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

Importance of Val567 on heme environment and substrate recognition of neuronal nitric oxide synthase

Nitric oxide (NO) produced by mammalian nitric oxide synthases (mNOSs) is an important mediator in a variety of physiological functions. Crystal structures of mNOSs have shown strong conservation of the active‐site residue Val567 (numbering for rat neuronal NOS, nNOS). NOS‐like proteins have been identified in several bacterial pathogens, and these display striking sequence identity to the oxygenase domain of mNOS (NOSoxy), with the exception of a Val to Ile mutation at the active site. Preliminary studies have highlighted the importance of this Val residue in NO‐binding, substrate recognition, and oxidation in mNOSs. To further elucidate the role of this valine in substrate and substrate analogue recognition, we generated five Val567 mutants of the oxygenase domain of the neuronal NOS (nNOSoxy) and used UV‐visible and EPR spectroscopy to investigate the effects of these mutations on the heme distal environment, the stability of the heme‐Fe^II‐CO complexes, and the binding of a series of substrate analogues. Our results are consistent with Val567 playing an important role in preserving the integrity of the active site for substrate binding, stability of heme‐bound gaseous ligands, and potential NO production.

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.

Oxygen activation in NO synthases: evidence for a direct role of the substrate

Nitric oxide (NO) and the other reactive nitrogen species (RNOS) play crucial patho‐physiological roles at the interface of oxidative stress and signalling processes. In mammals, the NO synthases (NOSs) are the source of these reactive nitrogen species, and so to understand the precise biological role of RNOS and NO requires elucidation of the molecular functioning of NOS. Oxygen activation, which is at the core of NOS catalysis, involves a sophisticated sequence of electron and proton transfers. While electron transfer in NOS has received much attention, the proton transfer processes has been scarcely investigated. Here, we report an original approach that combines fast‐kinetic techniques coupled to resonance Raman spectroscopy with the use of synthetic analogues of NOS substrate. We characterise Fe^II‐O₂ reaction intermediates in the presence of L‐arginine (Arg), alkyl‐ and aryl‐guanidines. The presence of new reaction intermediates, such as ferric haem‐peroxide, that was formerly postulated, was tracked by analysing the oxygen activation reaction at different times and with different excitation wavelengths. Our results suggest that Arg is not a proton donor, but indirectly intervenes in oxygen activation mechanism by modulating the distal H‐bond network and, in particular, by tuning the position and the role of the distal water molecule. This report supports a catalytic model with two proton transfers in step 1 (Arg hydroxylation) but only one proton transfer in step 2 (N^ω‐hydroxy‐L‐arginine oxidation).

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.

Detection of nitric oxide and superoxide radical anion by electron paramagnetic resonance spectroscopy from cells using spin traps

Reactive nitrogen/oxygen species (ROS/RNS) at low concentrations play an important role in regulating cell function, signaling, and immune response but in unregulated concentrations are detrimental to cell viability. While living systems have evolved with endogenous and dietary antioxidant defense mechanisms to regulate ROS generation, ROS are produced continuously as natural by-products of normal metabolism of oxygen and can cause oxidative damage to biomolecules resulting in loss of protein function, DNA cleavage, or lipid peroxidation, and ultimately to oxidative stress leading to cell injury or death. Superoxide radical anion (O2•-) is the major precursor of some of the most highly oxidizing species known to exist in biological systems such as peroxynitrite and hydroxyl radical. The generation of O2•- signals the first sign of oxidative burst, and therefore, its detection and/or sequestration in biological systems is important. In this demonstration, O2•- was generated from polymorphonuclear neutrophils (PMNs). Through chemotactic stimulation with phorbol-12-myristate-13-acetate (PMA), PMN generates O2•- via activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Nitric oxide (NO) synthase which comes in three isoforms, as inducible-, neuronal- and endothelial-NOS, or iNOS, nNOS or eNOS, respectively, catalyzes the conversion of L- arginine to L-citrulline, using NADPH to produce NO. Here, we generated NO from endothelial cells. Under oxidative stress conditions, eNOS for example can switch from producing NO to O2•- in a process called uncoupling, which is believed to be caused by oxidation of heme or the co-factor, tetrahydrobiopterin (BH4). There are only few reliable methods for the detection of free radicals in biological systems but are limited by specificity and sensitivity. Spin trapping is commonly used for the identification of free radicals and involves the addition reaction of a radical to a spin trap forming a persistent spin adduct which can be detected by electron paramagnetic resonance (EPR) spectroscopy. The various radical adducts exhibit distinctive spectrum which can be used to identify the radicals being generated and can provide a wealth of information about the nature and kinetics of radical production. The cyclic nitrones, 5,5-dimethyl-pyrroline-N-oxide, DMPO, the phosphoryl-substituted DEPMPO, and the ester-substituted, EMPO and BMPO, have been widely employed as spin traps--the latter spin traps exhibiting longer half-lives for O2•- adduct. Iron (II)-N-methyl-D-glucamine dithiocarbamate, Fe(MGD)2 is commonly used to trap NO due to high rate of adduct formation and the high stability of the spin adduct.

The conserved Trp-Cys hydrogen bond dampens the "push effect" of the heme cysteinate proximal ligand during the first catalytic cycle of nitric oxide synthase

Residues surrounding and interacting with the heme proximal ligand are important for efficient catalysis by heme proteins. The nitric oxide synthases (NOSs) are thiolate-coordinated enzymes that catalyze the hydroxylation of l-Arg in the first of the two catalytic cycles needed to synthesize nitric oxide. In NOSs, the indole NH group of a conserved tryptophan [W56 of the bacterial NOS-like protein from Staphylococcus aureus (saNOS)] forms a hydrogen bond with the heme proximal cysteinate ligand. The purpose of this study was to determine the impact of increasing (W56F and W56Y variants) or decreasing (W56H variant) the electron density of the proximal cysteinate ligand on molecular oxygen (O(2)) activation using saNOS as a model. We show that the removal of the indole NH···S(-) bond for W56F and W56Y caused an increase in the electron density of the cysteinate. This was probed by the decrease of the midpoint reduction potential (E(1/2)) along with weakened σ-bonding and strengthened π-backbonding with distal ligands (CO and O(2)). On the other hand, the W56H variant showed stronger Fe-OO and Fe-CO bonds (strengthened σ-bonding) along with an elevated E(1/2), which is consistent with the formation of a strong NH···S(-) hydrogen bond from H56. We also show here that changing the electron density of the proximal thiolate controls its "push effect"; whereas the rates of both O(2) activation and autoxidation of the Fe(II)O(2) complex increase with the stronger push effect created by removing the indole NH···S(-) hydrogen bond (W56F and W56Y variants), the W56H variant showed an increased stability of the complex against autoxidation and a slower rate of O(2) activation. These results are discussed with regard to the roles played by the conserved tryptophan-cysteinate interaction in the first catalytic cycle of NOS.

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.

Role of arginine guanidinium moiety in nitric-oxide synthase mechanism of oxygen activation

Nitric-oxide synthases (NOS) are highly regulated heme-thiolate enzymes that catalyze two oxidation reactions that sequentially convert the substrate L-Arg first to N(omega)-hydroxyl-L-arginine and then to L-citrulline and nitric oxide. Despite numerous investigations, the detailed molecular mechanism of NOS remains elusive and debatable. Much of the dispute in the various proposed mechanisms resides in the uncertainty concerning the number and sources of proton transfers. Although specific protonation events are key features in determining the specificity and efficiency of the two catalytic steps, little is known about the role and properties of protons from the substrate, cofactors, and H-bond network in the vicinity of the heme active site. In this study, we have investigated the role of the acidic proton from the L-Arg guanidinium moiety on the stability and reactivity of the ferrous heme-oxy complex intermediate by exploiting a series of L-Arg analogues exhibiting a wide range of guanidinium pK(a) values. Using electrochemical and vibrational spectroscopic techniques, we have analyzed the effects of the analogues on the heme, including characteristics of its proximal ligand, heme conformation, redox potential, and electrostatic properties of its distal environment. Our results indicate that the substrate guanidinium pK(a) value significantly affects the H-bond network near the heme distal pocket. Our results lead us to propose a new structural model where the properties of the guanidinium moiety finely control the proton transfer events in NOS and tune its oxidative chemistry. This model may account for the discrepancies found in previously proposed mechanisms of NOS oxidation processes.

4,4-Difluorinated analogues of l-arginine and N(G)-hydroxy-l-arginine as mechanistic probes for nitric oxide synthase

4,4-Difluoro-l-arginine and 4,4-difluoro-N(G)-hydroxy-l-arginine were synthesized and shown to be substrates for the inducible isoform of nitric oxide synthase (iNOS). Binding of both fluorinated analogues to the NOS active site was also investigated using a spectral binding assay employing a heme domain construct of the inducible NOS isoform (iNOS(heme)). 4,4-Difluoro-N(G)-hydroxy-arginine was found to bind at the NOS active site in a unique manner consistent with a model involving ligation of the Fe(III) heme center by the oxygen atom of the N(G)-hydroxy moiety.

N(G)-aminoguanidines from primary amines and the preparation of nitric oxide synthase inhibitors

A concise, general, and high-yielding method for the preparation of N(G)-aminoguanidines from primary amines is reported. Using available and readily prepared materials, primary amines are converted to protected N(G)-aminoguanidines in a one-pot procedure. The method has been successfully applied to a number of examples including the syntheses of four nitric oxide synthase (NOS) inhibitors. The inhibitors prepared were investigated as competitive inhibitors and as mechanistic inactivators of the inducible isoform of NOS (iNOS). In addition, one of the four inhibitors prepared, N(G)-amino-N(G)-2,2,2-trifluoroethyl-L-arginine 19, displays the unique ability to both inhibit NO formation and prevent NADPH consumption by iNOS without irreversible inactivation of the enzyme.

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.