Single-turnover of nitric-oxide synthase in the presence of 4-amino-tetrahydrobiopterin: proposed role for tetrahydrobiopterin as a proton donor

Cigarette Smoke-Induced Reactive Oxygen Species Formation: A Concise Review

Smoking is recognized as a significant risk factor for numerous disorders, including cardiovascular diseases, respiratory conditions, and various forms of cancer. While the exact pathogenic mechanisms continue to be explored, the induction of oxidative stress via the production of excess reactive oxygen species (ROS) is widely accepted as a primary molecular event that predisposes individuals to these smoking-related ailments. This review focused on how cigarette smoke (CS) promotes ROS formation rather than the pathophysiological repercussions of ROS and oxidative stress. A comprehensive analysis of existing studies revealed the following key ways through which CS imposes ROS burden on biological systems: (1) ROS, as well as radicals, are intrinsically present in CS, (2) CS constituents generate ROS through chemical reactions with biomolecules, (3) CS stimulates cellular ROS sources to enhance production, and (4) CS disrupts the antioxidant system, aggravating the ROS generation and its functions. While the evidence supporting these mechanisms is chiefly based on in vitro and animal studies, the direct clinical relevance remains to be fully elucidated. Nevertheless, this understanding is fundamental for deciphering molecular events leading to oxidative stress and for developing intervention strategies to counter CS-induced oxidative stress.

Theoretical studies of the second step of the nitric oxide synthase reaction: Electron tunneling prevents uncoupling

Nitric oxide (NO·) is a messenger molecule with diverse physiological roles including host defense, neurotransmission and vascular function. The synthesis of NO· from l-arginine is catalyzed by NO-synthases and occurs in two steps through the intermediary Nω-hydroxy-l-arginine (NHA). In both steps the P450-like reaction cycle is coupled with the redox cycle of the cofactor tetrahydrobiopterin (H4B). The mechanism of the second step is studied by Density Functional Theory calculations to ascertain the canonical sequence of proton and electron transfer (PT and ET) events. The proposed mechanism is controlled by the interplay of two electron donors, H4B and NHA. Consistent with experimental data, the catalytic cycle proceeds through the ferric-hydroperoxide complex (Cpd 0) and the following aqua-ferriheme resting state, and involves interim partial oxidation of H4B. The mechanism starts with formation of Cpd 0 from the ferrous-dioxy reactant complex by PT from the C-ring heme propionate coupled with hole transfer to H4B through the highest occupied π-orbital of NHA as a bridge. This enables PT from NHA+· to the proximal oxygen leading to the shallow ferriheme-H2O2 oxidant. Subsequent Fenton-like peroxide bond cleavage triggered by ET from the NHA-derived iminoxy-radical leads to the protonated Cpd II diradicaloid singlet stabilized by spin delocalization in H4B, and the closed-shell coordination complex of HO- with iminoxy-cation. The complex is converted to the transient C-adduct, which releases intended products upon PT to the ferriheme-HO- complex coupled with ET to the H4B+·. Deferred ET from the substrate or undue ET from/to the cofactor leads to side products.

Revisiting the Val/Ile Mutation in Mammalian and Bacterial Nitric Oxide Synthases: A Spectroscopic and Kinetic Study

Nitric oxide is produced in mammals by the nitric oxide synthase (NOS) isoforms at a catalytic site comprising a heme associated with a biopterin cofactor. Through genome sequencing, proteins that are highly homologous to the oxygenase domain of NOSs have been identified, in particular in bacteria. The active site is highly conserved except for a valine residue in the distal pocket that is replaced with an isoleucine in bacteria. This switch was previously reported to influence the kinetics of the reaction. We have used the V346I mutant of the mouse inducible NOS (iNOS) as well as the I224V mutant of the NOS from Bacillus subtilis (bsNOS) to study their spectroscopic signatures in solution and look for potential structural differences compared to their respective wild types. Both mutants seem destabilized in the absence of substrate and cofactor. When both substrate and cofactor are present, small differences can be detected with Nω-hydroxy-l-arginine compared to arginine, which is likely due to the differences in the hydrogen bonding network of the distal pocket. Stopped-flow experiments evidence significant changes in the kinetics of the reaction due to the mutation as was already known. We found these effects particularly marked for iNOS. On the basis of these results, we performed rapid freeze-quench experiments to trap the biopterin radical and found the same results that we had obtained for the wild types. Despite differences in kinetics, a radical could be trapped in both steps for the iNOS mutant but only for the first step in the mutant of bsNOS. This strengthens the hypothesis that mammalian and bacterial NOSs may have a different mechanism during the second catalytic step.

Neuronal NOS localises to human airway cilia

BACKGROUND

Airway NO synthase (NOS) isoenzymes are responsible for rapid and localised nitric oxide (NO) production and are expressed in airway epithelium. We sought to determine the localisation of neuronal NOS (nNOS) in airway epithelium due to the paucity of evidence.

METHODS AND RESULTS

Sections of healthy human bronchial tissue in glycol methacrylate resin and human nasal polyps in paraffin wax were immunohistochemically labelled and reproducibly demonstrated nNOS immunoreactivity, particularly at the proximal portion of cilia; this immunoreactivity was blocked by a specific nNOS peptide fragment. Healthy human epithelial cells differentiated at an air-liquid interface (ALI) confirmed the presence of all three NOS isoenzymes by immunofluorescence labelling. Only nNOS immunoreactivity was specific to the ciliary axonemeand co-localised with the cilia marker β-tubulin in the proximal part of the ciliary axoneme.

CONCLUSIONS

We report a novel localisation of nNOS at the proximal portion of cilia in airway epithelium and conclude that its independent and local regulation of NO levels is crucial for normal cilia function.

Comparative pharmacophore modeling and QSAR studies for structural requirements of some substituted 2-aminopyridines derivatives as inhibitors nitric oxide synthases

The present studies are an attempt in this direction seeking for the development and comparison of QSAR models of substituted 2-aminopyridines derivatives as inhibitors of nitric oxide synthases by different feature selection methods. Comparing the two different feature selection methods, it is implicit that the model built with the selected variables by simulated annealing (SA) method gives better prediction in case of 2D and 3D QSAR modeling. The QSAR study was carried out on V-life Molecular Design Suite software and the derived best QSAR model was derived by partial component regression (PCR) method. The statistically significant best model with high correlation coefficient (r2 = 0.8408) was selected for further study. The model was further validated by means of crossed squared correlation coefficient (q2 = 0.7270 and pred r2 = 0.7889) which shows model has good predictive ability. 3D-QSAR analysis has been performed on a series of substituted 2-aminopyridines derivatives as which were screened as inhibitors of nitric oxide synthases, using the simulated annealing and step wise k-nearest neighbour Molecular Field Analysis. The best QSAR model showed q2 = 0.8377, r2 = 0.8739 and standard error = 0.1954. It was observed that steric properties predicted by k-nearest neighbour MFA contours can be related to inhibitors of nitric oxide synthases. The predictive ability of the resultant model was evaluated using a test set molecules and the predicted r2 = 0.8159. The distances between the pharmacophore sites were measured in order to confirm their significance to the activities. The results reveal that the acceptor (acc), donor (don), aliphatic and aromatic pharmacophore properties are favorable contours sites for both the activities. The two dimensional and k-nearest neighbour contour plots required for further understanding of the relationship between structural features of substituted 2-aminopyridines derivatives and their activities which should be applicable to design newer potential inducible nitric oxide synthases.

Comparison of oxygen-induced radical intermediates in iNOS oxygenase domain with those from nNOS and eNOS

Inducible nitric-oxide synthase (iNOS) produces the reactive oxygen and nitrogen species (ROS/RNS) involved in bacteria killing and is crucial in the host defense mechanism. However, high level ROS/RNS can also be detrimental to normal cells and thus their production has to be tightly controlled. Availability or deficiency of tetrahydrobiopterin (BH4) cofactor and l-arginine substrate controls coupling or uncoupling of NOS catalysis. Fully coupled reaction, with abundant BH4 and l-arginine, produces NO whereas the uncoupled NOS (in the absence of BH4 and/or l-arginine) generates ROS/RNS. In the current work we focus on direct rapid freeze EPR to characterize the structure and kinetics of oxygen-induced radical intermediates produced by ferrous inducible NOS oxygenase domain (iNOSox) in the presence or absence of BH4 and/or l-arginine. Fully reconstituted iNOSox (+BH4, +L-Arg) forms a dimer and yields a typical BH4 radical that indicates coupled reaction. iNOSox (-BH4) remains mainly monomeric and produces exclusively superoxide, that is only marginally affected by the presence of l-arginine. iNOSox (+BH4, -L-Arg) exists as a monomer/dimer mixture and yields both BH4 radical and superoxide. Present study is a natural extension of our previous work on the ferrous endothelial NOSox (eNOSox) [V. Berka, G. Wu, H.C. Yeh, G. Palmer, A.L. Tsai, J. Biol. Chem. 279 (2004) 32243-32251] and ferrous neuronal NOSox (nNOSox) [V. Berka, L.H. Wang, A.L. Tsai, Biochemistry 47 (2008) 405-420]. Overall, our data suggests different regulatory roles of l-arginine and BH4 in the production of oxygen-induced radical intermediates in NOS isoforms which nicely serve individual functional role.

Tetrahydrobiopterin in nitric oxide synthase

SUMMARY

Nitric oxide synthase (NOS) is a critical enzyme for the production of the messenger molecule nitric oxide (NO) from L-arginine. NOS enzymes require tetrahydrobiopterin as a cofactor for NO synthesis. Besides being one of the few enzymes to use this cofactor, the role of tetrahydrobiopterin in NOS catalytic mechanism is different from other enzymes: during the catalytic cycle of NOS, tetrahydrobiopterin forms a radical species that is again reduced, thus effectively regenerating after each NO synthesis cycle. In this review, we summarize our current knowledge about the role of tetrahydrobiopterin in the structure, function, and catalytic mechanism of NOS enzymes.

Electron paramagnetic resonance characterization of tetrahydrobiopterin radical formation in bacterial nitric oxide synthase compared to mammalian nitric oxide synthase

H(4)B is an essential catalytic cofactor of the mNOSs. It acts as an electron donor and activates the ferrous heme-oxygen complex intermediate during Arg oxidation (first step) and NOHA oxidation (second step) leading to nitric oxide and citrulline as final products. However, its role as a proton donor is still debated. Furthermore, its exact involvement has never been explored for other NOSs such as NOS-like proteins from bacteria. This article proposes a comparative study of the role of H(4)B between iNOS and bsNOS. In this work, we have used freeze-quench to stop the arginine and NOHA oxidation reactions and trap reaction intermediates. We have characterized these intermediates using multifrequency electron paramagnetic resonance. For the first time, to our knowledge, we report a radical formation for a nonmammalian NOS. The results indicate that bsNOS, like iNOS, has the capacity to generate a pterin radical during Arg oxidation. Our current electron paramagnetic resonance data suggest that this radical is protonated indicating that H(4)B may not transfer any proton. In the 2nd step, the radical trapped for iNOS is also suggested to be protonated as in the 1st step, whereas it was not possible to trap a radical for the bsNOS 2nd step. Our data highlight potential differences for the catalytic mechanism of NOHA oxidation between mammalian and bacterial NOSs.

Arg375 tunes tetrahydrobiopterin functions and modulates catalysis by inducible nitric oxide synthase

NO synthase enzymes (NOS) support unique single-electron transitions of a bound H(4)B cofactor during catalysis. Previous studies showed that both the pterin structure and surrounding protein residues impact H(4)B redox function during catalysis. A conserved Arg residue (Arg375 in iNOS) forms hydrogen bonds with the H(4)B ring. In order to understand the role of this residue in modulating the function of H(4)B and overall NO synthesis of the enzyme, we generated and characterized three mutants R375D, R375K and R375N of the oxygenase domain of inducible NOS (iNOSoxy). The mutations affected the dimer stability of iNOSoxy and its binding affinity toward substrates and H(4)B to varying degrees. Optical spectra of the ferric, ferrous, ferrous dioxy, ferrous-NO, ferric-NO, and ferrous-CO forms of each mutant were similar to the wild-type. However, mutants displayed somewhat lower heme midpoint potentials and faster ferrous heme-NO complex reactivity with O(2). Unlike the wild-type protein, mutants could not oxidize NOHA to nitrite in a H(2)O(2)-driven reaction. Mutation could potentially change the ferrous dioxy decay rate, H(4)B radical formation rate, and the amount of the Arg hydroxylation during single turnover Arg hydroxylation reaction. All mutants were able to form heterodimers with the iNOS G450A full-length protein and displayed lower NO synthesis activities and uncoupled NADPH consumption. We conclude that the conserved residue Arg375 (1) regulates the tempo and extent of the electron transfer between H(4)B and ferrous dioxy species and (2) controls the reactivity of the heme-based oxidant formed after electron transfer from H(4)B during steady state NO synthesis and H(2)O(2)-driven NOHA oxidation. Thus, Arg375 modulates the redox function of H(4)B and is important in controlling the catalytic function of NOS enzymes.

Reactions between iron porphyrins and tetrahydropterins

Data from the last few years have revealed a novel biological role of the tetrahydrobiopterin ( H 4 B ) cofactor, in one-electron transfers to the heme of the active site of NO-synthases (NOSs) with intermediate formation of a H 4 B -derived radical. These electron transfers play a key role in the catalytic cycles of the two steps catalyzed by NOS, the N ω-hydroxylation of L-arginine, and the three-electron oxidation of N ω-hydroxyarginine to L-citrulline and NO. Recent experiments performed between various tetrahydropterins and iron porphyrins have shown that the one-electron transfer from tetrahydropterins, such as the natural cofactors H 4 B and tetrahydrofolate or the synthetic 6,7-dimethyltetrahydropterin (diMeH4P), to Fe(III) porphyrins of sufficiently high redox potentials (> about -100 mV versus NHE for the Fe(III)/Fe(II) couple) is a very general reaction that occurs with formation of a tetrahydropterin-derived radical. Reaction of diMeH4P with a stable porphyrin Fe(II)-O 2 complex leads to a diMeH4P-derived radical and a transient Fe(III)-OOH complex, mimicking the reaction between H 4 B and heme Fe(II)-O 2 in the NOS catalytic cycle. Tetrahydropterins such as diMeH4P also reduce hemeproteins Fe(III) of sufficiently high redox potentials, such as cytochromes c and b5 or metmyoglobin, to the corresponding hemeproteins Fe(II) .

The molecular mechanism of mammalian NO-synthases: a story of electrons and protons

Since its discovery, nitric oxide synthase (NOS), the enzyme responsible for NO biosynthesis in mammals, has been the subject of extensive investigations regarding its catalytic and molecular mechanisms. These studies reveal the high degree of sophistication of NOS functioning and regulation. However, the precise description of the NOS molecular mechanism and in particular of the oxygen activation chemistry is still lacking. The reaction intermediates implicated in NOS catalysis continue to elude identification and the current working paradigm is increasingly contested. Consequently, the last three years has seen the emergence of several competing models. All these models propose the same global reaction scheme consisting of two successive oxidation reactions but they diverge in the details of their reaction sequence. The major discrepancies concern the number, source and characteristics of proton and electron transfer processes. As a result each model proposes distinct reaction pathways with different implied oxidative species. This review aims to examine the different experimental evidence concerning NOS proton and electron transfer events and the role played by the substrates and cofactors in these processes. The resulting discussion should provide a comparative picture of all potential models for the NOS molecular mechanism.

Nitric oxide synthase stabilizes the tetrahydrobiopterin cofactor radical by controlling its protonation state

Nitric oxide synthase (NOS), a homodimeric enzyme with a flavin reductase domain and a P450-type heme-containing oxygenase domain, catalyzes the formation of NO from L-arginine, NADPH, and O(2) in a two-step reaction sequence. In the first step, a tetrahydrobiopterin (H(4)B) cofactor bound near one of the heme propionate groups acts as an electron donor to the P450-type heme active site, yielding a one-electron oxidized radical that is subsequently re-reduced. In solution, H(4)B undergoes two-electron oxidation, showing that the enzyme significantly alters the proton- and electron-transfer properties of the cofactor. Multifrequency EPR and ENDOR spectroscopy were used to determine magnetic parameters, and from them the (de)protonation state of the H(4)B radical in the oxygenase domain dimer of inducible NO synthase that was trapped by rapid freeze quench. From 9.5 and 330-416 GHz EPR and from 34 GHz (1)H ENDOR spectroscopy, the g tensor of the radical and the hyperfine tensors of several N and H nuclei in the radical were obtained. Density functional theory calculations at the PBE0/EPR-II level for H(4)B radical models predict different spin density distributions and g and hyperfine tensors for different protonation states. Comparison of the predicted and experimental values leads to the conclusion that the radical is cationic H(4)B(*+), suggesting that NOS stabilizes this protonated form to utilize the cofactor in a unique dual one-electron redox role, where it can deliver an electron to the active site for reductive oxygen activation and also remove an electron from the active site to generate NO and not NO(-). The protein environment also prevents further oxidation and subsequent loss of function of the cofactor, thus enabling the enzyme to perform the unusual catalytic one-electron chemistry.

NO synthase: structures and mechanisms

Production of NO from arginine and molecular oxygen is a complex chemical reaction unique to biology. Our understanding of the chemical and regulation mechanisms of the NO synthases has developed over the past two decades, uncovering some extraordinary features. This article reviews recent progress and highlights current issues and controversies. The structure of the enzyme has now been determined almost in entirety, although it is as a selection of fragments, which are difficult to assemble unambiguously. NO synthesis is driven by electron transfer through FAD and FMN cofactors, which is controlled by calmodulin binding in the constitutive mammalian enzymes. Many of the unique structural features involved have been characterised, but the mechanics of calmodulin-dependent activation are largely unresolved. Ultimately, NO is produced in the active site by the reaction of arginine with activated heme-bound oxygen in two distinct cycles. The unique role of the tetrahydrobiopterin cofactor as an electron donor in this process has now been established, but the subsequent chemical events are currently a matter of intense speculation and debate.

EPR and ENDOR characterization of the reactive intermediates in the generation of NO by cryoreduced oxy-nitric oxide synthase from Geobacillus stearothermophilus

Cryoreduction EPR/ENDOR/step-annealing measurements with substrate complexes of oxy-gsNOS (3; gsNOS is nitric oxide synthase from Geobacillus stearothermophilus) confirm that Compound I (6) is the reactive heme species that carries out the gsNOS-catalyzed (Stage I) oxidation of L-arginine to N-hydroxy-L-arginine (NOHA), whereas the active species in the (Stage II) oxidation of NOHA to citrulline and HNO/NO(-) is the hydroperoxy-ferric form (5). When 3 is reduced by tetrahydrobiopterin (BH4), instead of an externally supplied electron, the resulting BH4(+) radical oxidizes HNO/NO(-) to NO. In this report, radiolytic one-electron reduction of 3 and its complexes with Arg, Me-Arg, and NO(2)Arg was shown by EPR and (1)H and (14,15)N ENDOR spectroscopies to generate 5; in contrast, during cryoreduction of 3/NOHA, the peroxo-ferric-gsNOS intermediate (4/NOHA) was trapped. During annealing at 145 K, ENDOR shows that 5/Arg and 5/Me-Arg (but not 5/NO(2)Arg) generate a Stage I primary product species in which the OH group of the hydroxylated substrate is coordinated to Fe(III), characteristic of 6 as the active heme center. Analysis shows that hydroxylation of Arg and Me-Arg is quantitative. Annealing of 4/NOHA at 160 K converts it first to 5/NOHA and then to the Stage II primary enzymatic product. The latter contains Fe(III) coordinated by water, characteristic of 5 as the active heme center. It further contains quantitative amounts of citrulline and HNO/NO(-); the latter reacts with the ferriheme to form the NO-ferroheme upon further annealing. Stage I delivery of the first proton of catalysis to the (unobserved) 4 formed by cryoreduction of 3 involves a bound water that may convey a proton from L-Arg, while the second proton likely derives from the carboxyl side chain of Glu 248 or the heme carboxylates; the process also involves proton delivery by water(s). In the Stage II oxidation of NOHA, the proton that converts 4/NOHA to 5/NOHA likely is derived from NOHA itself, a conclusion supported by the pH invariance of the process. The present results illustrate how the substrate itself modulates the nature and reactivity of intermediates along the monooxygenase reaction pathway.

Comparative computational analysis of active and inactive cofactors of nitric oxide synthase

Nitric oxide synthases (NOSs) are heme proteins that catalyze the formation of nitric oxide from L-Arg in the presence of oxygen. Of the two electrons required for the first step of the reaction, the second is primarily donated by the tetrahydrobiopterin (H4B) cofactor bound adjacent to the heme, which is eventually reduced back to resting state by the ultimate electron source of the reaction, the flavins of the NOS reductase domain. Density functional theory calculations were carried out to identify those protonation states of different cofactor molecules that best support radicalization of the cofactor and the coupled increase in the electron density of the heme-bound oxygen molecule. Three cofactor molecules were studied, native H4B, an active analogue, 5-methyl-H4B, and the inactive 4-amino-H4B. Findings support the emerging model where H4B and 5-methyl-H4B are coupled proton/electron sources of NOS catalysis, while 4-amino-H4B is an inhibitor due to its inability to donate the catalytically required proton.

The solution electrochemistry of tetrahydrobiopterin revisited

Re-investigation of the electrochemical behavior of the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin on graphite electrodes has revealed drastic differences in reversibility of electron transfer (ET) depending on the type of electrode surface employed. In particular, slow electron transfer kinetics and quasireversibility on an unpolished glassy carbon electrode can mask underlying concerted two-electron transfer chemistry and cause the appearance of an apparent one-electron couple. Nonetheless, the thermodynamic instability of the radical intermediate prevents any detectable build-up of this intermediate under any conditions tested. Scan rate and pH-dependencies of the concerted two-electron couple indicate a kinetic barrier to formation of the radical that depends on proton availability. These observations resolve previous conflicting interpretations of tetrahydrobiopterin solution electrochemistry and comment on how NOS may stabilize the one-electron oxidized radical state that participates in enzymatic production of nitric oxide.

Stabilization and characterization of a heme-oxy reaction intermediate in inducible nitric-oxide synthase

Nitric-oxide synthases (NOS) are heme-thiolate enzymes that N-hydroxylate L-arginine (L-Arg) to make NO. NOS contain a unique Trp residue whose side chain stacks with the heme and hydrogen bonds with the heme thiolate. To understand its importance we substituted His for Trp188 in the inducible NOS oxygenase domain (iNOSoxy) and characterized enzyme spectral, thermodynamic, structural, kinetic, and catalytic properties. The W188H mutation had relatively small effects on l-Arg binding and on enzyme heme-CO and heme-NO absorbance spectra, but increased the heme midpoint potential by 88 mV relative to wild-type iNOSoxy, indicating it decreased heme-thiolate electronegativity. The protein crystal structure showed that the His188 imidazole still stacked with the heme and was positioned to hydrogen bond with the heme thiolate. Analysis of a single turnover L-Arg hydroxylation reaction revealed that a new heme species formed during the reaction. Its build up coincided kinetically with the disappearance of the enzyme heme-dioxy species and with the formation of a tetrahydrobiopterin (H4B) radical in the enzyme, whereas its subsequent disappearance coincided with the rate of l-Arg hydroxylation and formation of ferric enzyme. We conclude: (i) W188H iNOSoxy stabilizes a heme-oxy species that forms upon reduction of the heme-dioxy species by H4B. (ii) The W188H mutation hinders either the processing or reactivity of the heme-oxy species and makes these steps become rate-limiting for l-Arg hydroxylation. Thus, the conserved Trp residue in NOS may facilitate formation and/or reactivity of the ultimate hydroxylating species by tuning heme-thiolate electronegativity.

Profound biopterin oxidation and protein tyrosine nitration in tissues of ApoE-null mice on an atherogenic diet: contribution of inducible nitric oxide synthase

Diminished nitric oxide (NO) bioactivity and enhanced peroxynitrite formation have been implicated as major contributors to atherosclerotic vascular dysfunctions. Hallmark reactions of peroxynitrite include the accumulation of 3-nitrotyrosine (3-NT) in proteins and oxidation of the NO synthase (NOS) cofactor, tetrahydrobiopterin (BH(4)). The present study sought to 1) quantify the extent to which 3-NT accumulates and BH(4) becomes oxidized in organs of apolipoprotein E-deficient (ApoE(-/-)) atherosclerotic mice and 2) determine the specific contribution of inducible NOS (iNOS) to these processes. Whereas protein 3-NT and oxidized BH(4) were undetected or near the detection limit in heart, lung, and kidney of 3-wk-old ApoE(-/-) mice or ApoE(-/-) mice fed a regular chow diet for 24 wk, robust accumulation was evident after 24 wk on a Western (atherogenic) diet. Since 3-NT accumulation was diminished 3- to 20-fold in heart, lung, and liver in ApoE(-/-) mice missing iNOS, iNOS-derived species are involved in this reaction. In contrast, iNOS-derived species did not contribute to elevated protein 3-NT formation in kidney or brain. iNOS deletion also afforded marked protection against BH(4) oxidation in heart, lung, and kidney of atherogenic ApoE(-/-) mice but not in brain or liver. These findings demonstrate that iNOS-derived species are increased during atherogenesis in ApoE(-/-) mice and that these species differentially contribute to protein 3-NT accumulation and BH(4) oxidation in a tissue-selective manner. Since BH(4) oxidation can switch the predominant NOS product from NO to superoxide, we predict that progressive NOS uncoupling is likely to drive atherogenic vascular dysfunctions.

Insights into ligand selectivity in nitric oxide synthase isoforms: A molecular dynamics study

Molecular dynamics (MD) simulations were carried out for inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) isoforms complexed with substrate (L-arginine) and the iNOS specific inhibitor GW 273629, 2 for a time period of 1.2ns. The simulations were compared both within and across the isoforms. iNOS specificity of inhibitor 2 is attributed to water mediated interactions and cooperative hydrogen bond networks. Juxtaposition of the carboxylic and ammonium groups in the substrate and inhibitor serve as a modulating key in binding to the isoforms. Based on these investigations, molecules 3 and 4 were rationally designed to attain specificity among the isoforms. The capability of the designed ligands was theoretically tested through MD simulations to envisage binding patterns with both isoforms. A detailed analysis of the molecular recognition pattern shows molecule 4 to be more selective to iNOS when compared to eNOS.

Substrate-specific Interactions with the Heme-bound Oxygen Molecule of Nitric-oxide Synthase

We report the characterization by resonance Raman spectroscopy of the oxygenated complex (Fe(II)O(2)) of nitric-oxide synthases of Staphylococcus aureus (saNOS) and Bacillus subtilis (bsNOS) saturated with N(omega)-hydroxy-l-arginine. The frequencies of the nu(Fe-O) and nu(O-O) modes were 530 and 1135 cm(-), respectively, in both the presence and absence of tetrahydrobiopterin. On the basis of a comparison of these frequencies with those of saNOS and bsNOS saturated with l-arginine (nu(Fe-O) at 517 cm(-1) and nu(O-O) at 1123 cm(-1)) and those of substrate-free saNOS (nu(Fe-O) at 517 and nu(O-O) at 1135 cm(-1)) (Chartier, F. J. M., Blais, S. P., and Couture, M. (2006) J. Biol. Chem. 281, 9953-9962), we propose two models that account for the frequency shift of nu(Fe-O) (but not nu(O-O)) upon N(omega)-hydroxy-l-arginine binding as well as the frequency shift of nu(O-O) (but not nu(Fe-O)) upon l-arginine binding. The implications of these substrate-specific interactions with respect to catalysis by NOSs are discussed.

Substrate- and Isoform-Specific Dioxygen Complexes of Nitric Oxide Synthase

Nitric oxide synthase (NOS) catalyzes the formation of NO via a consecutive two-step reaction. In the first step, L-arginine (Arg) is converted to N-hydroxy-L-arginine (NOHA). In the second step, NOHA is further converted to citrulline and nitric oxide (NO). To assess the mechanistic differences between the two steps of the reaction, we have used resonance Raman spectroscopy combined with a homemade continuous-flow rapid solution mixer to study the structural properties of the metastable dioxygen-bound complexes of the oxygenase domain of inducible NOS (iNOSoxy). We identified the O-O stretching frequency of the substrate-free enzyme at 1133 cm-1. This frequency is insensitive to the presence of tetrahydrobiopterin, but it shifts to 1126 cm-1 upon binding of Arg, which we attribute to H-bonding interactions to the terminal oxygen atom of the heme iron-bound dioxygen. In contrast, the addition of NOHA to the enzyme did not bring about a shift in the frequency of the O-O stretching mode, because, unlike Arg, there is no H-bond associated with the terminal oxygen atom of the dioxygen. The substrate-specific H-bonding interactions play a critical role in determining the fate of the key peroxy intermediate. In the first step of the reaction, the H-bonds facilitate the rupture of the O-O bond, leading to the formation of the active ferryl species, which is essential for the oxidation of the Arg. On the other hand, in the second step of the reaction, the absence of the H-bonds prevents the premature O-O bond cleavage, such that the peroxy intermediate can perform a nucleophilic addition reaction to the substrate, NOHA.

Great adaptability of the heme-cysteinate monooxygenases family to very diverse substrates and sophisticated reactions

Heme-cysteinate proteins, such as cytochromes P 450( CYPs ) and nitric oxide synthases (NOSs), catalyze the monooxygenation of a huge number of substrates with very diverse structures. The ability of CYPs to oxidize a myriad of xenobiotics, in order to facilitate their elimination, plays a key role in the adaptation of aerobic organisms to their always changing chemical environment. Moreover, some members of the CYP superfamily and the NOSs are involved in the biosynthesis of key biological endogenous molecules, such as estrogens or NO, through the catalysis of highly sophisticated and regulated reactions. How can proteins using the same catalytic heme-cysteinate cofactor and mechanism of dioxygen activation oxidize such diverse and always changing substrates and catalyze different, sometimes very sophisticated reactions? Recent data on the first X-ray structures of mammalian cytochrome P 450-substrate complexes and on the mechanism of NO-synthases has permitted an understanding of this"double adaptation" of heme-cysteinate monooxygenases towards very diverse substrates and different reactions. These data show that cytochromes P 450 involved in the metabolism of xenobiotics are able to oxidize very different substrates by offering a great choice of very diverse and malleable active sites. They also show that heme-cysteinate monooxygenases are able to catalyze special, sophisticated reactions, such as the selective oxidation of L-arginine to NO, by using supplementary cofactors adapted for the required catalysis.

Nitric-oxide synthase: A cytochrome P450 family foster child

Nitric-oxide synthase (NOS), the enzyme responsible for mammalian NO generation, is no cytochrome P450, but there are striking similarities between both enzymes. First and foremost, both are heme-thiolate proteins, employing the same prosthetic group to perform similar chemistry. Moreover, they share the same redox partner, a diflavoprotein reductase, which in the case of NOS is incorporated with the oxygenase in one polypeptide chain. There are, however, also conspicuous differences, such as the presence in NOS of the additional cofactor tetrahydrobiopterin, which is applied as an auxiliary electron donor to prevent decay of the oxyferrous complex to ferric heme and superoxide. In this review similarities and differences between NOS and cytochrome P450 are analyzed in an attempt to explain why NOS requires BH4 and why NO synthesis is not catalyzed by a member of the cytochrome P450 family.

Interactions between substrates and the haem-bound nitric oxide of ferric and ferrous bacterial nitric oxide synthases

We report here the resonance Raman spectra of the FeIII-NO and FeII-NO complexes of the bacterial NOSs (nitric oxide synthases) from Staphylococcus aureus and Bacillus subtilis. The haem-NO complexes of these bacterial NOSs displayed Fe-N-O frequencies similar to those of the mammalian NOSs, in presence and absence of L-arginine, indicating that haem-bound NO and L-arginine had similar haem environments in bacterial and mammalian NOSs. The only notable difference between the two types of NOS was the lack of change in Fe-N-O frequencies of the FeIII-NO complexes upon (6R) 5,6,7,8-tetrahydro-L-biopterin binding to bacterial NOSs. We report, for the first time, the characterization of NO complexes with NOHA (N(omega)-hydroxy-L-arginine), the substrate used in the second half of the catalytic cycle of NOSs. In the FeIII-NO complexes, both L-arginine and NOHA induced the Fe-N-O bending mode at nearly the same frequency as a result of a steric interaction between the substrates and the haem-bound NO. However, in the FeII-NO complexes, the Fe-N-O bending mode was not observed and the nu(Fe-NO) mode displayed a 5 cm(-1) higher frequency in the complex with NOHA than in the complex with L-arginine as a result of direct interactions that probably involve hydrogen bonds. The different behaviour of the substrates in the FeII-NO complexes thus reveal that the interactions between haem-bound NO and the substrates are finely tuned by the geometry of the Fe-ligand structure and are relevant to the use of the FeII-NO complex as a model of the oxygenated complex of NOSs.

The role of tetrahydrobiopterin in catalysis by nitric oxide synthase

Electronic structure calculations show that the cofactor H4B can be a key factor in a proton transfer relay in nitric oxide synthase, and that 4-amino-H4B cannot fulfill this role.

Structural studies of constitutive nitric oxide synthases with diatomic ligands bound

Crystal structures are reported for the endothelial nitric oxide synthase (eNOS)-arginine-CO ternary complex as well as the neuronal nitric oxide synthase (nNOS) heme domain complexed with L: -arginine and diatomic ligands, CO or NO, in the presence of the native cofactor, tetrahydrobiopterin, or its oxidized analogs, dihydrobiopterin and 4-aminobiopterin. The nature of the biopterin has no influence on the diatomic ligand binding. The binding geometries of diatomic ligands to nitric oxide synthase (NOS) follow the {MXY}(n) formalism developed from the inorganic diatomic-metal complexes. The structures reveal some subtle structural differences between eNOS and nNOS when CO is bound to the heme which correlate well with the differences in CO stretching frequencies observed by resonance Raman techniques. The detailed hydrogen-bonding geometries depicted in the active site of nNOS structures indicate that it is the ordered active-site water molecule rather than the substrate itself that would most likely serve as a direct proton donor to the diatomic ligands (CO, NO, as well as O(2)) bound to the heme. This has important implications for the oxygen activation mechanism critical to NOS catalysis.

Nitrosyl-heme structures of Bacillus subtilis nitric oxide synthase have implications for understanding substrate oxidation

The crystal structures of nitrosyl-heme complexes of a prokaryotic nitric oxide synthase (NOS) from Bacillus subtilis (bsNOS) reveal changes in active-site hydrogen bonding in the presence of the intermediate N(omega)-hydroxy-l-arginine (NOHA) compared to the substrate l-arginine (l-Arg). Correlating with a Val-to-Ile residue substitution in the bsNOS heme pocket, the Fe(II)-NO complex with both l-Arg and NOHA is more bent than the Fe(II)-NO, l-Arg complex of mammalian eNOS [Li, H., Raman, C. S., Martasek, P., Masters, B. S. S., and Poulos, T. L. (2001) Biochemistry 40, 5399-5406]. Structures of the Fe(III)-NO complex with NOHA show a nearly linear nitrosyl group, and in one subunit, partial nitrosation of bound NOHA. In the Fe(II)-NO complexes, the protonated NOHA N(omega) atom forms a short hydrogen bond with the heme-coordinated NO nitrogen, but active-site water molecules are out of hydrogen bonding range with the distal NO oxygen. In contrast, the l-Arg guanidinium interacts more weakly and equally with both NO atoms, and an active-site water molecule hydrogen bonds to the distal NO oxygen. This difference in hydrogen bonding to the nitrosyl group by the two substrates indicates that interactions provided by NOHA may preferentially stabilize an electrophilic peroxo-heme intermediate in the second step of NOS catalysis.

Structures of the high-valent metal-ion haem–oxygen intermediates in peroxidases, oxygenases and catalases

Peroxidases, oxygenases and catalases have similar high-valent metal-ion intermediates in their respective reaction cycles. In this review, haem-based examples will be discussed. The intermediates of the haem-containing enzymes have been extensively studied for many years by different spectroscopic methods like UV-Vis, EPR (electron paramagnetic resonance), resonance Raman, Mössbauer and MCD (magnetic circular dichroism). The first crystal structure of one of these high-valent intermediates was on cytochrome c peroxidase in 1987. Since then, structures have appeared for catalases in 1996, 2002, 2003, putatively for cytochrome P450 in 2000, for myoglobin in 2002, for horseradish peroxidase in 2002 and for cytochrome c peroxidase again in 1994 and 2003. This review will focus on the most recent structural investigations for the different intermediates of these proteins. The structures of these intermediates will also be viewed in light of quantum mechanical (QM) calculations on haem models. In particular quantum refinement, which is a combination of QM calculations and crystallography, will be discussed. Only small structural changes accompany the generation of these intermediates. The crystal structures show that the compound I state, with a so called pi-cation radical on the haem group, has a relatively short iron-oxygen bond (1.67-1.76A) in agreement with a double-bond character, while the compound II state or the compound I state with a radical on an amino acid residue have a relatively long iron-oxygen bond (1.86-1.92A) in agreement with a single-bond character where the oxygen-atom is protonated.

A Weak Fe–O Bond in the Oxygenated Complex of the Nitric-oxide Synthase of Staphylococcus aureus

Little is known about the intermediates formed during catalysis by nitric-oxide synthase (NOS). We report here the characterization by resonance Raman spectroscopy of the oxygenated complex of the NOS from Staphylococcus aureus (saNOS) as well as the kinetics of formation and decay of the complex. An oxygenated complex transiently formed after mixing reduced saNOS with oxygen and decayed to the ferric enzyme with kinetics that were dependent on the substrate L-arginine and the cofactor H(4)B. The oxygenated complex displayed a Soret absorption band centered at 430 nm. Resonance Raman spectroscopy revealed that it can be described as a ferric superoxide form (Fe(III)O(2)(-)) with a single nu(O-O) mode at 1135 cm(-1). In the presence of L-arginine, an additional nu(O-O) mode at 1123 cm(-1) was observed, indicating an increased pi back-bonding electron donation to the bound oxygen induced by the substrate. With saNOS, this is the first time that the nu(Fe-O) mode of a NOS has been observed. The low frequency of this mode, at 517 cm(-1), points to an oxygenated complex that differs from that of P450(cam). The electronic structure of the oxygenated complex and the effect of L-arginine are discussed in relation to the kinetic properties of saNOS and other NOS.

Differential Effects of Alkyl- and Arylguanidines on the Stability and Reactivity of Inducible NOS Heme−Dioxygen Complexes

NO-Synthases are heme proteins that catalyze the oxidation of L-arginine into NO and L-citrulline. Some non-amino acid alkylguanidines may serve as substrates of inducible NOS (iNOS), while no NO* production is obtained from arylguanidines. All studied guanidines induce uncoupling between electrons transferred from the reductase domain and those required for NO formation. This uncoupling becomes critical with arylguanidines, leading to the exclusive formation of superoxide anion O2*- as well as hydrogen peroxide H2O2. To understand these different behaviors, we have conducted rapid scanning stopped-flow experiments with dihydrobiopterin (BH2) and tetrahydrobiopterin (BH4) to study, respectively, the (i) autoxidation and (ii) activation processes of heme ferrous-O2 complexes (Fe(II)O2) in the presence of eight alkyl- and arylguanidines. The Fe(II)O2 complex is more easily autooxidized by alkylguanidines (10-fold) and arylguanidines (100-fold) compared to L-arginine. In the presence of alkylguanidines and BH4, the oxygen-activation kinetics are very similar to those observed with L-arginine. Conversely, in the presence of arylguanidines, no Fe(II)O2 intermediate is detected. To understand such variations in reactivity and stability of Fe(II)O2 complex, we have characterized the effects of alkyl- and arylguanidines on Fe(II)O2 structure using the Fe(II)CO complex as a mimic. Resonance Raman and FTIR spectroscopies show that the two classes of guanidine derivatives induce different polar effects on Fe(II)CO environment. Our data suggest that the structure of the substituted guanidine can modulate the stability and the reactivity of heme-dioxygen complexes. We thus propose differential mechanisms for the electron- and proton-transfer steps in the NOS-dependent, oxygen-activation process, contingent upon whether alkyl- or arylguanidines are bound.

High-pressure studies of the reaction mechanism of nitric-oxide synthase

Nitric-oxide synthase (NOS) generates nitric oxide from l-arginine in two reaction cycles with N(omega)-hydroxy-l-arginine as an obligate intermediate. Although much progress has been made in recent years in the elucidation of the reaction mechanism of NOS, many questions remain to be answered. The use of low temperature has been instrumental in the revelation of the mechanism of NO synthesis, particularly regarding the role of the cofactor 5,6,7,8-tetrahydrobopterin (BH4). High-pressure studies may be expected to be similarly useful, but have been very few so far. In this short review, we depict the present state of knowledge about the reaction mechanism of NO synthesis, and the role(s) BH4 plays in it. This exposition is followed by a summary of the results obtained thus far in high-pressure studies and of the conclusions that can be drawn from them.

Structure and reactivity of a thermostable prokaryotic nitric-oxide synthase that forms a long-lived oxy-heme complex

In an effort to generate more stable reaction intermediates involved in substrate oxidation by nitric-oxide synthases (NOSs), we have cloned, expressed, and characterized a thermostable NOS homolog from the thermophilic bacterium Geobacillus stearothermophilus (gsNOS). As expected, gsNOS forms nitric oxide (NO) from l-arginine via the stable intermediate N-hydroxy l-arginine (NOHA). The addition of oxygen to ferrous gsNOS results in long-lived heme-oxy complexes in the presence (Soret peak 427 nm) and absence (Soret peak 413 nm) of substrates l-arginine and NOHA. The substrate-induced red shift correlates with hydrogen bonding between substrate and heme-bound oxygen resulting in conversion to a ferric heme-superoxy species. In single turnover experiments with NOHA, NO forms only in the presence of H(4)B. The crystal structure of gsNOS at 3.2 AA of resolution reveals great similarity to other known bacterial NOS structures, with the exception of differences in the distal heme pocket, close to the oxygen binding site. In particular, a Lys-356 (Bacillus subtilis NOS) to Arg-365 (gsNOS) substitution alters the conformation of a conserved Asp carboxylate, resulting in movement of an Ile residue toward the heme. Thus, a more constrained heme pocket may slow ligand dissociation and increase the lifetime of heme-bound oxygen to seconds at 4 degrees C. Similarly, the ferric-heme NO complex is also stabilized in gsNOS. The slow kinetics of gsNOS offer promise for studying downstream intermediates involved in substrate oxidation.

Reactivity of the heme-dioxygen complex of the inducible nitric oxide synthase in the presence of alternative substrates

Single turnover reactions of the inducible nitric oxide synthase oxygenase domain (iNOSoxy) in the presence of several non alpha-amino acid N-hydroxyguanidines and guanidines were studied by stopped-flow visible spectroscopy, and compared with reactions using the native substrates L-arginine (L-arg) or N(omega)-hydroxy-L-arginine (NOHA). In experiments containing dihydrobiopterin, a catalytically incompetent pterin, and each of the studied substrates, L-arg, butylguanidine (BuGua), para-fluorophenylguanidine (FPhGua), NOHA, N-butyl- and N-(para-fluorophenyl)-N'-hydroxyguanidines (BuNOHG and FPhNOHG), the formation of a iron(II) heme-dioxygen intermediate (Fe(II)O2) was always observed. The Fe(II)O2 species then decayed to iron(III) iNOSoxy at rates that were dependent on the nature of the substrate. Identical reactions containing the catalytically competent cofactor tetrahydrobiopterin (BH4), iNOSoxy and the three N-hydroxyguanidines, all exhibited an initial formation of an Fe(II)O2 species that was successively converted to an Fe(III)NO complex and eventually to high-spin iron(III) iNOSoxy. The formation and decay kinetics of the Fe(III)NO complex did not vary greatly as a function of the N-hydroxyguanidine structure, but the formation of Fe(III)NO was substoichiometric in the cases of BuNOHG and FPhNOHG. Reactions between BH4-containing iNOSoxy and BuGua exhibited kinetics similar to those of the corresponding reaction with L-arginine, with formation of an Fe(II)O2 intermediate that was directly converted to high-spin iron(III) iNOSoxy. In contrast, no Fe(II)O2 intermediate was observed in the reaction of BH4-containing iNOSoxy and FPhGua. Multi-turnover reaction of iNOS with FPhGua did not lead to formation of NO or to hydroxylation of the substrate, contrary to reactions with BuGua or L-arg. Our results reveal how different structural and chemical properties of NOS substrate analogues can impact on the kinetics and reactivity of the Fe(II)O2 intermediate, and support an important role for substrate pKa during NOS oxygen activation.

Neural tube closure depends on nitric oxide synthase activity

Neural tube (NT) closure is a multifactorial process that involves yet unresolved molecular mechanisms. It had been shown previously that high levels of nitric oxide (NO) block the process of NT closure in the chick embryo by inhibiting methionine synthase (MS). The MS inhibition and its effect on NT closure could be alleviated by folic acid, suggesting the involvement of the folate-methionine pathway in the process. Here we test the hypothesis that endogenous nitric oxide synthase (NOS) activity regulates the MS activity required in the process of NT closure. The experiments described here reveal that NOS activity per se, is indeed critical for NT closure in the chick embryo. Inhibition of NOS activity with either 2,4-diamino-6-hydroxypyrimidine (DAHP), which blocks biosynthesis of the NOS co-factor tetrahydrobiopterin (BH4), or with calmidazolium, which blocks calcium-calmodulin binding to NOS, resulted in reduced MS activity and consequently ablated NT closure. Addition of BH4 or the calcium ionophore A23187 restored NOS and MS activities, resulting in NT closure. The results described here imply that NOS and MS activities can serve as functional markers in this developmental process as they are essential in the process of NT closure.

Exploring the redox reactions between heme and tetrahydrobiopterin in the nitric oxide synthases

The NO synthases (NOSs) catalyze a two-step oxidation of L-arginine (Arg) to generate nitric oxide (NO) plus L-citrulline. Because NOSs are the only hemeproteins known to contain tetrahydrobiopterin (H4B) as a bound cofactor, the function and role of H4B in their heme-based oxygen activation and catalysis is of current interest. Distinct oxidative and reductive transitions of bound H4B cofactor occur during catalysis and are associated with distinct redox transitions of the NOS heme and flavin prosthetic groups. In this perspective, we discuss the redox transitions of H4B and heme with regard to their kinetics, regulation, role in the catalytic mechanism, and how and why they may be linked.

Hunting oxygen complexes of nitric oxide synthase at low temperature and high pressure

The reaction of nitric oxide synthase (NOS) with oxygen is fast and takes place within several steps, separated by ephemeral intermediates. The use of extreme experimental conditions, such as low temperature and high pressure, associated to rapid kinetic analysis, has proven to be a convenient tool to study this complex reaction. Stopped-flow experiments under high pressure indicated that oxygen binding occurred in more than one step. This was further corroborated by the detection of two short-lived oxy-compounds, differing in their spectral and electronic properties. Oxy-I resembles the ferrous oxygen complex known for cytochrome P450, whereas oxy-II appears to be locked in the superoxide form. Subzero temperature spectroscopy, together with an analytical separation method, revealed that the subsequent one-electron reduction of the oxygen complex is carried out by the NOS cofactor tetrahydrobiopterin (BH4). The low-temperature stabilized oxidation product of BH4 was found to be a protonated BH3 radical. Finally, work in the presence of a BH4 analog indicated that proton transfer to the activated oxygen complex is a second essential function of BH4.

Tetrahydrobiopterin as combined electron/proton donor in nitric oxide biosynthesis: cryogenic UV-Vis and EPR detection of reaction intermediates

The role of tetrahydrobiopterin (BH4) as a cofactor in nitric oxide synthase (NOS) has been the object of intense research in the last few years. It was found that in addition to its established effects on the NOS heme spin state, substrate affinity, and enzyme dimerization, BH4 is required as a one-electron donor to oxyferrous [Fe(II).O2] heme that is formed as an intermediate in the catalytic cycle. Cryogenic spectroscopic techniques proved particularly useful in the identification of this role of BH4 in NO synthesis. With these methods, the mechanism of fast reactions, such as the reaction of ferrous NOS with O2, can be unraveled by lowering the reaction temperature to subzero values. This may not only reduce the rate to such an extent that the reaction can be followed on a time scale from seconds to minutes, but intermediates may be observed that do not accumulate at higher temperatures. Cryogenic ultraviolet-visible (UV-vis) and electron paramagnetic resonance spectroscopy have been applied to clarify why the BH4 analogue 4-amino-tetrahydrobiopterin (4-amino-BH4) is unable to support NO synthesis. In the course of these studies, evidence was gathered supporting a role for BH4 as an obligate proton and electron donor. It is believed that the inhibitory action of 4-amino-BH4 derives from an inability to serve as a proton donor, even though it is perfectly able to serve as an electron donor. In this chapter, the suitability, drawbacks, and advantages of cryogenic methods are discussed.

Regioselective Nitration of Tryptophan by a Complex between Bacterial Nitric-oxide Synthase and Tryptophanyl-tRNA Synthetase

Bacterial nitric-oxide synthase proteins (NOSs) from certain Streptomyces strains have been shown to participate in biosynthetic nitration of tryptophanyl moieties in vivo (Kers, J. A., Wach, M. J., Krasnoff, S. B., Cameron, K. D., Widom, J., Bukhaid, R. A., Gibson, D. M., and Crane, B. R., and Loria, R. (2004) Nature 429, 79-82). We report that the complex between Deinococcus radiodurans NOS (deiNOS) and an unusual tryptophanyl-tRNA synthetase (TrpRS II) catalyzes the regioselective nitration of tryptophan (Trp) at the 4-position. Unlike non-enzymatic Trp nitration, and similar reactions catalyzed by globins and peroxidases, deiNOS only produces the otherwise unfavorable 4-nitro-Trp isomer. Although deiNOS alone will catalyze 4-nitro-Trp production, yields are significantly enhanced by TrpRS II and ATP. 4-Nitro-Trp formation exhibits saturation behavior with Trp (but not tyrosine) and is completely inhibited by the addition of the mammalian NOS cofactor (6R)-5,6,7,8-tetrahydro-l-biopterin (H(4)B). Trp stimulates deiNOS oxidation of substrate l-arginine (Arg) to the same degree as H(4)B. These observations are consistent with a mechanism where Trp or a derivative thereof binds in the NOS pterin site, participates in Arg oxidation, and becomes nitrated at the 4-position.

Thermodynamic and kinetic analysis of the nitrosyl, carbonyl, and dioxy heme complexes of neuronal nitric-oxide synthase. The roles of substrate and tetrahydrobiopterin in oxygen activation

Mammalian NO synthases catalyze the monooxygenation of L-arginine (L-Arg) to N-hydroxyarginine (NOHA) and the subsequent monooxygenation of this to NO and citrulline. Both steps proceed via formation of an oxyferrous heme complex and may ultimately lead to a ferrous NO complex, from which NO must be released. Electrochemical reduction of NO-bound neuronal nitricoxide synthase (nNOS) oxygenase domain was used to form the ferrous heme NO complex, which was found to be stable only in the presence of low NO concentrations, due to catalytic degradation of NO at the nNOS heme site. The reduction potential for the heme-NO complex was approximately -140 mV, which shifted to 0 mV in the presence of either L-Arg or NOHA. This indicates that the complex is stabilized by 14 kJ mol(-1) in the presence of substrate, consistent with a strong H-bonding interaction between NO and the guanidino group. Neither substrate influenced the reduction potential of the ferrous heme CO complex, however. Both L-Arg and NOHA appear to interact with bound NO in a similar way, indicating that both bind as guanidinium ions. The dissociation constant for NO bound to ferrous heme in the presence of l-Arg was determined electrochemically to be 0.17 nM, and the rate of dissociation was estimated to be 10(-4) s(-1), which is much slower than the rate of catalysis. Stopped-flow kinetic analysis of oxyferrous formation and decay showed that both l-Arg and NOHA also stabilize the ferrous heme dioxy complex, resulting in a 100-fold decrease in its rate of decay. Electron transfer from the active-site cofactor tetrahydrobiopterin (H4B) has been proposed to trigger the monoxygenation process. Consistent with this, substitution by the analogue/inhibitor 4-amino-H4B stabilized the oxyferrous complex by a further two orders of magnitude. H4B is required, therefore, to break down both the oxyferrousand ferrous nitrosyl complexes of nNOS during catalysis. The energetics of these processes necessitates an electron donor/acceptor operating within a specific reduction potential range, defining the role of H4B.

Alternative nitric oxide-producing substrates for NO synthases

Nitric oxide (NO) is a key inter- and intracellular molecule involved in the maintenance of vascular tone, neuronal signaling, and host response to infection. The biosynthesis of NO in mammals involves a two-step oxidation of L-arginine (L-Arg) to citrulline and NO catalyzed by a particular class of heme-thiolate proteins, called NO-synthases (NOSs). The NOSs successively catalyze the Nomega-hydroxylation of the guanidine group of L-Arg with formation of Nomega-hydroxy-L-arginine (NOHA) and the oxidative cleavage of the CN(OH) bond of NOHA with formation of citrulline and NO. During the last decade, a great number of compounds bearing a CNH or CNOH function have been synthesized and studied as possible NO-producing substrates of recombinant NOSs. This includes derivatives of L-Arg and NOHA, N-alkyl (or aryl) guanidines, N,N'- or N,N-disubstituted guanidines, N-alkyl (or aryl) N'-hydroxyguanidines, N- (or O-) disubstituted N'-hydroxyguanidines, as well as amidoximes, ketoximes, and aldoximes. However, only those involving the NHC(NH2)=NH (or NOH) moiety have led to a significant formation of NO. All the N-monosubstituted N'-hydroxyguanidines that are well recognized by the NOS active site lead to NO with catalytic efficiences (kcat/Km) up to 50% of that of NOHA. This is the case of many N-aryl and N-alkyl N'-hydroxyguanidines, provided that the aryl or alkyl substituent is small enough to be accommodated by a NOS hydrophobic site located in close proximity of the NOS "guanidine binding site." As far as N-substituted guanidines are concerned, few compounds bearing a small alkyl group have been found to act as NO-producing substrates. The kcat value found for the best compound may reach 55% of the kcat of L-Arg oxidation. However, the best catalytic efficiency (kcat/Km) that was obtained with N-(4,4,4-trifluorobutyl) guanidine is only 100-fold lower than that of L-Arg. In a general manner, NOS II is a better catalyst that NOS I and III for the oxidation of exogenous guanidines and N-hydroxyguanidines to NO. This is particularly true for guanidines as the ones acting as substrates for NOS II have been found to be almost inactive for NOS I and NOS III. Thus, a good NO-producing guanidine substrate for the two latter isozymes remains to be found.

Structural and functional diversity in heme monooxygenases

Recent advances in understanding structure-function relationships in cytochrome P450 (P450), nitric-oxide synthase (NOS), and heme oxygenase are summarized. Of particular importance is the role that dynamics plays in P450 function, where the active site undergoes large open/close motions to enable substrates to bind and products to leave. In sharp contrast, the heme-containing active site of NOS is rigid and remains relatively exposed compared with P450s. This difference in dynamics and active site exposure requires that the O(2) activation machinery operate somewhat differently in P450 and NOS. Owing to the open NOS active site, the NOS-oxy complex could be subject to nonspecific protonation that short-circuits the normal reaction path. One working hypothesis holds that NOS recruited the cofactor, tetrahydrobiopterin, to bind near the heme for very rapid coupled electron/proton transfer to the oxy complex, which avoids indiscriminate reaction with bulk solvent. Despite these differences, P450, NOS, and also heme oxygenase use a very similar network of H-bonded water molecules in the active site that are required for oxygen activation. Both P450 and NOS are important drug targets. With NOS, the structural basis for isoform-selective inhibition by a class of dipeptide inhibitors has been worked out, thus providing the basis for structure-based drug design.

Evidence of Two Distinct Oxygen Complexes of Reduced Endothelial Nitric Oxide Synthase

Oxygen binding to the oxygenase domain of reduced endothelial nitric oxide synthase (eNOS) results in two distinct species differing in their Soret and visible absorbance maxima and in their capacity to exchange oxygen by CO. At 7 degrees C, heme-oxy I (with maxima at 420 and 560 nm) is formed very rapidly (k(on) approximately 2.5.10(6) m(-1).s(-1)) in the absence of substrate but in the presence of pterin cofactor. It is capable of exchanging oxygen with CO at -30 degrees C. Heme-oxy II is formed more slowly (k(on) approximately equal to 3.10(5) m(-1).s(-1)) in the presence of substrate, regardless of the presence of pterin. It is also formed in the absence of both substrate and pterin. In contrast to heme-oxy I, it cannot exchange oxygen with CO at cryogenic temperature. In the presence of arginine, heme-oxy II is characterized by absorbance maxima near 432, 564, and 597 nm. When arginine is replaced by N-hydroxyarginine, and also in the absence of both substrate and pterin, its absorbance maxima are blue-shifted to 428, 560, and 593 nm. Heme-oxy I seems to resemble the ferrous dioxygen complex observed in many hemoproteins, including cytochrome P450. Heme-oxy II, which is the oxygen complex competent for product formation, appears to represent a distinct conformation in which the electronic configuration is essentially locked in the ferric superoxide complex.

A conserved Val to Ile switch near the heme pocket of animal and bacterial nitric-oxide synthases helps determine their distinct catalytic profiles

Nitric oxide (NO) release from nitric oxide synthases (NOSs) is largely dependent on the dissociation of an enzyme ferric heme-NO product complex (Fe(III)NO). Although the NOS-like protein from Bacillus subtilis (bsNOS) generates Fe(III)NO from the reaction intermediate N-hydroxy-l-arginine (NOHA), its NO dissociation is about 20-fold slower than in mammalian NOSs. Crystal structures suggest that a conserved Val to Ile switch near the heme pocket of bsNOS might determine its kinetic profile. To test this we generated complementary mutations in the mouse inducible NOS oxygenase domain (iNOSoxy, V346I) and in bsNOS (I224V) and characterized the kinetics and extent of their NO synthesis from NOHA and their NO-binding kinetics. The mutations did not greatly alter binding of Arg, (6R)-tetrahydrobiopterin, or alter the electronic properties of the heme or various heme-ligand complexes. Stopped-flow spectroscopy was used to study heme transitions during single turnover NOHA reactions. I224V bsNOS displayed three heme transitions involving four species as typically occurs in wild-type NOS, the beginning ferrous enzyme, a ferrous-dioxy (Fe(II)O(2)) intermediate, Fe(III)NO, and an ending ferric enzyme. The rate of each transition was increased relative to wild-type bsNOS, with Fe(III)NO dissociation being 3.6 times faster. In V346I iNOSoxy we consecutively observed the beginning ferrous, Fe(II)O(2), a mixture of Fe(III)NO and ferric heme species, and ending ferric enzyme. The rate of each transition was decreased relative to wild-type iNOSoxy, with the Fe(III)NO dissociation being 3 times slower. An independent measure of NO binding kinetics confirmed that V346I iNOSoxy has slower NO binding and dissociation than wild-type. Citrulline production by both mutants was only slightly lower than wild-type enzymes, indicating good coupling. Our data suggest that a greater shielding of the heme pocket caused by the Val/Ile switch slows down NO synthesis and NO release in NOS, and thus identifies a structural basis for regulating these kinetic variables.