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.

Instability of the Human Cytochrome P450 Reductase A287P Variant Is the Major Contributor to Its Antley-Bixler Syndrome-like Phenotype.

Human NADPH-cytochrome P450 oxidoreductase (POR) gene mutations are associated with severe skeletal deformities and disordered steroidogenesis. The human POR mutation A287P presents with disordered sexual development and skeletal malformations. Difficult recombinant expression and purification of this POR mutant suggested that the protein was less stable than WT. The activities of cytochrome P450 17A1, 19A1, and 21A2, critical in steroidogenesis, were similar using our purified, full-length, unmodified A287P or WT POR, as were those of several xenobiotic-metabolizing cytochromes P450, indicating that the A287P protein is functionally competent in vitro, despite its functionally deficient phenotypic behavior in vivo Differential scanning calorimetry and limited trypsinolysis studies revealed a relatively unstable A287P compared with WT protein, leading to the hypothesis that the syndrome observed in vivo results from altered POR protein stability. The crystal structures of the soluble domains of WT and A287P reveal only subtle differences between them, but these differences are consistent with the differential scanning calorimetry results as well as the differential susceptibility of A287P and WT observed with trypsinolysis. The relative in vivo stabilities of WT and A287P proteins were also examined in an osteoblast cell line by treatment with cycloheximide, a protein synthesis inhibitor, showing that the level of A287P protein post-inhibition is lower than WT and suggesting that A287P may be degraded at a higher rate. Current studies demonstrate that, unlike previously described mutations, A287P causes POR deficiency disorder due to conformational instability leading to proteolytic susceptibility in vivo, rather than through an inherent flavin-binding defect.

Deciphering the binding of caveolin-1 to client protein endothelial nitric-oxide synthase (eNOS): scaffolding subdomain identification, interaction modeling, and biological significance.

Caveolin-1 (Cav-1) gene inactivation interferes with caveolae formation and causes a range of cardiovascular and pulmonary complications in vivo. Recent evidence suggests that blunted Cav-1/endothelial nitric-oxide synthase (eNOS) interaction, which occurs specifically in vascular endothelial cells, is responsible for the multiple phenotypes observed in Cav-1-null animals. Under basal conditions, Cav-1 binds eNOS and inhibits nitric oxide (NO) production via the Cav-1 scaffolding domain (CAV; amino acids 82-101). Although we have recently shown that CAV residue Phe-92 is responsible for eNOS inhibition, the "inactive" F92A Cav-1 mutant unexpectedly retains its eNOS binding ability and can increase NO release, indicating the presence of a distinct eNOS binding domain within CAV. Herein, we identified and characterized a small 10-amino acid CAV subsequence (90-99) that accounted for the majority of eNOS association with Cav-1 (Kd = 49 nM), and computer modeling of CAV(90-99) docking to eNOS provides a rationale for the mechanism of eNOS inhibition by Phe-92. Finally, using gene silencing and reconstituted cell systems, we show that intracellular delivery of a F92A CAV(90-99) peptide can promote NO bioavailability in eNOS- and Cav-1-dependent fashions. To our knowledge, these data provide the first detailed analysis of Cav-1 binding to one of its most significant client proteins, eNOS.

Oxidative stress induces phosphorylation of neuronal NOS in cardiomyocytes through AMP-activated protein kinase (AMPK).

Neuronal nitric oxide synthase (nNOS) plays a critical role in regulating cardiomyocyte function. nNOS was reported to decrease superoxide production in the myocardium by inhibiting the function of xanthine oxidoreductase. However, the effect of oxidative stress on nNOS in cardiomyocytes has not been determined. We report here that brief exposure of HL-1 cardiomyocytes to hydrogen peroxide (H2O2) induces phosphorylation of nNOS at serine 1412. This increase in phosphorylation was concomitant with increased nitric oxide (NO) production. Prolonged exposure to the oxidant, however, resulted in decreased expression of the protein. H2O2 treatment for short periods also stimulated phosphorylation of AKT and AMPK. H2O2-induced phosphorylation of nNOS was reduced when AMPK activity was inhibited by compound C, suggesting that AMPK is a mediator of oxidative stress-induced phosphorylation of nNOS. However, inhibition of AKT activity by the pan AKT inhibitor, AKTi, had no effect on nNOS phosphorylation caused by H2O2. These data demonstrate the novel regulation of nNOS phosphorylation and expression by oxidative stress.

Probing the Hydrogen Bonding of the Ferrous-NO Heme Center of nNOS by Pulsed Electron Paramagnetic Resonance.

Oxidation of L-arginine (L-Arg) to nitric oxide (NO) by NO synthase (NOS) takes place at the heme active site. It is of current interest to study structures of the heme species that activates O2 and transforms the substrate. The NOS ferrous-NO complex is a close mimic of the obligatory ferric (hydro)peroxo intermediate in NOS catalysis. In this work, pulsed electron-nuclear double resonance (ENDOR) spectroscopy was used to probe the hydrogen bonding of the NO ligand in the ferrous-NO heme center of neuronal NOS (nNOS) without a substrate and with L-Arg or N-hydroxy-L-arginine (NOHA) substrates. Unexpectedly, no H-bonding interaction connecting the NO ligand to the active site water molecule or the Arg substrate was detected, in contrast to the results obtained by X-ray crystallography for the Arg-bound nNOS heme domain [Li et al. J. Biol. Inorg. Chem. 2006, 11, 753-768]. The nearby exchangeable proton in both the no-substrate and Arg-containing nNOS samples is located outside the H-bonding range and, on the basis of the obtained structural constraints, can belong to the active site water (or OH). On the contrary, in the NOHA-bound sample, the nearby exchangeable hydrogen forms an H-bond with the NO ligand (on the basis of its distance from the NO ligand and a nonzero isotropic hfi constant), but it does not belong to the active site water molecule because the water oxygen atom (detected by (17)O ENDOR) is too far. This hydrogen should therefore come from the NOHA substrate, which is in agreement with the X-ray crystallography work [Li et al. Biochemistry 2009, 48, 10246-10254]. The nearby nonexchangeable hydrogen atom assigned as H(ε) of Phe584 was detected in all three samples. This hydrogen atom may have a stabilizing effect on the NO ligand and probably determines its position.

Holoenzyme structures of endothelial nitric oxide synthase - an allosteric role for calmodulin in pivoting the FMN domain for electron transfer.

While the three-dimensional structures of heme- and flavin-binding domains of the NOS isoforms have been determined, the structures of the holoenzymes remained elusive. Application of electron cryo-microscopy and structural modeling of the bovine endothelial nitric oxide synthase (eNOS) holoenzyme produced detailed models of the intact holoenzyme in the presence and absence of Ca(2+)/calmodulin (CaM). These models accommodate the cross-electron transfer from the reductase in one monomer to the heme in the opposite monomer. The heme domain acts as the anchoring dimeric structure for the entire enzyme molecule, while the FMN domain is activated by CaM to move flexibly to bridge the distance between the reductase and oxygenase domains. Our results indicate that the key regulatory role of CaM involves the stabilization of structural intermediates and precise positioning of the pivot for the FMN domain tethered shuttling motion to accommodate efficient and rapid electron transfer in the homodimer of eNOS.

Nox4 NADPH oxidase mediates peroxynitrite-dependent uncoupling of endothelial nitric-oxide synthase and fibronectin expression in response to angiotensin II: role of mitochondrial reactive oxygen species.

Activation of glomerular mesangial cells (MCs) by angiotensin II (Ang II) leads to extracellular matrix accumulation. Here, we demonstrate that, in MCs, Ang II induces endothelial nitric-oxide synthase (eNOS) uncoupling with enhanced generation of reactive oxygen species (ROS) and decreased production of NO. Ang II promotes a rapid increase in 3-nitrotyrosine formation, and uric acid attenuates Ang II-induced decrease in NO bioavailability, demonstrating that peroxynitrite mediates the effects of Ang II on eNOS dysfunction. Ang II rapidly up-regulates Nox4 protein. Inhibition of Nox4 abolishes the increase in ROS and peroxynitrite generation as well as eNOS uncoupling triggered by Ang II, indicating that Nox4 is upstream of eNOS. This pathway contributes to Ang II-mediated fibronectin accumulation in MCs. Ang II also elicits an increase in mitochondrial abundance of Nox4 protein, and the oxidase contributes to ROS production in mitochondria. Overexpression of mitochondrial manganese superoxide dismutase prevents the stimulatory effects of Ang II on mitochondrial ROS production, loss of NO availability, and MC fibronectin accumulation, whereas manganese superoxide dismutase depletion increases mitochondrial ROS, NO deficiency, and fibronectin synthesis basally and in cells exposed to Ang II. This work provides the first evidence that uncoupled eNOS is responsible for Ang II-induced MC fibronectin accumulation and identifies Nox4 and mitochondrial ROS as mediators of eNOS dysfunction. These data shed light on molecular processes underlying the oxidative signaling cascade engaged by Ang II and identify potential targets for intervention to prevent renal fibrosis.

Combination of chiral linkers with thiophenecarboximidamide heads to improve the selectivity of inhibitors of neuronal nitric oxide synthase.

To develop potent and selective nNOS inhibitors, a new series of double-headed molecules with chiral linkers that derive from natural amino acid derivatives have been designed and synthesized. The new structures integrate a thiophenecarboximidamide head with two types of chiral linkers, presenting easy synthesis and good inhibitory properties. Inhibitor (S)-9b exhibits a potency of 14.7 nM against nNOS and is 1134 and 322-fold more selective for nNOS over eNOS and iNOS, respectively. Crystal structures show that the additional binding between the aminomethyl moiety of 9b and propionate A on the heme and tetrahydrobiopterin (H4B) in nNOS, but not eNOS, contributes to its high selectivity. This work demonstrates the advantage of integrating known structures into structure optimization, and it should be possible to more readily develop compounds that incorporate bioavailability with these advanced features. Moreover, this integrative strategy is a general approach in new drug discovery.

Methylated N(ω)-hydroxy-L-arginine analogues as mechanistic probes for the second step of the nitric oxide synthase-catalyzed reaction.

Nitric oxide synthase (NOS) catalyzes the conversion of L-arginine to L-citrulline through the intermediate N(ω)-hydroxy-L-arginine (NHA), producing nitric oxide, an important mammalian signaling molecule. Several disease states are associated with improper regulation of nitric oxide production, making NOS a therapeutic target. The first step of the NOS reaction has been well-characterized and is presumed to proceed through a compound I heme species, analogous to the cytochrome P450 mechanism. The second step, however, is enzymatically unprecedented and is thought to occur via a ferric peroxo heme species. To gain insight into the details of this unique second step, we report here the synthesis of NHA analogues bearing guanidinium methyl or ethyl substitutions and their investigation as either inhibitors of or alternate substrates for NOS. Radiolabeling studies reveal that N(ω)-methoxy-L-arginine, an alternative NOS substrate, produces citrulline, nitric oxide, and methanol. On the basis of these results, we propose a mechanism for the second step of NOS catalysis in which a methylated nitric oxide species is released and is further metabolized by NOS. Crystal structures of our NHA analogues bound to nNOS have been determined, revealing the presence of an active site water molecule only in the presence of singly methylated analogues. Bulkier analogues displace this active site water molecule; a different mechanism is proposed in the absence of the water molecule. Our results provide new insights into the steric and stereochemical tolerance of the NOS active site and substrate capabilities of NOS.

Neuronal nitric oxide synthase is phosphorylated in response to insulin stimulation in skeletal muscle.

Type 2 Diabetes (T2DM) is the seventh leading cause of death in the United States, and is quickly becoming a global pandemic. T2DM results from reduced insulin sensitivity coupled with a relative failure of insulin secretion. Reduced insulin sensitivity has been associated with reduced nitric oxide synthase (NOS) activity and impaired glucose uptake in T2DM skeletal muscle. Upon insulin stimulation, NO synthesis increases in normal adult skeletal muscle, whereas no such increase is observed in T2DM adults. Endothelial NOS is activated by phosphorylation in the C-terminal tail in response to insulin. Neuronal NOS (nNOS), the primary NOS isoform in skeletal muscle, contains a homologous phosphorylation site, raising the possibility that nNOS, too, may undergo an activating phosphorylation event upon insulin treatment. Yet it remains unknown if or how nNOS is regulated by insulin in skeletal muscle. Data shown herein indicate that nNOS is phosphorylated in response to insulin in skeletal muscle and that this phosphorylation event occurs rapidly in C2C12 myotubes, resulting in increased NO production. In vivo phosphorylation of nNOS was also observed in response to insulin in mouse skeletal muscle. These results indicate, for the first time, that nNOS is phosphorylated in skeletal muscle in response to insulin and in association with increased NO production.

Intra- and inter-molecular effects of a conserved arginine residue of neuronal and inducible nitric oxide synthases on FMN and calmodulin binding.

Nitric oxide synthases (NOSs) synthesize nitric oxide (NO), a signaling molecule, from l-arginine, utilizing electrons from NADPH. NOSs are flavo-hemo proteins, with two flavin molecules (FAD and FMN) and one heme per monomer, which require the binding of calcium/calmodulin (Ca(2+)/CaM) to produce NO. It is therefore important to understand the molecular factors influencing CaM binding from a structure/function perspective. A crystal structure of the CaM-bound iNOS FMN-binding domain predicted a salt bridge between R536 of human iNOS and E47 of CaM. To characterize the interaction between the homologous Arg of rat nNOS (R753) and murine iNOS (R530) with CaM, the Arg was mutated to Ala and, in iNOS, to Glu. The mutation weakens the interaction between nNOS and CaM, decreasing affinity by ~3-fold. The rate of electron transfer from FMN is greatly attenuated; however, little effect on electron transfer from FAD is observed. The mutated proteins showed reduced FMN binding, from 20% to 60%, suggesting an influence of this residue on FMN incorporation. The weakened FMN binding may be due to conformational changes caused by the arginine mutation. Our data show that this Arg residue plays an important role in CaM binding and influences FMN binding.

Chiral linkers to improve selectivity of double-headed neuronal nitric oxide synthase inhibitors.

To develop potent and selective nNOS inhibitors, new double-headed molecules with chiral linkers that derive from natural amino acids or their derivatives have been designed. The new structures contain two ether bonds, which greatly simplifies the synthesis and accelerates structure optimization. Inhibitor (R)-6b exhibits a potency of 32nM against nNOS and is 475 and 244 more selective for nNOS over eNOS and iNOS, respectively. Crystal structures show that the additional binding between the aminomethyl moiety of 6b and the two heme propionates in nNOS, but not eNOS, is the structural basis for its high selectivity. This work demonstrates the importance of stereochemistry in this class of molecules, which significantly influences the potency and selectivity of the inhibitors. The structure-activity information gathered here provides a guide for future structure optimization.

In search of potent and selective inhibitors of neuronal nitric oxide synthase with more simple structures.

In certain neurodegenerative diseases damaging levels of nitric oxide (NO) are produced by neuronal nitric oxide synthase (nNOS). It, therefore, is important to develop inhibitors selective for nNOS that do not interfere with other NOS isoforms, especially endothelial NOS (eNOS), which is critical for proper functioning of the cardiovascular system. While we have been successful in developing potent and isoform-selective inhibitors, such as lead compounds 1 and 2, the ease of synthesis and bioavailability have been problematic. Here we describe a new series of compounds including crystal structures of NOS-inhibitor complexes that integrate the advantages of easy synthesis and good biological properties compared to the lead compounds. These results provide the basis for additional structure-activity relationship (SAR) studies to guide further improvement of isozyme selective inhibitors.

Cyclopropyl- and methyl-containing inhibitors of neuronal nitric oxide synthase.

Inhibitors of neuronal nitric oxide synthase have been proposed as therapeutics for the treatment of different types of neurological disorders. On the basis of a cis-3,4-pyrrolidine scaffold, a series of trans-cyclopropyl- and methyl-containing nNOS inhibitors have been synthesized. The insertion of a rigid electron-withdrawing cyclopropyl ring decreases the basicity of the adjacent amino group, which resulted in decreased inhibitory activity of these inhibitors compared to the parent compound. Nonetheless, three of them exhibited double-digit nanomolar inhibition with high nNOS selectivity on the basis of in vitro enzyme assays. Crystal structures of nNOS and eNOS with these inhibitors bound provide a basis for detailed structure-activity relationship (SAR) studies. The conclusions from these studies will be used as a guide in the future development of selective NOS inhibitors.

Selective monocationic inhibitors of neuronal nitric oxide synthase. Binding mode insights from molecular dynamics simulations.

The reduction of pathophysiologic levels of nitric oxide through inhibition of neuronal nitric oxide synthase (nNOS) has the potential to be therapeutically beneficial in various neurodegenerative diseases. We have developed a series of pyrrolidine-based nNOS inhibitors that exhibit excellent potencies and isoform selectivities (J. Am. Chem. Soc. 2010, 132, 5437). However, there are still important challenges, such as how to decrease the multiple positive charges derived from basic amino groups, which contribute to poor bioavailability, without losing potency and/or selectivity. Here we present an interdisciplinary study combining molecular docking, crystallography, molecular dynamics simulations, synthesis, and enzymology to explore potential pharmacophoric features of nNOS inhibitors and to design potent and selective monocationic nNOS inhibitors. The simulation results indicate that different hydrogen bond patterns, electrostatic interactions, hydrophobic interactions, and a water molecule bridge are key factors for stabilizing ligands and controlling ligand orientation. We find that a heteroatom in the aromatic head or linker chain of the ligand provides additional stability and blocks the substrate binding pocket. Finally, the computational insights are experimentally validated with double-headed pyridine analogues. The compounds reported here are among the most potent and selective monocationic pyrrolidine-based nNOS inhibitors reported to date, and 10 shows improved membrane permeability.

Intramolecular hydrogen bonding: a potential strategy for more bioavailable inhibitors of neuronal nitric oxide synthase.

Selective neuronal nitric oxide synthase (nNOS) inhibitors have therapeutic applications in the treatment of numerous neurodegenerative diseases. Here we report the synthesis and evaluation of a series of inhibitors designed to have increased cell membrane permeability via intramolecular hydrogen bonding. Their potencies were examined in both purified enzyme and cell-based assays; a comparison of these results demonstrates that two of the new inhibitors display significantly increased membrane permeability over previous analogs. NMR spectroscopy provides evidence of intramolecular hydrogen bonding under physiological conditions in two of the inhibitors. Crystal structures of the inhibitors in the nNOS active site confirm the predicted non-intramolecular hydrogen bonded binding mode. Intramolecular hydrogen bonding may be an effective approach for increasing cell membrane permeability without affecting target protein binding.

Potent, highly selective, and orally bioavailable gem-difluorinated monocationic inhibitors of neuronal nitric oxide synthase.

In our efforts to discover neuronal isoform selective nitric oxide synthase (NOS) inhibitors, we have developed a series of compounds containing a pyrrolidine ring with two stereogenic centers. The enantiomerically pure compounds, (S,S) versus (R,R), exhibited two different binding orientations, with (R,R) inhibitors showing much better potency and selectivity. To improve the bioavailability of these inhibitors, we have introduced a CF(2) moiety geminal to an amino group in the long tail of one of these inhibitors, which reduced its basicity, resulting in compounds with monocationic character under physiological pH conditions. Biological evaluations have led to a nNOS inhibitor with a K(i) of 36 nM and high selectivity for nNOS over eNOS (3800-fold) and iNOS (1400-fold). MM-PBSA calculations indicated that the low pK(a) NH is, at least, partially protonated when bound to the active site. A comparison of rat oral bioavailability of the difluorinated compound to the parent molecule shows 22% for the difluorinated compound versus essentially no oral bioavailability for the parent compound. This indicates that the goal of this research to make compounds with only one protonated nitrogen atom at physiological pH to allow for membrane permeability, but which can become protonated when bound to NOS, has been accomplished.

Heme-coordinating inhibitors of neuronal nitric oxide synthase. Iron-thioether coordination is stabilized by hydrophobic contacts without increased inhibitor potency.

The heme-thioether ligand interaction often occurs between heme iron and native methionine ligands, but thioether-based heme-coordinating (type II) inhibitors are uncommon due to the difficulty in stabilizing the Fe-S bond. Here, a thioether-based inhibitor (3) of neuronal nitric oxide synthase (nNOS) was designed, and its binding was characterized by spectrophotometry and crystallography. A crystal structure of inhibitor 3 coordinated to heme iron was obtained, representing, to our knowledge, the first crystal structure of a thioether inhibitor complexed to any heme enzyme. A series of related potential inhibitors (4-8) also were evaluated. Compounds 4-8 were all found to be type I (non-heme-coordinating) inhibitors of ferric nNOS, but 4 and 6-8 were found to switch to type II upon heme reduction to the ferrous state, reflecting the higher affinity of thioethers for ferrous heme than for ferric heme. Contrary to what has been widely thought, thioether-heme ligation was found not to increase inhibitor potency, illustrating the intrinsic weakness of the thioether-ferric heme linkage. Subtle changes in the alkyl groups attached to the thioether sulfur caused drastic changes in the binding conformation, indicating that hydrophobic contacts play a crucial role in stabilizing the thioether-heme coordination.

Selective neuronal nitric oxide synthase inhibitors and the prevention of cerebral palsy.

OBJECTIVE: To design a new class of selective neuronal nitric oxide synthase (NOS) inhibitors, and demonstrate that administration in a rabbit model for cerebral palsy (CP) prevents hypoxia-ischemia-induced deaths and reduces the number of newborn kits exhibiting signs of CP. METHODS: We used a novel computer-based drug design method called fragment hopping to identify new chemical entities, synthesized them, and conducted in vitro enzyme inhibition studies with the three isozymes of NOS and in vivo experiments to monitor cardiovascular effects on pregnant rabbit dams, NOS activity, and NO(x) (NO and NO(2)) concentration in fetal brain, and assess neurobehavioral effects on kits born to saline- and compound treated dams. RESULTS: The computer-based design led to the development of powerful and highly selective compounds for inhibition of neuronal NOS over the other isozymes. After maternal administration in a rabbit model of CP, these compounds were found to distribute to fetal brain, to be nontoxic, without cardiovascular effects, inhibit fetal brain NOS activity in vivo, reduce NO concentration in fetal brain, and dramatically ameliorate deaths and number of newborn kits exhibiting signs of CP. INTERPRETATION: This approach may lead to new preventive strategies for CP.

Peripheral but crucial: a hydrophobic pocket (Tyr(706), Leu(337), and Met(336)) for potent and selective inhibition of neuronal nitric oxide synthase.

Selective inhibition of the neuronal isoform of nitric oxide synthase (nNOS) over endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) has become a promising strategy for the discovery of new therapeutic agents for neurodegenerative diseases. However, because of the high sequence homology of different isozymes in the substrate binding pocket, developing inhibitors with both potency and excellent isoform selectivity remains a challenging problem. Herein, we report the evaluation of a recently discovered peripheral hydrophobic pocket (Tyr(706), Leu(337), and Met(336)) that opens up upon inhibitor binding and its potential in designing potent and selective nNOS inhibitors using three compounds, 2a, 2b, and 3. Crystal structure results show that inhibitors 2a and 3 adopted the same binding mode as lead compound 1. We also found that hydrophobic interactions between the 4-methyl group of the aminopyridine ring of these compounds with the side chain of Met(336), as well as the π-π stacking interaction between the pyridinyl motif and the side chain of Tyr(706) are important for the high potency and selectivity of these nNOS inhibitors.