Molecular architecture of mammalian nitric oxide synthases

Tetrahydrobiopterin redox cycling in nitric oxide synthase: evidence supports a through-heme electron delivery

The nitric oxide synthases (NOS) catalyze a two-step oxidation of l-arginine (Arg) to generate NO. In the first step, O2 activation involves one electron being provided to the heme by an enzyme-bound 6R-tetrahydro-l-biopterin cofactor (H4 B), and the H4 B radical must be reduced back to H4 B in order for NOS to continue catalysis. Although an NADPH-derived electron is used to reduce the H4 B radical, how this occurs is unknown. We hypothesized that the NOS flavoprotein domain might reduce the H4 B radical by utilizing the NOS heme porphyrin as a conduit to deliver the electron. This model predicts that factors influencing NOS heme reduction should also influence the extent and rate of H4 B radical reduction in kind. To test this, we utilized single catalytic turnover and stop-freeze methods, along with electron paramagnetic resonance spectroscopy, to measure the rate and extent of reduction of the 5-methyl-H4 B radical formed in neuronal NOS (nNOS) during Arg hydroxylation. We used several nNOS variants that supported either a slower or faster than normal rate of ferric heme reduction. We found that the rates and extents of nNOS heme reduction correlated well with the rates and extents of 5-methyl-H4 B radical reduction among the various nNOS enzymes. This supports a model where the heme porphyrin transfers an electron from the NOS flavoprotein to the H4 B radical formed during catalysis, revealing that the heme plays a dual role in catalyzing O2 activation or electron transfer at distinct points in the reaction cycle.

Nitric oxide synthase and structure-based inhibitor design

Once it was discovered that the enzyme nitric oxide synthase (NOS) is responsible for the biosynthesis of NO, NOS became a drug target. Particularly important is the over production of NO by neuronal NOS (nNOS) in various neurodegenerative disorders. After the various NOS isoforms were identified, inhibitor development proceeded rapidly. It soon became evident, however, that isoform selectivity presents a major challenge. All 3 human NOS isoforms, nNOS, eNOS (endothelial NOS), and iNOS (inducible NOS) have nearly identical active site structures thus making selective inhibitor design especially difficult. Of particular importance is the avoidance of inhibiting eNOS owing to its vital role in the cardiovascular system. This review summarizes some of the history of NOS inhibitor development and more recent advances in developing isoform selective inhibitors using primarily structure-based approaches.

Nitric oxide-polyamines cross-talk during dormancy release and germination of apple embryos

Nitric oxide (NO) and polyamines (PAs) belong to plant growth and development regulators. These compounds play a key role in numerous physiological processes e.g. seed germination. Based on the suggestion of overlapping of NO and PAs biosynthetic pathways, we demonstrated a cross-talk of NO and PAs in regulation of embryonic dormancy release. The aim of the work was to investigate an impact of PAs (Put, Spd and Spm) or NO short-term fumigation on nitrite, urea, Arg and ornithine (Orn) content, NO synthase-like (NOS-like) and arginase activity in axes of apple (Malus domestica Borkh.) embryos during dormancy alleviation and at the stage of termination of germination sensu stricto. NO, Put/Spd induced dormancy breakage and germination of apple embryos corresponded to stimulation of urea cycle and high free Arg pool in seedlings roots. After two days of the culture Put and Spd stimulated Arg dependent NO formation, inhibition of which was observed after Spm application. Put or Spd application as well as NO short-term pretreatment of apple embryos influenced level of ubiquitin-conjugated proteins. Higher abundance of such modified proteins correlated well to the declined content of nitrated proteins, suggesting their important role in regulation of embryo germination. NO led to stimulation of embryos germination by increasing level of free PAs (mostly Put). While transcriptomic approach showed down regulation of Spm synthesis and up-regulation of Spm degradation by NO, confirming negative role of Spm over-accumulation in embryo dormancy removal. Our data clearly indicate positive relationship of NO-Put/Spd acting as dormancy removing factors.

Modification of the endogenous NO level influences apple embryos dormancy by alterations of nitrated and biotinylated protein patterns

NO donors and Arg remove dormancy of apple embryos and stimulate germination. Compounds lowering NO level (cPTIO, L -NAME, CAN) strengthen dormancy. Embryo transition from dormancy state to germination is linked to increased nitric oxide synthase (NOS)-like activity. Germination of embryos is associated with declined level of biotin containing proteins and nitrated proteins in soluble protein fraction of root axis. Pattern of nitrated proteins suggest that storage proteins are putative targets of nitration. Nitric oxide (NO) acts as a key regulatory factor in removal of seed dormancy and is a signal necessary for seed transition from dormant state into germination. Modulation of NO concentration in apple (Malus domestica Borkh.) embryos by NO fumigation, treatment with NO donor (S-nitroso-N-acetyl-D,L-penicillamine, SNAP), application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), N ω-nitro-L-arginine methyl ester (L-NAME), canavanine (CAN) or arginine (Arg) allowed us to investigate the NO impact on seed dormancy status. Arg analogs and NO scavenger strengthened embryo dormancy by lowering reactive nitrogen species level in embryonic axes. This effect was accompanied by strong inhibition of NOS-like activity, without significant influence on tissue NO2 (-) concentration. Germination sensu stricto of apple embryos initiated by dormancy breakage via short term NO treatment or Arg supplementation were linked to a reduced level of biotinylated proteins in root axis. Decrease of total soluble nitrated proteins was observed at the termination of germination sensu stricto. Also modulation of NO tissue status leads to modification in nitrated protein pattern. Among protein bands that correspond to molecular mass of approximately 95 kDa, storage proteins (legumin A-like and seed biotin-containing protein) were identified, and can be considered as good markers for seed dormancy status. Moreover, pattern of nitrated proteins suggest that biotin containing proteins are also targets of nitration.

Role of a Conserved Tyrosine Residue in the FMN-Heme Interdomain Electron Transfer in Inducible Nitric Oxide Synthase

The interdomain electron transfer (IET) between the flavin mononucleotide (FMN) and heme domains is essential in the biosynthesis of nitric oxide (NO) by the NO synthase (NOS) enzymes. A conserved tyrosine residue in the FMN domain (Y631 in human inducible NOS) was proposed to be a key part of the electron transfer pathway in the FMN/heme docked complex model. In the present study, the FMN-heme IET kinetics in the Y631F mutant and wild type of a bidomain oxygenase/FMN construct of human inducible NOS were determined by laser flash photolysis. The rate constant of the Y631F mutant is significantly decreased by ∼75% (compared to the wild type), showing that the tyrosine residue indeed facilitates the FMN-heme IET through the protein medium. The IET rate constant of the wild type protein decreases from 345 to 242 s^-1 on going from H₂O to 95% D₂O, giving a solvent kinetic isotope effect of 1.4. In contrast, no deuterium isotope effect was observed for the Tyr-to-Phe mutant. Moreover, an appreciable change in the wild type iNOS IET rate constant value was observed upon changing pH. These results indicate that the FMN-heme IET is proton coupled, in which the conserved tyrosine residue may play an important role.

Nitric oxide synthase in plants: Where do we stand?

Over the past twenty years, nitric oxide (NO) has emerged as an important player in various plant physiological processes. Although many advances in the understanding of NO functions have been made, the question of how NO is produced in plants is still challenging. It is now generally accepted that the endogenous production of NO is mainly accomplished through the reduction of nitrite via both enzymatic and non-enzymatic mechanisms which remain to be fully characterized. Furthermore, experimental arguments in favour of the existence of plant nitric oxide synthase (NOS)-like enzymes have been reported. However, recent investigations revealed that land plants do not possess animal NOS-like enzymes while few algal species do. Phylogenetic and structural analyses reveals interesting features specific to algal NOS-like proteins.

Phosphorylation Controls Endothelial Nitric-oxide Synthase by Regulating Its Conformational Dynamics

The activity of endothelial NO synthase (eNOS) is triggered by calmodulin (CaM) binding and is often further regulated by phosphorylation at several positions in the enzyme. Phosphorylation at Ser¹¹⁷⁹ occurs in response to diverse physiologic stimuli and increases the NO synthesis and cytochrome c reductase activities of eNOS, thereby enhancing its participation in biological signal cascades. Despite its importance, the mechanism by which Ser¹¹⁷⁹ phosphorylation increases eNOS activity is not understood. To address this, we used stopped-flow spectroscopy and computer modeling approaches to determine how the phosphomimetic mutation (S1179D) may impact electron flux through eNOS and the conformational behaviors of its reductase domain, both in the absence and presence of bound CaM. We found that S1179D substitution in CaM-free eNOS had multiple effects; it increased the rate of flavin reduction, altered the conformational equilibrium of the reductase domain, and increased the rate of its conformational transitions. We found these changes were equivalent in degree to those caused by CaM binding to wild-type eNOS, and the S1179D substitution together with CaM binding caused even greater changes in these parameters. The modeling indicated that the changes caused by the S1179D substitution, despite being restricted to the reductase domain, are sufficient to explain the stimulation of both the cytochrome c reductase and NO synthase activities of eNOS. This helps clarify how Ser¹¹⁷⁹ phosphorylation regulates eNOS and provides a foundation to compare its regulation by other phosphorylation events.

Correlating Calmodulin Landscapes with Chemical Catalysis in Neuronal Nitric Oxide Synthase using Time-Resolved FRET and a 5-Deazaflavin Thermodynamic Trap

A major challenge in enzymology is the need to correlate the dynamic properties of enzymes with, and understand the impact on, their catalytic cycles. This is especially the case with large, multicenter enzymes such as the nitric oxide synthases (NOSs), where the importance of dynamics has been inferred from a variety of structural, single-molecule, and ensemble spectroscopic approaches but where motions have not been correlated experimentally with mechanistic steps in the reaction cycle. Here we take such an approach. Using time-resolved spectroscopy employing absorbance and Förster resonance energy transfer (FRET) and exploiting the properties of a flavin analogue (5-deazaflavin mononucleotide (5-dFMN)) and isotopically labeled nicotinamide coenzymes, we correlate the timing of CaM structural changes when bound to neuronal nitric oxide synthase (nNOS) with the nNOS catalytic cycle. We show that remodeling of CaM occurs early in the electron transfer sequence (FAD reduction), not at later points in the reaction cycle (e.g., FMN reduction). Conformational changes are tightly correlated with FAD reduction kinetics and reflect a transient “opening” and then “closure” of the bound CaM molecule. We infer that displacement of the C-terminal tail on binding NADPH and subsequent FAD reduction are the likely triggers of conformational change. By combining the use of cofactor/coenzyme analogues and time-resolved FRET/absorbance spectrophotometry, we show how the reaction cycles of complex enzymes can be simplified, enabling a detailed study of the relationship between protein dynamics and reaction cycle chemistry—an approach that can also be used with other complex multicenter enzymes.

Inhibitory action on the production of advanced glycation end products (AGEs) and suppression of free radicals in vitro by a Sri Lankan polyherbal formulation Nawarathne Kalka

Background

Advanced glycation end products (AGEs) and free radicals are inflammatory mediators and are implicated in many diseases such as diabetes, cancer, rheumatoid arthritis etc. Multi targeted poly herbal drug systems like Nawarathne Kalka (NK) are able to quench the overall effect of these mediators as they contain good combinations of phytochemicals that have least side effects in contrast to modern medicinal drugs. The objectives of this study were to evaluate phytochemical composition, free radical scavenging activity, cytotoxicity and the inhibitory action on the formation of AGEs by aqueous extract of NK.

Methods

Total phenolic content (TPC) and total flavonoid content (TFC) were determined using Folin ciocalteu method and aluminium chloride assay respectively. Free radical scavenging activity was assessed by DPPH radical scavenging assay (DRSA), phosphomolybdenum reduction antioxidant assay (PRAA) and nitric oxide (NO) scavenging assay. Brine Shrimp Lethality (BSL) bioassay was performed as preliminary screening for cytotoxic activity. Inhibitory action on AGE formation was evaluated using fructose mediated glycation of bovine serum albumin using fluorescence spectroscopic method.

Results

The TPC and TFC were 75.1 ± 3.0 mg/g gallic acid equivalents and 68.7 ± 7.8 mg/g epigallocatechin gallate equivalents. The DRSA yielded EC₅₀ of 19.15 ± 2.24 μg mL⁻¹ for NK. DRSA of NK extract was greater than butylated hydroxy toluene (EC₅₀ = 96.50 ± 4.51 μg mL⁻¹) but lesser than L-ascorbic acid (EC₅₀ = 5.60 ± 0.51 μg mL⁻¹). The total antioxidant capacity of NK as evidenced by PRAA was 106.4 ± 8.2 mg/g L-ascorbic acid equivalents. NK showed EC₅₀ value of 99.3 ± 8.4 μg mL⁻¹ in the NO scavenging assay compared to the standard ascorbic acid (EC₅₀ = 7.3 ± 0.3 μg mL⁻¹). The extract indicated moderate cytotoxic activity in the BSL bioassay. The extract showed effective inhibitory action on the formation of AGEs with EC₅₀ values of 116 ± 19 μg mL⁻¹, 125 ± 35 μg mL⁻¹ and 84 ± 28 μg mL⁻¹ in data obtained over three consecutive weeks respectively. Comparatively the reference standard, aminoguanidine at a concentration of 500 μg mL⁻¹ demonstrated 65 % inhibition on the formation of AGE after one week of sample incubation.

Conclusions

The results proved the potential of NK as a free radical scavenger, moderate cytotoxic agent and an inhibitor on the formation of advanced glycation end-products.

Dual inhibition of nitric oxide and prostaglandin E2 production by polysubstituted 2-aminopyrimidines

The present in vitro experiments demonstrate inhibitory effects of polysubstituted 2-aminopyrimidines on high output production of nitric oxide (NO) and prostaglandin E2 (PGE2) stimulated by interferon-γ and lipopolysaccharide (LPS) in peritoneal macrophages of mouse and rat origin. PGE2 production was inhibited also in LPS-activated human peripheral blood mononuclear cells. A tight dependence of the suppressive activities on chemical structure of pyrimidines was observed. Derivatives containing hydroxyl groups at the C-4 and C-6 positions of pyrimidine ring were devoid of any influence on NO and PGE2. Remarkable inhibitory potential was acquired by the replacement of hydroxyl groups with chlorine, the 4,6-dichloro derivatives being more effective than the monochloro analogues. The effects were further intensified by modification of the amino group at the C-2 position, changing it to the (N,N-dimethylamino)methyleneamino or the formamido ones. There was no substantial difference in the expression of NO-inhibitory effects among derivatives containing distinct types of substituents at the C-5 position (hydrogen, methyl, ethyl, propyl, butyl, phenyl, and benzyl). In contrast to NO, larger substituents then methyl were required to inhibit PGE2 production. Overall, no significant correlation between the extent of NO and PGE2 suppression was observed. The IC50s of derivatives with the strongest effects on both NO and PGE2 were within the range of 2-10 μM. Their NO-inhibitory potential of pyrimidines was stronger than that of non-steroidal anti-inflammatory drugs (NSAIDs) aspirin and indomethacin. The PGE2-inhibitory effectiveness of pyrimidines was about the same as that of aspirin, but weaker as compared to indomethacin. The NO- and PGE2-inhibitory activity of tested pyrimidines has been found associated with decreased expression of iNOS mRNA and COX-2 mRNA, respectively, and with post-translation interactions. Selected NO-/PGE2-inhibitory derivatives decreased severity of intestinal inflammation in murine model of ulcerative colitis.

Engineering nitric oxide synthase chimeras to function as NO dioxygenases

Nitric oxide synthases (NOSs) catalyze a two-step oxidation of l-arginine to form nitric oxide (NO) and l-citrulline. NOS contains a N-terminal oxygenase domain (NOSoxy) that is the site of NO synthesis, and a C-terminal reductase domain (NOSred) that binds nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN) and provides electrons to the NOSoxy heme during catalysis. The three NOS isoforms in mammals inducible NOS (iNOS), neuronal NOS (nNOS), and endothelial NOS (eNOS) share high structural similarity but differ in NO release rates and catalytic properties due to differences in enzyme kinetic parameters. These parameters must be balanced for NOS enzymes to release NO, rather than consume it in a competing, inherent NO dioxygenase reaction. To improve understanding, we drew on a global catalytic model and previous findings to design three NOS chimeras that may predominantly function as NO dioxygenases: iNOSoxy/nNOSred (Wild type (WT) chimera), V346I iNOSoxy/nNOSred (V346I chimera) and iNOSoxy/S1412D nNOSred (S1412D chimera). The WT and S1412D chimeras had higher NO release than the parent iNOS, while the V346I chimera exhibited much lower NO release, consistent with expectations. Measurements indicated that a greater NO dioxygenase activity was achieved, particularly in the V346I chimera, which dioxygenated an estimated two to four NO per NO that it released, while the other chimeras had nearly equivalent NO dioxygenase and NO release activities. Computer simulations of the global catalytic model using the measured kinetic parameters produced results that mimicked the measured outcomes, and this provided further insights on the catalytic behaviors of the chimeras and basis of their increased NO dioxygenase activities.

Elucidating nitric oxide synthase domain interactions by molecular dynamics

Nitric oxide synthase (NOS) is a multidomain enzyme that catalyzes the production of nitric oxide (NO) by oxidizing L-Arg to NO and L-citrulline. NO production requires multiple interdomain electron transfer steps between the flavin mononucleotide (FMN) and heme domain. Specifically, NADPH-derived electrons are transferred to the heme-containing oxygenase domain via the flavin adenine dinucleotide (FAD) and FMN containing reductase domains. While crystal structures are available for both the reductase and oxygenase domains of NOS, to date there is no atomic level structural information on domain interactions required for the final FMN-to-heme electron transfer step. Here, we evaluate a model of this final electron transfer step for the heme-FMN-calmodulin NOS complex based on the recent biophysical studies using a 105-ns molecular dynamics trajectory. The resulting equilibrated complex structure is very stable and provides a detailed prediction of interdomain contacts required for stabilizing the NOS output state. The resulting equilibrated complex model agrees well with previous experimental work and provides a detailed working model of the final NOS electron transfer step required for NO biosynthesis.

Single-molecule spectroscopy reveals how calmodulin activates NO synthase by controlling its conformational fluctuation dynamics

Mechanisms that regulate the nitric oxide synthase enzymes (NOS) are of interest in biology and medicine. Although NOS catalysis relies on domain motions, and is activated by calmodulin binding, the relationships are unclear. We used single-molecule fluorescence resonance energy transfer (FRET) spectroscopy to elucidate the conformational states distribution and associated conformational fluctuation dynamics of the two electron transfer domains in a FRET dye-labeled neuronal NOS reductase domain, and to understand how calmodulin affects the dynamics to regulate catalysis. We found that calmodulin alters NOS conformational behaviors in several ways: It changes the distance distribution between the NOS domains, shortens the lifetimes of the individual conformational states, and instills conformational discipline by greatly narrowing the distributions of the conformational states and fluctuation rates. This information was specifically obtainable only by single-molecule spectroscopic measurements, and reveals how calmodulin promotes catalysis by shaping the physical and temporal conformational behaviors of NOS.

Real-time analysis of conformational control in electron transfer reactions of human cytochrome P450 reductase with cytochrome c

Protein domain dynamics and electron transfer chemistry are often associated, but real‐time analysis of domain motion in enzyme‐catalysed reactions and the elucidation of mechanistic schemes that relate these motions to the reaction chemistry are major challenges for biological catalysis research. Previously we suggested that reduction of human cytochrome P450 reductase with the reducing coenzyme NADPH is accompanied by major structural re‐orientation of the FMN‐ and FAD‐binding domains through an inferred dynamic cycle of ‘open’ and ‘closed’ conformations of the enzyme (PLoS Biol, 2011, e1001222). However, these studies were restricted to stopped‐flow/FRET analysis of the reductive half‐reaction, and were compromised by fluorescence quenching of the acceptor by the flavin cofactors. Here we have improved the design of the FRET system, by using dye pairs with near‐IR fluorescence, and extended studies on human cytochrome P450 reductase to the oxidative half‐reaction using a double‐mixing stopped‐flow assay, thereby analysing in real‐time conformational dynamics throughout the complete catalytic cycle. We correlate redox changes accompanying the reaction chemistry with protein dynamic changes observed by FRET, and show that redox chemistry drives a major re‐orientation of the protein domains in both the reductive and oxidative half‐reactions. Our studies using the tractable (soluble) surrogate electron acceptor cytochrome c provide a framework for analysing mechanisms of electron transfer in the endoplasmic reticulum between cytochrome P450 reductase and cognate P450 enzymes. More generally, our work emphasizes the importance of protein dynamics in intra‐ and inter‐protein electron transfer, and establishes methodology for real‐time analysis of structural changes throughout the catalytic cycle of complex redox proteins.

Structural Basis of Neuronal Nitric-oxide Synthase Interaction with Dystrophin Repeats 16 and 17

Duchenne muscular dystrophy is a lethal genetic defect that is associated with the absence of dystrophin protein. Lack of dystrophin protein completely abolishes muscular nitric-oxide synthase (NOS) function as a regulator of blood flow during muscle contraction. In normal muscles, nNOS function is ensured by its localization at the sarcolemma through an interaction of its PDZ domain with dystrophin spectrin-like repeats R16 and R17. Early studies suggested that repeat R17 is the primary site of interaction but ignored the involved nNOS residues, and the R17 binding site has not been described at an atomic level. In this study, we characterized the specific amino acids involved in the binding site of nNOS-PDZ with dystrophin R16-17 using combined experimental biochemical and structural in silico approaches. First, 32 alanine-scanning mutagenesis variants of dystrophin R16-17 indicated the regions where mutagenesis modified the affinity of the dystrophin interaction with the nNOS-PDZ. Second, using small angle x-ray scattering-based models of dystrophin R16-17 and molecular docking methods, we generated atomic models of the dystrophin R16-17·nNOS-PDZ complex that correlated well with the alanine scanning identified regions of dystrophin. The structural regions constituting the dystrophin interaction surface involve the A/B loop and the N-terminal end of helix B of repeat R16 and the N-terminal end of helix A' and a small fraction of helix B' and a large part of the helix C' of repeat R17. The interaction surface of nNOS-PDZ involves its main β-sheet and its specific C-terminal β-finger.

Insight into structural rearrangements and interdomain interactions related to electron transfer between flavin mononucleotide and heme in nitric oxide synthase: A molecular dynamics study

Calmodulin (CaM) binding to nitric oxide synthase (NOS) enables a conformational change, in which the FMN domain shuttles between the FAD and heme domains to deliver electrons to the active site heme center. A clear understanding of this large conformational change is critical, since this step is the rate-limiting in NOS catalysis. Herein molecular dynamics simulations were conducted on a model of an oxygenase/FMN (oxyFMN) construct of human inducible NOS (iNOS). This is to investigate the structural rearrangements and the domain interactions related to the FMN-heme interdomain electron transfer (IET). We carried out simulations on the iNOS oxyFMN·CaM complex models in [Fe(III)][FMNH(-)] and [Fe(II)][FMNH] oxidation states, the pre- and post-IET states. The comparison of the dynamics and conformations of the iNOS construct at the two oxidation states has allowed us to identify key factors related to facilitating the FMN-heme IET process. The computational results demonstrated, for the first time, that the conformational change is redox-dependent. Predictions of the key interacting sites in optimal interdomain FMN/heme docking are well supported by experimental data in the literature. An intra-subunit pivot region is predicted to modulate the FMN domain motion and correlate with existence of a bottleneck in the conformational sampling that leads to the electron transfer-competent state. Interactions of the residues identified in this work are proposed to ensure that the FMN domain moves with appropriate degrees of freedom and docks to proper positions at the heme domain, resulting in efficient IET and nitric oxide production.

Biochemical investigations of the mechanism of action of small molecules ZL006 and IC87201 as potential inhibitors of the nNOS-PDZ/PSD-95-PDZ interactions

ZL006 and IC87201 have been presented as efficient inhibitors of the nNOS/PSD-95 protein-protein interaction and shown great promise in cellular experiments and animal models of ischemic stroke and pain. Here, we investigate the proposed mechanism of action of ZL006 and IC87201 using biochemical and biophysical methods, such as fluorescence polarization (FP), isothermal titration calorimetry (ITC), and (1)H-(15)N HSQC NMR. Our data show that under the applied in vitro conditions, ZL006 and IC87201 do not interact with the PDZ domains of nNOS or PSD-95, nor inhibit the nNOS-PDZ/PSD-95-PDZ interface by interacting with the β-finger of nNOS-PDZ. Our findings have implications for further medicinal chemistry efforts of ZL006, IC87201 and analogues, and challenge the general and widespread view on their mechanism of action.

Fluorescence quenching studies of structure and dynamics in calmodulin-eNOS complexes

Activation of endothelial nitric oxide synthase (eNOS) by calmodulin (CaM) facilitates formation of a sequence of conformational states that is not well understood. Fluorescence decays of fluorescently labeled CaM bound to eNOS reveal four distinct conformational states and single-molecule fluorescence trajectories show multiple fluorescence states with transitions between states occurring on time scales of milliseconds to seconds. A model is proposed relating fluorescence quenching states to enzyme conformations. Specifically, we propose that the most highly quenched state corresponds to CaM docked to an oxygenase domain of the enzyme. In single-molecule trajectories, this state occurs with time lags consistent with the oxygenase activity of the enzyme.

Nitric oxide synthase in innate and adaptive immunity: an update

Thirty years after the discovery of its production by activated macrophages, our appreciation of the diverse roles of nitric oxide (NO) continues to grow. Recent findings have not only expanded our understanding of the mechanisms controlling the expression of NO synthases (NOS) in innate and adaptive immune cells, but have also revealed new functions and modes of action of NO in the control and escape of infectious pathogens, in T and B cell differentiation, and in tumor defense. I discuss these findings, in the context of a comprehensive overview of the various sources and multiple reaction partners of NO, and of the regulation of NOS2 by micromilieu factors, antisense RNAs, and 'unexpected' cytokines.

Towards the free energy landscape for catalysis in mammalian nitric oxide synthases

The general requirement for conformational sampling in biological electron transfer reactions catalysed by multi-domain redox systems has been emphasized in recent years. Crucially, we lack insight into the extent of the conformational space explored and the nature of the energy landscapes associated with these reactions. The nitric oxide synthases (NOS) produce the signalling molecule NO through a series of complex electron transfer reactions. There is accumulating evidence that protein domain dynamics and calmodulin binding are implicated in regulating electron flow from NADPH, through the FAD and FMN cofactors, to the haem oxygenase domain, where NO is generated. Simple models based on static crystal structures of the isolated reductase domain have suggested a role for large-scale motions of the FMN-binding domain in shuttling electrons from the reductase domain to the oxygenase domain. However, detailed insight into the higher-order domain architecture and dynamic structural transitions in NOS enzymes during enzyme turnover is lacking. In this review, we discuss the recent advances made towards mapping the catalytic free energy landscapes of NOS enzymes through integration of both structural techniques (e.g. cryo-electron microscopy) and biophysical techniques (e.g. pulsed-electron paramagnetic resonance). The general picture that emerges from these experiments is that NOS enzymes exist in an equilibrium of conformations, comprising a 'rugged' or 'frustrated' energy landscape, with a key regulatory role for calmodulin in driving vectorial electron transfer by altering the conformational equilibrium. A detailed understanding of these landscapes may provide new opportunities for discovery of isoform-specific inhibitors that bind at the dynamic interfaces of these multi-dimensional energy landscapes.