric-oxide synthase (nNOS) specifically expressed in the central nervous system having a 105-amino acid deletion in the heme-binding domain as a result of in-frame mutation by specific alternative splicing. The mouse nNOS2 cDNA gene was heterologously expressed in Escherichia coli, and the resultant product was characterized spectroscopically in detail. Purified recombinant nNOS2 contained heme but showed no L-arginine- and NADPH-dependent citrulline-forming activity in the presence of Ca2+-promoted calmodulin, elicited a sharp electron paramagnetic resonance (EPR) signal at g = 6.0 indicating the presence of a high spin ferriheme as isolated and showed a peak at around 420 nm in the CO difference spectrum, instead of a 443-nm peak detected with the recombinant wild-type nNOS1 enzyme. Thus, although the heme domain of nNOS2 is capable of binding heme, the heme coordination geometry is highly abnormal in that it probably has a proximal non-cysteine thiolate ligand both in the ferric and ferrous states. Moreover, negligible spectral perturbation of the nNOS2 ferriheme was detected upon addition of either L-arginine or imidazole. These provide a possible rational explanation for the inability of nNOS2 to catalyze the cytochrome P450-type monooxygenase reaction.

Catalysis by nitric oxide synthase

The enzyme nitric oxide synthase catalyzes the oxidation of the amino acid L-arginine to L-citrulline and nitric oxide in an NADPH-dependent reaction. Nitric oxide plays a critical role in signal transduction pathways in the cardiovascular and nervous systems and is a key component of the cytostatic/cytotoxic function of the immune system. Characterization of nitric oxide synthase substrates and cofactors has outlined the broad details of the overall reaction and suggested possibilities for chemical steps in the reaction; however, the molecular details of the reaction mechanism are still poorly understood. Recent evidence suggests a role for the reduced bound pterin in the first step of the reaction--the hydroxylation of L-arginine.

The influence of chaotropic reagents on neuronal nitric oxide synthase and its flavoprotein module. Urea and guanidine hydrochloride stimulate NADPH-cytochrome c reductase activity of both proteins

Changes in flavin and protein fluorescence of neuronal nitric oxide synthase (nNOS) and its flavoprotein module were studied in the presence of urea and compared with those previously reported for cytochrome P450 reductase (CPR) [R. Narayanasami, P. M. Horowitz, and B. S. S. Masters (1995) Arch. Biochem. Biophys. 316, 267-274]. As in the case of CPR, FMN was relatively loosely bound to nNOS and the flavoprotein module, but FAD remained bound at concentrations of up to 2 M urea Protein fluorescence increased progressively with increasing urea concentration, but could not be correlated with changes in flavin binding. NADPH-cytochrome c reductase activity of both nNOS and the flavoprotein module, but not that of CPR, was stimulated at early time points by both urea and guanidine hydrochloride (GnHCl), with levels of initial activity returning to baseline values within 60 min after addition of the chaotropic agent. Thus, at 3-4 M urea, enhancements of reductase activities of 20- and 5-fold with nNOS and the flavoprotein module, respectively, were obtained. Comparable enhancements of 12- and 6- to 7-fold, respectively, were obtained with calmodulin (CaM)/ CaCl2 and 0.5 M GnHCl. Thus, the effects of urea and GnHCl mimicked the stimulating effects of CaM. Separate preincubations of nNOS and cytochrome c with urea or GnHCl prior to initiation of the reductase assay showed that sensitivity to chaotropic agent under these conditions was a property of nNOS and not of cytochrome c. Moreover, when the nonprotein electron acceptor 2,6-dichlorophenolindophenol was employed in place of cytochrome c, comparable stimulation of reductase activity was observed in the presence of either urea or GnHCl. Fluorescence of 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfate in the presence of either nNOS or the flavoprotein module was increased optimally between 3 and 4 M urea, consistent with simultaneous exposure of hydrophobic regions of both proteins to solvent and optimization of reductase activity. FMN release from nNOS, but not from the flavoprotein module, was enhanced by CaM. Addition of FMN or FMN + FAD to nNOS, in the presence or absence of urea, brought about a doubling of initial cytochrome c reductase activity, but did not prevent the eventual decline in activity to basal levels. These data are consistent with conformational changes which favor increased electron transfer similar to that achieved with nNOS in the presence of CaM.

Structure-function aspects in the nitric oxide synthases

Research on the biological roles of nitric oxide has revealed that it functions as an important signal and effector molecule in a variety of physiologic and pathologic settings. In animals, nitric oxide is synthesized enzymatically from L-arginine through the actions of the nitric oxide synthases (NOSs). The three known NOS isoforms are all dimeric, bi-domain enzymes that contain iron protoporphyrin IX, flavin adenine dinucleotide, flavin mononucleotide, and tetrahydrobiopterin as bound prosthetic groups. This chapter summarizes information regarding the structure-function aspects of the NOSs, which includes composition of the domains, the protein residues and regions involved in prosthetic group binding, catalytic properties of the domains, the relationship between dimeric structure and prosthetic group binding and function, and factors that control assembly of NOS in cells. A general model for NOS structure and assembly is presented.