Differential activation of nitric-oxide synthase isozymes by calmodulin-troponin C chimeras

Calmodulin-induced Conformational Control and Allostery Underlying Neuronal Nitric Oxide Synthase Activation

Nitric oxide synthase (NOS) is the primary generator of nitric oxide signals controlling diverse physiological processes such as neurotransmission and vasodilation. NOS activation is contingent on Ca²⁺/calmodulin binding at a linker between its oxygenase and reductase domains to induce large conformational changes that orchestrate inter-domain electron transfer. However, the structural dynamics underlying activation of full-length NOS remain ambiguous. Employing hydrogen-deuterium exchange mass spectrometry, we reveal mechanisms underlying neuronal NOS activation by calmodulin and regulation by phosphorylation. We demonstrate that calmodulin binding orders the junction between reductase and oxygenase domains, exposes the FMN subdomain, and elicits a more dynamic oxygenase active site. Furthermore, we demonstrate that phosphorylation partially mimics calmodulin activation to modulate neuronal NOS activity via long-range allostery. Calmodulin binding and phosphorylation ultimately promote a more dynamic holoenzyme while coordinating inter-domain communication and electron transfer.

A cross-domain charge interaction governs the activity of NO synthase

NO synthase (NOS) enzymes perform interdomain electron transfer reactions during catalysis that may rely on complementary charge interactions at domain-domain interfaces. Guided by our previous results and a computer-generated domain-docking model, we assessed the importance of cross-domain charge interactions in the FMN-to-heme electron transfer in neuronal NOS (nNOS). We reversed the charge of three residues (Glu-762, Glu-816, and Glu-819) that form an electronegative triad on the FMN domain and then individually reversed the charges of three electropositive residues (Lys-423, Lys-620, and Lys-660) on the oxygenase domain (NOSoxy), to potentially restore a cross-domain charge interaction with the triad, but in reversed polarity. Charge reversal of the triad completely eliminated heme reduction and NO synthesis in nNOS. These functions were partly restored by the charge reversal at oxygenase residue Lys-423, but not at Lys-620 or Lys-660. Full recovery of heme reduction was probably muted by an accompanying change in FMN midpoint potential that made electron transfer to the heme thermodynamically unfavorable. Our results provide direct evidence that cross-domain charge pairing is required for the FMN-to-heme electron transfer in nNOS. The unique ability of charge reversal at position 423 to rescue function indicates that it participates in an essential cross-domain charge interaction with the FMN domain triad. This supports our domain-docking model and suggests that it may depict a productive electron transfer complex formed during nNOS catalysis.

The FNR modules contribute to control nitric oxide synthase catalysis revealed by chimera enzymes

The reductase domains of neuronal NOS, endothelial NOS and two constitutive nitric oxide synthase (cNOS) share higher sequence similarity (>60%). In order to evaluate the role of ferredoxin‑NADP+ reductase (FNR) module in adjusting NOS catalytic activities, chimeras were by interchanging the FNR‑like module between endothelial NOS and neuronal NOS in the present study. The assays of steady‑state enzymatic activities for cytochrome c and ferricyanide reduction, NO synthesis and NADPH oxidation were performed spectrophotometrically. The two NOS FNR modules transferred their ferricyanide reductase character to the chimera enzymes. Results showed that the FNR module was important in adjusting electrons flow through the reductase domain and out of the FMN module. Results indicated that the FNR module was critical in controlling the electron transfer capacities of the FMN module.

A versatile cell-penetrating peptide-adaptor system for efficient delivery of molecular cargos to subcellular destinations

Cell penetrating peptides have long held great potential for delivery of biomolecular cargos for research, therapeutic and diagnostic purposes. They allow rapid, relatively nontoxic passage of a wide variety of biomolecules through the plasma membranes of living cells. However, CPP-based research tools and therapeutics have been stymied by poor efficiency in release from endosomes and a great deal of effort has been made to solve this ‘endosomal escape problem.’ Previously, we showed that use of a reversible, noncovalent coupling between CPP and cargo using calmodulin and a calmodulin binding motif allowed efficient delivery of cargo proteins to the cytoplasm in baby hamster kidney and other mammalian cell lines. The present report demonstrates the efficacy of our CPP-adaptor scheme for efficient delivery of model cargos to the cytoplasm using a variety of CPPs and adaptors. Effective overcoming of the endosomal escape problem is further demonstrated by the delivery of cargo to the nucleus, endoplasmic reticulum and peroxisomes by addition of appropriate subcellular localization signals to the cargos. CPP-adaptors were also used to deliver cargo to myotubes, demonstrating the feasibility of the system as an alternative to transfection for the manipulation of hard-to-transfect cells.

Arabidopsis CaM1 and CaM4 Promote Nitric Oxide Production and Salt Resistance by Inhibiting S-Nitrosoglutathione Reductase via Direct Binding

Salt is a major threat to plant growth and crop productivity. Calmodulin (CaM), the most important multifunctional Ca2+ sensor protein in plants, mediates reactions against environmental stresses through target proteins; however, direct proof of the participation of CaM in salt tolerance and its corresponding signaling pathway in vivo is lacking. In this study, we found that AtCaM1 and AtCaM4 produced salt-responsive CaM isoforms according to real-time reverse transcription-polymerase chain reaction analyses; this result was verified based on a phenotypic analysis of salt-treated loss-of-function mutant and transgenic plants. We also found that the level of nitric oxide (NO), an important salt-responsive signaling molecule, varied in response to salt treatment depending on AtCaM1 and AtCaM4 expression. GSNOR is considered as an important and widely utilized regulatory component of NO homeostasis in plant resistance protein signaling networks. In vivo and in vitro protein-protein interaction assays revealed direct binding between AtCaM4 and S-nitrosoglutathione reductase (GSNOR), leading to reduced GSNOR activity and an increased NO level. Overexpression of GSNOR intensified the salt sensitivity of cam4 mutant plants accompanied by a reduced internal NO level, whereas a gsnor deficiency increased the salt tolerance of cam4 plants accompanied by an increased internal NO level. Physiological experiments showed that CaM4-GSNOR, acting through NO, reestablished the ion balance to increase plant resistance to salt stress. Together, these data suggest that AtCaM1 and AtCaM4 serve as signals in plant salt resistance by promoting NO accumulation through the binding and inhibition of GSNOR. This could be a conserved defensive signaling pathway in plants and animals.

The exchanged EF-hands in calmodulin and troponin C chimeras impair the Ca²⁺-induced hydrophobicity and alter the interaction with Orai1: a spectroscopic, thermodynamic and kinetic study

Background

Calmodulin (CaM) plays an important role in Ca²⁺-dependent signal transduction. Ca²⁺ binding to CaM triggers a conformational change, forming a hydrophobic patch that is important for target protein recognition. CaM regulates a Ca²⁺-dependent inactivation process in store-operated Ca²⁺ entry, by interacting Orai1. To understand the relationship between Ca²⁺-induced hydrophobicity and CaM/Orai interaction, chimera proteins constructed by exchanging EF-hands of CaM with those of Troponin C (TnC) are used as an informative probe to better understand the functionality of each EF-hand.

Results

ANS was used to assess the context of the induced hydrophobic surface on CaM and chimeras upon Ca²⁺ binding. The exchanged EF-hands from TnC to CaM resulted in reduced hydrophobicity compared with wild-type CaM. ANS lifetime measurements indicated that there are two types of ANS molecules with rather distinct fluorescence lifetimes, each specifically corresponding to one lobe of CaM or chimeras. Thermodynamic studies indicated the interaction between CaM and a 24-residue peptide corresponding to the CaM-binding domain of Orail1 (Orai-CMBD) is a 1:2 CaM/Orai-CMBD binding, in which each peptide binding yields a similar enthalpy change (ΔH = −5.02 ± 0.13 kcal/mol) and binding affinity (K_a = 8.92 ± 1.03 × 10⁵ M⁻¹). With the exchanged EF1 and EF2, the resulting chimeras noted as CaM(1TnC) and CaM(2TnC), displayed a two sequential binding mode with a one-order weaker binding affinity and lower ΔH than that of CaM, while CaM(3TnC) and CaM(4TnC) had similar binding thermodynamics as CaM. The dissociation rate constant for CaM/Orai-CMBD was determined to be 1.41 ± 0.08 s⁻¹ by rapid kinetics. Stern-Volmer plots of Orai-CMBD Trp76 indicated that the residue is located in a very hydrophobic environment but becomes more solvent accessible when EF1 and EF2 were exchanged.

Conclusions

Using ANS dye to assess induced hydrophobicity showed that exchanging EFs for all Ca²⁺-bound chimeras impaired ANS fluorescence and/or binding affinity, consistent with general concepts about the inadequacy of hydrophobic exposure for chimeras. However, such ANS responses exhibited no correlation with the ability to interact with Orai-CMBD. Here, the model of 1:2 binding stoichiometry of CaM/Orai-CMBD established in solution supports the already published crystal structure.

Endothelial nitric oxide synthase is regulated by ERK phosphorylation at Ser602

eNOS (endothelial nitric oxide synthase) contains a MAPK (mitogen-activated protein kinase)-binding site associated with a major eNOS control element. Purified ERK (extracellular-signal-regulated kinase) phosphorylates eNOS with a stoichiometry of 2–3 phosphates per eNOS monomer. Phosphorylation decreases NO synthesis and cytochrome c reductase activity. Three sites of phosphorylation were detected by MS. All sites matched the SP and TP MAPK (mitogen-activated protein kinase) phosphorylation motif. Ser⁶⁰² lies at the N-terminal edge of the 42-residue eNOS AI (autoinhibitory) element. The pentabasic MAPK-binding site lies at the opposite end of the AI, and other critical regulatory features are between them. Thr⁴⁶ and Ser⁵⁸ are located in a flexible region associated with the N terminus of the oxygenase domain. In contrast with PKC (protein kinase C), phosphorylation by ERK did not significantly interfere with CaM (calmodulin) binding as analysed by optical biosensing. Instead, ERK phosphorylation favours a state in which FMN and FAD are in close association and prevents conformational changes that expose reduced FMN to acceptors. The close associations between control sites in a few regions of the molecule suggest that control of signal generation is modulated by multiple inputs interacting directly on the surface of eNOS via overlapping binding domains and tightly grouped targets.

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.

Pulsed electron paramagnetic resonance study of domain docking in neuronal nitric oxide synthase: the calmodulin and output state perspective

The binding of calmodulin (CaM) to neuronal nitric oxide synthase (nNOS) enables formation of the output state of nNOS for nitric oxide production. Essential to NOS function is the geometry and dynamics of CaM docking to the NOS oxygenase domain, but little is known about these details. In the present work, the domain docking in a CaM-bound oxygenase/FMN (oxyFMN) construct of nNOS was investigated using the relaxation-induced dipolar modulation enhancement (RIDME) technique, which is a pulsed electron paramagnetic resonance technique sensitive to the magnetic dipole interaction between the electron spins. A cysteine was introduced at position 110 of CaM, after which a nitroxide spin label was attached at the position. The RIDME study of the magnetic dipole interaction between the spin label and the ferric heme centers in the oxygenase domain of nNOS revealed that, with increasing [Ca(2+)], the concentration of nNOS·CaM complexes increases and reaches a maximum at [Ca(2+)]/[CaM] ≥ 4. The RIDME kinetics of CaM-bound nNOS represented monotonous decays without well-defined oscillations. The analysis of these kinetics based on the structural models for the open and docked states has shown that only about 15 ± 3% of the CaM-bound nNOS is in the docked state at any given time, while the remaining 85 ± 3% of the protein is in the open conformations characterized by a wide distribution of distances between the bound CaM and the oxygenase domain. The results of this investigation are consistent with a model that the Ca(2+)-CaM interaction causes CaM docking with the oxygenase domain. The low population of the docked state indicates that the CaM-controlled docking between the FMN and heme domains is highly dynamic.

Dissecting regulation mechanism of the FMN to heme interdomain electron transfer in nitric oxide synthases

Nitric oxide synthase (NOS), a flavo-hemoprotein, is responsible for biosynthesis of nitric oxide (NO) in mammals. Three NOS isoforms, iNOS, eNOS and nNOS (inducible, endothelial, and neuronal NOS), achieve their biological functions by tight control of interdomain electron transfer (IET) process through interdomain interactions. In particular, the FMN-heme IET is essential in coupling electron transfer in the reductase domain with NO synthesis in the heme domain by delivery of electrons required for O2 activation at the catalytic heme site. Emerging evidence indicates that calmodulin (CaM) activates NO synthesis in eNOS and nNOS by a conformational change of the FMN domain from its shielded electron-accepting (input) state to a new electron-donating (output) state, and that CaM is also required for proper alignment of the FMN and heme domains in the three NOS isoforms. In the absence of a structure of full-length NOS, an integrated approach of spectroscopic, rapid kinetic and mutagenesis methods is required to unravel regulation mechanism of the FMN-heme IET process. This is to investigate the roles of the FMN domain motions and the docking between the primary functional FMN and heme domains in regulating NOS activity. The recent developments in this area that are driven by the combined approach are the focuses of this review. A better understanding of the roles of interdomain FMN/heme interactions and CaM binding may serve as a basis for the rational design of new selective modulators of the NOS enzymes.

Calmodulin activates neuronal nitric oxide synthase by enabling transitions between conformational states

We recently showed that inducible nitric oxide synthase conformational intermediates can be resolved via FMN fluorescence lifetimes. Here we show that neuronal NOS activation by calmodulin removes constraints favoring a closed 'input state', increasing occupation of other states and facilitating conformational transitions. The 90 ps FMN input state lifetime distinguishes it from ∼4 ns 'open' states in which FMN does not interact strongly with other groups, or 0.9 ns output states in which FMN interacts with ferriheme. Enablement of the conformational cycle is an important paradigm for control in nNOS and related enzymes, and may extend to other control modalities.

Lobe-specific calcium binding in calmodulin regulates endothelial nitric oxide synthase activation

BACKGROUND

Human endothelial nitric oxide synthase (eNOS) requires calcium-bound calmodulin (CaM) for electron transfer but the detailed mechanism remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using a series of CaM mutants with E to Q substitution at the four calcium-binding sites, we found that single mutation at any calcium-binding site (B1Q, B2Q, B3Q and B4Q) resulted in ∼2-3 fold increase in the CaM concentration necessary for half-maximal activation (EC50) of citrulline formation, indicating that each calcium-binding site of CaM contributed to the association between CaM and eNOS. Citrulline formation and cytochrome c reduction assays revealed that in comparison with nNOS or iNOS, eNOS was less stringent in the requirement of calcium binding to each of four calcium-binding sites. However, lobe-specific disruption with double mutations in calcium-binding sites either at N- (B12Q) or at C-terminal (B34Q) lobes greatly diminished both eNOS oxygenase and reductase activities. Gel mobility shift assay and flavin fluorescence measurement indicated that N- and C-lobes of CaM played distinct roles in regulating eNOS catalysis; the C-terminal EF-hands in its calcium-bound form was responsible for the binding of canonical CaM-binding domain, while N-terminal EF-hands in its calcium-bound form controlled the movement of FMN domain. Limited proteolysis studies further demonstrated that B12Q and B34Q induced different conformational change in eNOS.

CONCLUSIONS

Our results clearly demonstrate that CaM controls eNOS electron transfer primarily through its lobe-specific calcium binding.

FMN fluorescence in inducible NOS constructs reveals a series of conformational states involved in the reductase catalytic cycle

Nitric oxide synthases (NOSs) produce NO as a molecular signal in the nervous and cardiovascular systems and as a cytotoxin in the immune response. NO production in the constitutive isoforms is controlled by calmodulin regulation of electron transfer. In the tethered shuttle model for NOS reductase function, the FMN domain moves between NADPH dehydrogenase and oxygenase catalytic centers. Crystal structures of neuronal NOS reductase domain and homologs correspond to an 'input state', with FMN in close contact with FAD. We recently produced two domain 'output state' (oxyFMN) constructs showing calmodulin dependent FMN domain association with the oxygenase domain. FMN fluorescence is sensitive to enzyme conformation and calmodulin binding. The inducible NOS (iNOS) oxyFMN construct is more fluorescent than iNOS holoenzyme. The difference in steady state fluorescence is rationalized by the observation of a series of characteristic states in the two constructs, which we assign to FMN in different environments. OxyFMN and holoenzyme share open conformations with an average lifetime of ~4.3 ns. The majority state in holoenzyme has a short lifetime of ~90 ps, probably because of FAD-FMN interactions. In oxyFMN about 25-30% of the FMN is in a state with a lifetime of 0.9 ns, which we attribute to quenching by heme in the output state. Occupancy of the output state together with our previous kinetic results yields a heme edge to FMN distance estimate of 12-15 Å. These results indicate that FMN fluorescence is a valuable tool to study conformational states involved in the NOS reductase catalytic cycle.

Nitric oxide inhibits androgen receptor-mediated collagen production in human gingival fibroblasts

Lin S-J, Lu H-K, Lee H-W, Chen Y-C, Li C-L, Wang L-F. Nitric oxide inhibits androgen receptor-mediated collagen production in human gingival fibroblasts. J Periodont Res 2012; 47: 701-710. © 2012 John Wiley & Sons A/S Background and Objective:  In our previous study, we found that flutamide [an androgen receptor (AR) antagonist] inhibited the up-regulation of collagen induced by interleukin (IL)-1β and/or nifedipine in gingival fibroblasts. The present study attempted to verify the role of nitric oxide (NO) in the IL-1β/nifedipine-AR pathway in gingival overgrowth.

MATERIAL AND METHODS

  Confluent gingival fibroblasts derived from healthy individuals (n = 4) and those with dihydropyridine-induced gingival overgrowth (DIGO) (n = 6) were stimulated for 48 h with IL-1β (10 ng/mL), nifedipine (0.34 μm) or IL-1β + nifedipine. Gene and protein expression were analyzed with real-time RT-PCR and western blot analyses, respectively. Meanwhile, Sircol dye-binding and the Griess reagent were, respectively, used to detect the concentrations of total soluble collagen and nitrite in the medium.

RESULTS

  IL-1β and nifedipine simultaneously up-regulated the expression of the AR and type-I collagen α1 [Colα1(I)] genes and the total collagen concentration in DIGO cells (p < 0.05). IL-1β strongly increased the expression of inducible nitric oxide synthase (iNOS) mRNA and the nitrite concentration in both healthy and DIGO cells (p < 0.05). However, co-administration of IL-1β and nifedipine largely abrogated the expression of iNOS mRNA and the nitrite concentration with the same treatment. Spearman's correlation coefficients revealed a positive correlation between the AR and total collagen (p < 0.001), but they both showed a negative correlation with iNOS expression and the NO concentration (p < 0.001). The iNOS inhibitor, 1400W, enhanced IL-1β-induced AR expression; furthermore, the NO donor, NONOate, diminished the expression of the AR to a similar extent in gingival fibroblasts derived from both healthy patients and DIGO patients (p < 0.05).

CONCLUSION

  IL-1β-induced NO attenuated AR-mediated collagen production in human gingival fibroblasts. The iNOS/NO system down-regulated the axis of AR/Colα1(I) mRNA expression and the production of AR/total collagen proteins by DIGO cells.

Structure and dynamics of calmodulin (CaM) bound to nitric oxide synthase peptides: effects of a phosphomimetic CaM mutation

Nitric oxide synthase (NOS) plays a major role in a number of key physiological and pathological processes. Knowledge of how this is regulated is important. The small acidic calcium binding protein, calmodulin (CaM), is required to fully activate the enzyme. The exact mechanism of how CaM activates NOS is not fully understood. Studies have shown CaM to act like a switch that causes a conformational change in NOS to allow for the transfer of an electron between the reductase and oxygenase domains through a process that is thought to be highly dynamic. To investigate the dynamic properties of CaM-NOS interactions, we determined the solution structure of CaM bound to the inducible NOS (iNOS) and endothelial NOS (eNOS) CaM binding region peptides. In addition, we investigated the effect of CaM phosphorylation. Tyrosine 99 (Y99) of CaM is reported to be phosphorylated in vivo. We have produced a phosphomimetic Y99E CaM to investigate the structural and functional effects that the phosphorylation of this residue may have on nitric oxide production. All three mammalian NOS isoforms were included in the investigation. Our results show that a phosphomimetic Y99E CaM significantly reduces the maximal synthase activity of eNOS by 40% while having little effect on nNOS or iNOS activity. A comparative nuclear magnetic resonance study between phosphomimetic Y99E CaM and wild-type CaM bound to the eNOS CaM binding region peptide was performed. This investigation provides important insights into how the increased electronegativity of a phosphorylated CaM protein affects the binding, dynamics, and activation of the NOS enzymes.

Role of an isoform-specific serine residue in FMN-heme electron transfer in inducible nitric oxide synthase

In the crystal structure of a calmodulin (CaM)-bound FMN domain of human inducible nitric oxide synthase (NOS), the CaM-binding region together with CaM forms a hinge, and pivots on an R536(NOS)/E47(CaM) pair (Xia et al. J Biol Chem 284:30708-30717, 2009). Notably, isoform-specific human inducible NOS S562 and C563 residues form hydrogen bonds with the R536 residue through their backbone oxygens. In this study, we investigated the roles of the S562 and C563 residues in the NOS FMN-heme interdomain electron transfer (IET), the rates of which can be used to probe the interdomain FMN/heme alignment. Human inducible NOS S562K and C563R mutants of an oxygenase/FMN (oxyFMN) construct were made by introducing charged residues at these sites as found in human neuronal NOS and endothelial NOS isoforms, respectively. The IET rate constant of the S562K mutant is notably decreased by one third, and its flavin fluorescence intensity per micromole per liter is diminished by approximately 24 %. These results suggest that a positive charge at position 562 destabilizes the hydrogen-bond-mediated NOS/CaM alignment, resulting in slower FMN-heme IET in the mutant. On the other hand, the IET rate constant of the C563R mutant is similar to that of the wild-type, indicating that the mutational effect is site-specific. Moreover, the human inducible NOS oxyFMN R536E mutant was constructed to disrupt the bridging CaM/NOS interaction, and its FMN-heme IET rate was decreased by 96 %. These results demonstrated a new role of the isoform-specific serine residue of the key CaM/FMN(NOS) bridging site in regulating the FMN-heme IET (possibly by tuning the alignment of the FMN and heme domains).

MAP kinases bind endothelial nitric oxide synthase

Endothelial nitric oxide synthase (eNOS) contains a motif similar to recognition sequences in known MAPK binding partners. In optical biosensing experiments, eNOS bound p38 and ERK with ∼100 nM affinity and complex kinetics. Binding is diffusion-limited (k_on ∼ .15 × 10⁶ M⁻¹ s⁻¹). Neuronal NOS also bound p38 but exhibited much slower and weaker binding. p38-eNOS binding was inhibited by calmodulin. Evidence for a ternary complex was found when eNOS bound p38 was exposed to CaM, increasing the apparent dissociation rate. These observations strongly suggest a direct role for MAPK in regulation of NOS with implications for signaling pathways including angiogenesis and control of vascular tone.

Mechanism of Nitric Oxide Synthase Regulation: Electron Transfer and Interdomain Interactions

Nitric oxide synthase (NOS), a flavo-hemoprotein, tightly regulates nitric oxide (NO) synthesis and thereby its dual biological activities as a key signaling molecule for vasodilatation and neurotransmission at low concentrations, and also as a defensive cytotoxin at higher concentrations. Three NOS isoforms, iNOS, eNOS and nNOS (inducible, endothelial, and neuronal NOS), achieve their key biological functions by tight regulation of interdomain electron transfer (IET) process via interdomain interactions. In particular, the FMN-heme IET is essential in coupling electron transfer in the reductase domain with NO synthesis in the heme domain by delivery of electrons required for O(2) activation at the catalytic heme site. Compelling evidence indicates that calmodulin (CaM) activates NO synthesis in eNOS and nNOS through a conformational change of the FMN domain from its shielded electron-accepting (input) state to a new electron-donating (output) state, and that CaM is also required for proper alignment of the domains. Another exciting recent development in NOS enzymology is the discovery of importance of the the FMN domain motions in modulating reactivity and structure of the catalytic heme active site (in addition to the primary role of controlling the IET processes). In the absence of a structure of full-length NOS, an integrated approach of spectroscopic (e.g. pulsed EPR, MCD, resonance Raman), rapid kinetics (laser flash photolysis and stopped flow) and mutagenesis methods is critical to unravel the molecular details of the interdomain FMN/heme interactions. This is to investigate the roles of dynamic conformational changes of the FMN domain and the docking between the primary functional FMN and heme domains in regulating NOS activity. The recent developments in understanding of mechanisms of the NOS regulation that are driven by the combined approach are the focuses of this review. An improved understanding of the role of interdomain FMN/heme interaction and CaM binding may serve as the basis for the design of new selective inhibitors of NOS isoforms.

Intraprotein electron transfer between the FMN and heme domains in endothelial nitric oxide synthase holoenzyme

Intraprotein electron transfer (IET) from flavin mononucleotide (FMN) to heme is an essential step in nitric oxide (NO) synthesis by NO synthase (NOS). The IET kinetics in neuronal and inducible NOS (nNOS and iNOS) holoenzymes have been previously determined in our laboratories by laser flash photolysis [reviewed in: C.J. Feng, G. Tollin, Dalton Trans., (2009) 6692-6700]. Here we report the kinetics of the IET in a bovine endothelial NOS (eNOS) holoenzyme in the presence and absence of added calmodulin (CaM). The IET rate constant in the presence of CaM is estimated to be ~4.3s(-1). No IET was observed in the absence of CaM, indicating that CaM is the primary factor in controlling the FMN-heme IET in the eNOS enzyme. The IET rate constant value for the eNOS holoenzyme is approximately 10 times smaller than those obtained for the iNOS and CaM-bound nNOS holoenzymes. Possible mechanisms underlying the difference in IET kinetics among the NOS isoforms are discussed. Because the rate-limiting step in the IET process in these enzymes is the conformational change from input state to output state, a slower conformational change (than in the other isoforms) is most likely to cause the slower IET in eNOS.

Pulsed EPR determination of the distance between heme iron and FMN centers in a human inducible nitric oxide synthase

Mammalian nitric oxide synthase (NOS) is a homodimeric flavo-hemoprotein that catalyzes the oxidation of L-arginine to nitric oxide (NO). Regulation of NO biosynthesis by NOS is primarily through control of interdomain electron transfer (IET) processes in NOS catalysis. The IET from the flavin mononucleotide (FMN) to heme domains is essential in the delivery of electrons required for O(2) activation in the heme domain and the subsequent NO synthesis by NOS. The NOS output state for NO production is an IET-competent complex of the FMN-binding domain and heme domain, and thereby it facilitates the IET from the FMN to the catalytic heme site. The structure of the functional output state has not yet been determined. In the absence of crystal structure data for NOS holoenzyme, it is important to experimentally determine the Fe...FMN distance to provide a key calibration for computational docking studies and for the IET kinetics studies. Here we used the relaxation-induced dipolar modulation enhancement (RIDME) technique to measure the electron spin echo envelope modulation caused by the dipole interactions between paramagnetic FMN and heme iron centers in the [Fe(III)][FMNH(*)] (FMNH(*): FMN semiquinone) form of a human inducible NOS (iNOS) bidomain oxygenase/FMN construct. The FMNH(*)...Fe distance has been directly determined from the RIDME spectrum. This distance (18.8 +/- 0.1 A) is in excellent agreement with the IET rate constant measured by laser flash photolysis [Feng, C. J.; Dupont, A.; Nahm, N.; Spratt, D.; Hazzard, J. T.; Weinberg, J.; Guillemette, J.; Tollin, G.; Ghosh, D. K. J. Biol. Inorg. Chem. 2009, 14, 133-142].

Electron transfer in a human inducible nitric oxide synthase oxygenase/FMN construct co-expressed with the N-terminal globular domain of calmodulin

The FMN-heme intraprotein electron transfer (IET) kinetics in a human inducible NOS (iNOS) oxygenase/FMN (oxyFMN) construct co-expressed with NCaM, a truncated calmodulin (CaM) construct that includes only its N-terminal globular domain consisting of residues 1-75, were determined by laser flash photolysis. The IET rate constant is significantly decreased by nearly fourfold (compared to the iNOS oxyFMN co-expressed with full length CaM). This supports an important role of full length CaM in proper interdomain FMN/heme alignment in iNOS. The IET process was not observed with added excess EDTA, suggesting that Ca(2+) depletion results in the FMN domain moving away from the heme domain. The results indicate that a Ca(2+)-dependent reorganization of the truncated CaM construct could cause a major modification of the NCaM/iNOS association resulting in a loss of the IET.

Nitric oxide functions as a signal and acts upstream of AtCaM3 in thermotolerance in Arabidopsis seedlings

To characterize the role of nitric oxide (NO) in the tolerance of Arabidopsis (Arabidopsis thaliana) to heat shock (HS), we investigated the effects of heat on three types of Arabidopsis seedlings: wild type, noa1(rif1) (for nitric oxide associated1/resistant to inhibition by fosmidomycin1) and nia1nia2 (for nitrate reductase [NR]-defective double mutant), which both exhibit reduced endogenous NO levels, and a rescued line of noa1(rif1). After HS treatment, the survival ratios of the mutant seedlings were lower than those of wild type; however, they were partially restored in the rescued line. Treatment of the seedlings with sodium nitroprusside or S-nitroso-N-acetylpenicillamine revealed that internal NO affects heat sensitivity in a concentration-dependent manner. Calmodulin 3 (CaM3) is a key component of HS signaling in Arabidopsis. Real-time reverse transcription-polymerase chain reaction analysis after HS treatment revealed that the AtCaM3 mRNA level was regulated by the internal NO level. Sodium nitroprusside enhanced the survival of the wild-type and noa1(rif1) seedlings; however, no obvious effects were observed for cam3 single or cam3noa1(rif1) double mutant seedlings, suggesting that AtCaM3 is involved in NO signal transduction as a downstream factor. This point was verified by phenotypic analysis and thermotolerance testing using seedlings of three AtCaM3-overexpressing transgenic lines in an noa1(rif1) background. Electrophoretic mobility-shift and western-blot analyses demonstrated that after HS treatment, NO stimulated the DNA-binding activity of HS transcription factors and the accumulation of heat shock protein 18.2 (HSP18.2) through AtCaM3. These data indicate that NO functions in signaling and acts upstream of AtCaM3 in thermotolerance, which is dependent on increased HS transcription factor DNA-binding activity and HSP accumulation.

A bridging interaction allows calmodulin to activate NO synthase through a bi-modal mechanism

Calmodulin (CaM) activates the nitric-oxide synthases (NOS) by a mechanism that is not completely understood. A recent crystal structure showed that bound CaM engages in a bridging interaction with the NOS FMN subdomain. We investigated its importance in neuronal NOS (nNOS) by mutating the two residues that primarily create the bridging interaction (Arg(752) in the FMN subdomain and Glu(47) in CaM). Mutations designed to completely destroy the bridging interaction prevented bound CaM from increasing electron flux through the FMN subdomain and diminished the FMN-to-heme electron transfer by 90%, whereas mutations that partly preserve the interaction had intermediate effects. The bridging interaction appeared to control FMN subdomain interactions with both its electron donor (NADPH-FAD subdomain) and electron acceptor (heme domain) partner subdomains in nNOS. We conclude that the Arg(752)-Glu(47) bridging interaction is the main feature that enables CaM to activate nNOS. The mechanism is bi-modal and links a single structural aspect of CaM binding to specific changes in nNOS protein conformational and electron transfer properties that are essential for catalysis.

Ultraviolet B light-induced nitric oxide/peroxynitrite imbalance in keratinocytes--implications for apoptosis and necrosis

Elevation of nitric oxide (NO*) can either promote or inhibit ultraviolet B light (UVB)-induced apoptosis. In this study, we determined real-time concentration of NO* and peroxynitrite (ONOO(-)) and their role in regulation of membrane integrity and apoptosis. Nanosensors (diameter 300-500 nm) were used for direct in situ simultaneous measurements of NO* and ONOO(-) generated by UVB in cultured keratinocytes and mice epidermis. An exposure of keratinocytes to UVB immediately generated ONOO(-) at maximal concentration of 190 nm followed by NO(*) release with a maximal concentration of 91 nm. The kinetics of UVB-induced NO*/ONOO(-) was in contrast to cNOS agonist stimulated NO*/ONOO(-) from keratinocytes. After stimulating cNOS by calcium ionophore (CaI), NO* release from keratinocytes was followed by ONOO(-) production. The [NO*] to [ONOO(-)] ratio generated by UVB decreased below 0.5 indicating a serious imbalance between cytoprotective NO* and cytotoxic ONOO(-)-a main component of nitroxidative stress. The NO*/ONOO(-) imbalance increased membrane damage and cell apoptosis was partially reversed in the presence of free radical scavenger. The results suggest that UVB-induced and cNOS-produced NO* is rapidly scavenged by photolytically and enzymatically generated superoxide (O(2) (-)) to produce high levels of ONOO(-), which enhances oxidative injury and apoptosis of the irradiated cells.

Structural and mechanistic aspects of flavoproteins: electron transfer through the nitric oxide synthase flavoprotein domain

Nitric oxide synthases belong to a family of dual-flavin enzymes that transfer electrons from NAD(P)H to a variety of heme protein acceptors. During catalysis, their FMN subdomain plays a central role by acting as both an electron acceptor (receiving electrons from FAD) and an electron donor, and is thought to undergo large conformational movements and engage in two distinct protein-protein interactions in the process. This minireview summarizes what we know about the many factors regulating nitric oxide synthase flavoprotein domain function, primarily from the viewpoint of how they impact electron input/output and conformational behaviors of the FMN subdomain.

Regulation of interdomain electron transfer in the NOS output state for NO production

There is still much that is unknown about how nitric oxide (NO) biosynthesis by NO synthase (NOS) isoform is tightly regulated at the molecular level. This is remarkable because deviated NO production in vivo has been implicated in an increasing number of diseases that currently lack effective treatments, including stroke and cancer. Given the significant public health burden of these diseases, the NOS enzyme family is a key target for development of new pharmaceuticals. Three NOS isoforms, inducible, endothelial and neuronal NOS (iNOS, eNOS and nNOS, respectively), achieve their key biological functions via stringent regulations of interdomain electron transfer (IET) processes. Unlike iNOS, eNOS and nNOS isoforms are controlled by calmodulin (CaM) binding through facilitating catalytically significant IET processes. The CaM-modulated NOS output state is an IET-competent complex between the flavin mononucleotide (FMN) domain and the catalytic heme domain. The output state facilitates the catalytically essential FMN-heme IET, and thereby enables NO production by NOS. Due to lack of reliable techniques for specifically determining the inter-domain FMN-heme interactions and their direct effects on the catalytic heme center, the molecular mechanism that underlies the output state formation remains elusive. The recent developments in our understanding of mechanisms of the NOS output state formation that are driven by a combination of molecular biology, laser flash photolysis, and spectroscopic techniques are the subject of this perspective.

Inhibition of nitric oxide synthase by cobalamins and cobinamides

Cobalamins are important cofactors for methionine synthase and methylmalonyl-CoA mutase. Certain corrins also bind nitric oxide (NO), quenching its bioactivity. To determine if corrins would inhibit NO synthase (NOS), we measured their effects on -L-[(14)C]arginine-to-L-[(14)C]citrulline conversion by NOS1, NOS2, and NOS3. Hydroxocobalamin (OH-Cbl), cobinamide, and dicyanocobinamide (CN(2)-Cbi) potently inhibited all isoforms, whereas cyanocobalamin, methylcobalamin, and adenosylcobalamin had much less effect. OH-Cbl and CN(2)-Cbi prevented binding of the oxygen analog carbon monoxide (CO) to the reduced NOS1 and NOS2 heme active site. CN(2)-Cbi did not react directly with NO or CO. Spectral perturbation analysis showed that CN(2)-Cbi interacted directly with the purified NOS1 oxygenase domain. NOS inhibition by corrins was rapid and not reversed by dialysis with L-arginine or tetrahydrobiopterin. Molecular modeling indicated that corrins could access the unusually large heme- and substrate-binding pocket of NOS. Best fits were obtained in the "base-off" conformation of the lower axial dimethylbenzimidazole ligand. CN(2)-Cbi inhibited interferon-gamma-activated Raw264.7 mouse macrophage NO production. We show for the first time that certain corrins directly inhibit NOS, suggesting that these agents (or their derivatives) may have pharmacological utility. Endogenous cobalamins and cobinamides might play important roles in regulating NOS activity under normal and pathological conditions.

Regulation of FMN subdomain interactions and function in neuronal nitric oxide synthase

Nitric oxide synthases (NOS) are modular, calmodulin- (CaM-) dependent, flavoheme enzymes that catalyze oxidation of l-arginine to generate nitric oxide (NO) and citrulline. During catalysis, the FMN subdomain cycles between interaction with an NADPH-FAD subdomain to receive electrons and interaction with an oxygenase domain to deliver electrons to the NOS heme. This process can be described by a three-state, two-equilibrium model for the conformation of the FMN subdomain, in which it exists in two distinct bound states (FMN-shielded) and one common unbound state (FMN-deshielded). We studied how each partner subdomain, the FMN redox state, and CaM binding may regulate the conformational equilibria of the FMN module in rat neuronal NOS (nNOS). We utilized four nNOS protein constructs of different subdomain composition, including the isolated FMN subdomain, and determined changes in the conformational state by measuring the degree of FMN shielding by fluorescence, electron paramagnetic resonance, or stopped-flow spectroscopic techniques. Our results suggest the following: (i) The NADPH-FAD subdomain has a far greater capacity to interact with the FMN subdomain than does the oxygenase domain. (ii) CaM binding has no direct effects on the FMN subdomain. (iii) CaM destabilizes interaction of the FMN subdomain with the NADPH-FAD subdomain but does not measurably increase its interaction with the oxygenase domain. Our results imply that a different set point and CaM regulation exists for either conformational equilibrium of the FMN subdomain. This helps to explain the unique electron transfer and catalytic behaviors of nNOS, relative to other dual-flavin enzymes.

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.

Intraprotein electron transfer in inducible nitric oxide synthase holoenzyme

Intraprotein electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in NO synthesis by NO synthase (NOS). Our previous laser flash photolysis studies provided a direct determination of the kinetics of the FMN-heme IET in a truncated two-domain construct (oxyFMN) of murine inducible NOS (iNOS), in which only the oxygenase and FMN domains along with the calmodulin (CaM) binding site are present (Feng et al. J. Am. Chem. Soc. 128, 3808-3811, 2006). Here we report the kinetics of the IET in a human iNOS oxyFMN construct, a human iNOS holoenzyme, and a murine iNOS holoenzyme, using CO photolysis in comparative studies on partially reduced NOS and a NOS oxygenase construct that lacks the FMN domain. The IET rate constants for the human and murine iNOS holoenzymes are 34 +/- 5 and 35 +/- 3 s(-1), respectively, thereby providing a direct measurement of this IET between the catalytically significant redox couples of FMN and heme in the iNOS holoenzyme. These values are approximately an order of magnitude smaller than that in the corresponding iNOS oxyFMN construct, suggesting that in the holoenzyme the rate-limiting step in the IET is the conversion of the shielded electron-accepting (input) state to a new electron-donating (output) state. The fact that there is no rapid IET component in the kinetic traces obtained with the iNOS holoenzyme implies that the enzyme remains mainly in the input state. The IET rate constant value for the iNOS holoenzyme is similar to that obtained for a CaM-bound neuronal NOS holoenzyme, suggesting that CaM activation effectively removes the inhibitory effect of the unique autoregulatory insert in neuronal NOS.

Regulation of mammalian nitric oxide synthases by electrostatic interactions in the linker region of calmodulin

Calmodulin (CaM), the ubiquitous Ca(2+)-sensing protein, consists of two globular domains separated by a flexible central linker that properly orients CaM's globular domains to bind and regulate various intracellular proteins, including the nitric oxide synthase (NOS) enzymes. In the present study we determined that the charge and length of the central linker of CaM has an effect on the binding and activation of the NOS isozymes by using a variety of charge CaM mutants (T79D, S81D, T79D/S81D, S101D and E84R/E87K) and CaM mutants with residues removed (Delta84, Delta83-84, and Delta81-84). Our kinetic and spectropolarimetry results demonstrate that the NOS enzymes are not adversely affected by the CaM mutants with the exceptions of S101D, E84R/E87K and the deletion of residue 84. Electrostatic interactions in the central linker between residues 82-87 in combination with hydrophobic interactions in the globular domains of CaM are important for its tight association to inducible NOS.

Calcium-deficient calmodulin binding and activation of neuronal and inducible nitric oxide synthases

The nitric oxide synthase (NOS) enzymes are bound and activated by the Ca(2+)-binding protein, calmodulin (CaM). We have utilized CaM mutants deficient in binding Ca(2+) with mutations in the N-lobe (CaM(12)), the C-lobe (CaM(34)), or both lobes of CaM (CaM(1234)) to determine their effect on the binding and activation of the Ca(2+)-dependent neuronal (nNOS) and Ca(2+)-independent inducible NOS (iNOS) isoforms. Four different kinetic assays were employed to monitor the effect of these CaM mutants on electron transfer rates in NOS. Protein-protein interactions between CaM and NOS were studied using steady-state fluorescence and spectropolarimetry to monitor the binding of these CaM mutants to nNOS and iNOS CaM-binding domain peptides. The CaM mutants were unable to activate nNOS, however, our CD results show that the C-terminal lobe of CaM is capable of binding to nNOS peptide in the presence of Ca(2+). Our results prove for the first time without the use of chelators that apo-CaM is capable of binding to iNOS peptides and holoenzymes.

Role of TGF-beta1 and nitric oxide in the bystander response of irradiated glioma cells

The radiation-induced bystander effect (RIBE) increases the probability of cellular response and therefore has important implications for cancer risk assessment following low-dose irradiation and for the likelihood of secondary cancers after radiotherapy. However, our knowledge of bystander signaling factors, especially those having long half-lives, is still limited. The present study found that, when a fraction of cells within a glioblastoma population were individually irradiated with helium ions from a particle microbeam, the yield of micronuclei (MN) in the nontargeted cells was increased, but these bystander MN were eliminated by treating the cells with either aminoguanidine (an inhibitor of inducible nitric oxide (NO) synthase) or anti-transforming growth factor beta1 (anti-TGF-beta1), indicating that NO and TGF-beta1 are involved in the RIBE. Intracellular NO was detected in the bystander cells, and additional TGF-beta1 was detected in the medium from irradiated T98G cells, but it was diminished by aminoguanidine. Consistent with this, an NO donor, diethylamine nitric oxide (DEANO), induced TGF-beta1 generation in T98G cells. Conversely, treatment of cells with recombinant TGF-beta1 could also induce NO and MN in T98G cells. Treatment of T98G cells with anti-TGF-beta1 inhibited the NO production when only 1% of cells were targeted, but not when 100% of cells were targeted. Our results indicate that, downstream of radiation-induced NO, TGF-beta1 can be released from targeted T98G cells and plays a key role as a signaling factor in the RIBE by further inducing free radicals and DNA damage in the nontargeted bystander cells.

Surface charge interactions of the FMN module govern catalysis by nitric-oxide synthase

The FMN module of nitric-oxide synthase (NOS) plays a pivotal role by transferring NADPH-derived electrons to the enzyme heme for use in oxygen activation. The process may involve a swinging mechanism in which the same face of the FMN module accepts and provides electrons during catalysis. Crystal structure shows that this face of the FMN module is electronegative, whereas the complementary interacting surface is electropositive, implying that charge interactions enable function. We used site-directed mutagenesis to investigate the roles of six electronegative surface residues of the FMN module in electron transfer and catalysis in neuronal NOS. Results are interpreted in light of crystal structures of NOS and related flavoproteins. Neutralizing or reversing the negative charge of each residue altered the NO synthesis, NADPH oxidase, and cytochrome c reductase activities of neuronal NOS and also altered heme reduction. The largest effects occurred at the NOS-specific charged residue Glu(762). Together, the results suggest that electrostatic interactions of the FMN module help to regulate electron transfer and to minimize flavin autoxidation and the generation of reactive oxygen species during NOS catalysis.

Calcium Fluxes Modulate the Radiation-Induced Bystander Responses in Targeted Glioma and Fibroblast Cells

Bystander responses have been reported to be a major determinant of the response of cells to radiation exposure at low doses, including those of relevance to therapy. This study investigated the role of changes in calcium levels in bystander responses leading to chromosomal damage in nonirradiated T98G glioma cells and AG01522 fibroblasts that had been either exposed to conditioned medium from irradiated cells or co-cultured with a population where a fraction of cells were individually targeted through the nucleus or cytoplasm with a precise number of microbeam helium-3 particles. After the recipient cells were treated with conditioned medium from T98G or AG01522 cells that had been irradiated through either nucleus or cytoplasm, rapid calcium fluxes were monitored in the nonirradiated recipient cells. Their characteristics were dependent on the source of the conditioned medium but had no dependence on radiation dose. When recipient cells were co-cultured with an irradiated population of either T98G or AG01522 cells, micronuclei were induced in the nonirradiated cells, but this response was eliminated by treating the cells with calcicludine (CaC), a potent blocker of Ca(2+) channels. Moreover, both the calcium fluxes and the bystander effect were inhibited when the irradiated T98G cells were treated with aminoguanidine, an inhibitor of nitric oxide synthase (NOS), and when the irradiated AG01522 cells were treated with DMSO, a scavenger of reactive oxygen species (ROS), which indicates that NO and ROS were involved in the bystander responses generated from irradiated T98G and AG01522 cells, respectively. Our findings indicate that calcium signaling may be an early response in radiation-induced bystander effects leading to chromosome damage.

Electron Transfer by Neuronal Nitric-oxide Synthase Is Regulated by Concerted Interaction of Calmodulin and Two Intrinsic Regulatory Elements

The nitric-oxide synthases (NOSs) are modular, cofactor-containing enzymes, divided into a heme-containing oxygenase domain and an FMN- and FAD-containing reductase domain. The domains are connected by a calmodulin (CaM)-binding sequence, occupancy of which is required for nitric oxide (NO) production. Two additional CaM-modulated regulatory elements are present in the reductase domains of the constitutive isoforms, the autoregulatory region (AR) and the C-terminal tail region. Deletion of the AR reduces CaM stimulation of electron flow through the reductase domain from 10-fold in wild-type nNOS to 2-fold in the mutant. Deletion of the C terminus yields an enzyme with greatly enhanced reductase activity in the absence of CaM but with activity equivalent to that of wild-type enzyme in its presence. A mutant in which both the AR and C terminus were deleted completely loses CaM modulation through the reductase domain. Thus, transduction of the CaM effect through the reductase domain of nNOS is dependent on these elements. Formation of nitric oxide is, however, still stimulated by CaM in all three mutants. A CaM molecule in which the N-terminal lobe was replaced by the C-terminal lobe (CaM-CC) supported NO synthesis by the deletion mutants but not by wild-type nNOS. We propose a model in which the AR, the C-terminal tail, and CaM interact directly to regulate the conformational state of the reductase domain of nNOS.

Nitric-oxide Synthase Output State

Mammalian nitric-oxide synthases are large modular enzymes that evolved from independently expressed ancestors. Calmodulin-controlled isoforms are signal generators; calmodulin activates electron transfer from NADPH through three reductase domains to an oxygenase domain. Structures of the reductase unit and its homologs show FMN and FAD in contact but too isolated from the protein surface to permit exit of reducing equivalents. To study states in which FMN/heme electron transfer is feasible, we designed and produced constructs including only oxygenase and FMN binding domains, eliminating strong internal reductase complex interactions. Constructs for all mammalian isoforms were expressed and purified as dimers. All synthesize NO with peroxide as the electron donor at rates comparable with corresponding oxygenase constructs. All bind cofactors nearly stoichiometrically and have native catalytic sites by spectroscopic criteria. Modest differences in electrochemistry versus independently expressed heme and FMN binding domains suggest interdomain interactions. These interactions can be convincingly demonstrated via calmodulin-induced shifts in high spin ferriheme EPR spectra and through mutual broadening of heme and FMNH. radical signals in inducible nitric-oxide synthase constructs. Blue neutral FMN semiquinone can be readily observed; potentials of one electron couple (in inducible nitric-oxide synthase oxygenase FMN, FMN oxidized/semiquinone couple = +70 mV, FMN semiquinone/hydroquinone couple = -180 mV, and heme = -180 mV) indicate that FMN is capable of serving as a one electron heme reductant. The construct will serve as the basis for future studies of the output state for NADPH derived reducing equivalents.

Binding and activation of nitric oxide synthase isozymes by calmodulin EF hand pairs

Calmodulin (CaM) is a cytosolic Ca(2+) signal-transducing protein that binds and activates many different cellular enzymes with physiological relevance, including the nitric oxide synthase (NOS) isozymes. CaM consists of two globular domains joined by a central linker; each domain contains an EF hand pair. Four different mutant CaM proteins were used to investigate the role of the two CaM EF hand pairs in the binding and activation of the mammalian inducible NOS (iNOS) and the constitutive NOS (cNOS) enzymes, endothelial NOS (eNOS) and neuronal NOS (nNOS). The role of the CaM EF hand pairs in different aspects of NOS enzymatic function was monitored using three assays that monitor electron transfer within a NOS homodimer. Gel filtration studies were used to determine the effect of Ca(2+) on the dimerization of iNOS when coexpressed with CaM and the mutant CaM proteins. Gel mobility shift assays were performed to determine binding stoichiometries of CaM proteins to synthetic NOS CaM-binding domain peptides. Our results show that the N-terminal EF hand pair of CaM contains important binding and activating elements for iNOS, whereas the N-terminal EF hand pair in conjunction with the central linker region is required for cNOS enzyme binding and activation. The iNOS enzyme must be coexpressed with wild-type CaM in vitro because of its propensity to aggregate when residues of the highly hydrophobic CaM-binding domain are exposed to an aqueous environment. A possible role for iNOS aggregation in vivo is also discussed.

Kinetics of inflammatory response of astrocytes induced by TLR 3 and TLR4 ligation

Toll-like receptors (TLRs) are sentinels of the innate immune system that recognize an array of exogenous and endogenous pathogenic molecules. The ligation of the receptors triggers inflammatory response necessary for pathogen elimination and for the healing process. In the present study we examined inflammatory response of astrocytes elicited by the ligation of TLR3 and TLR4. Astrocytic cultures established from newborn rat brains were exposed to double stranded RNA (dsRNA) and lipopolysaccharide (LPS), the ligands for TLR3 and TLR4, respectively. The expression of cytokine genes was determined by RNase protection assay, and the generation of nitric oxide (NO) was measured by Griess technique. Both ligands upregulated the expression of several cytokines (i.e., IL-1alpha, IL-1beta, IL-6, TNFalpha, GM-CSF, LTbeta, and TGFbeta3) and downregulated the expression of MIF, but have no effect on the expression of IL-2, IL-3, IL-4, IL-5, IL-10, TGFbeta1, TGFbeta2, TNFbeta, and IFNgamma. Although dsRNA upregulated the expression of IFNbeta, LPS did not indicating that the TRIF-dependent branch of TLR4 signaling is inactive in astrocytes. Proinflammatory response as seen from upregulated cytokine expression and NO generation reached a peak within the first day of exposure, and was subsequently abrogated. The cells also became refractory to subsequent stimulation by the ligands indicating the existence of negative feedback mechanisms that control proinflammatory response in astrocytes.

C-terminal tail residue Arg1400 enables NADPH to regulate electron transfer in neuronal nitric-oxide synthase

The neuronal nitric-oxide synthase (nNOS) flavoprotein domain (nNOSr) contains regulatory elements that repress its electron flux in the absence of bound calmodulin (CaM). The repression also requires bound NADP(H), but the mechanism is unclear. The crystal structure of a CaM-free nNOSr revealed an ionic interaction between Arg(1400) in the C-terminal tail regulatory element and the 2'-phosphate group of bound NADP(H). We tested the role of this interaction by substituting Ser and Glu for Arg(1400) in nNOSr and in the full-length nNOS enzyme. The CaM-free nNOSr mutants had cytochrome c reductase activities that were less repressed than in wild-type, and this effect could be mimicked in wild-type by using NADH instead of NADPH. The nNOSr mutants also had faster flavin reduction rates, greater apparent K(m) for NADPH, and greater rates of flavin auto-oxidation. Single-turnover cytochrome c reduction data linked these properties to an inability of NADP(H) to cause shielding of the FMN module in the CaM-free nNOSr mutants. The full-length nNOS mutants had no NO synthesis in the CaM-free state and had lower steady-state NO synthesis activities in the CaM-bound state compared with wild-type. However, the mutants had faster rates of ferric heme reduction and ferrous heme-NO complex formation. Slowing down heme reduction in R1400E nNOS with CaM analogues brought its NO synthesis activity back up to normal level. Our studies indicate that the Arg(1400)-2'-phosphate interaction is a means by which bound NADP(H) represses electron transfer into and out of CaM-free nNOSr. This interaction enables the C-terminal tail to regulate a conformational equilibrium of the FMN module that controls its electron transfer reactions in both the CaM-free and CaM-bound forms of nNOS.

The function of the small insertion in the hinge subdomain in the control of constitutive mammalian nitric-oxide synthases

Control of nitric oxide (NO) synthesis in the constitutive nitric-oxide synthases (NOS) by calcium/calmodulin is exerted through the regulation of electron transfer from NADPH through the reductase domains. This process has been shown previously to involve the calmodulin binding site, the autoinhibitory insertion in the FMN binding domain, and the C-terminal tail. Smaller sequence elements also appear to correlate with control. Although some of these elements appear well positioned to function in control, they are poorly conserved; their role in control is neither well established nor defined by available information. In this study mutations have been induced in the small insertion of the hinge subdomain, which has been shown recently to form a beta hairpin in structural studies of the neuronal NOS reductase domains adjacent to the calmodulin site and the autoinhibitory element. Modification of the small insertion in neuronal NOS tends to increase cytochrome c reduction but not NO synthetic activity; some modifications or deletions in the corresponding region in endothelial NOS modestly increase activity under some conditions. Unexpectedly, some minor changes in the sequence introduce a loss in the content of heme relative to flavin cofactors. Taken together, these results suggest that the small insertion protects the calmodulin binding site and that it may be a modulator of NOS activity.