Catalytic intermediates of inducible nitric-oxide synthase stabilized by the W188H mutation

The eNOS isoform exhibits increased expression and activation in the main olfactory bulb of nNOS knock-out mice

The main olfactory bulb (MOB) is a neural structure that processes olfactory information. Among the neurotransmitters present in the MOB, nitric oxide (NO) is particularly relevant as it performs a wide variety of functions. In this structure, NO is produced mainly by neuronal nitric oxide synthase (nNOS) but also by inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS). The MOB is considered a region with great plasticity and the different NOS also show great plasticity. Therefore, it could be considered that this plasticity could compensate for various dysfunctional and pathological alterations. We examined the possible plasticity of iNOS and eNOS in the MOB in the absence of nNOS. For this, wild-type and nNOS knock-out (nNOS-KO) mice were used. We assessed whether the absence of nNOS expression could affect the olfactory capacity of mice, followed by the analysis of the expression and distribution of the NOS isoforms using qPCR and immunofluorescence. NO production in MOB was examined using both the Griess and histochemical NADPH-diaphorase reactions. The results indicate nNOS-KO mice have reduced olfactory capacity. We observed that in the nNOS-KO animal, there is an increase both in the expression of eNOS and NADPH-diaphorase, but no apparent change in the level of NO generated in the MOB. It can be concluded that the level of eNOS in the MOB of nNOS-KO is related to the maintenance of normal levels of NO. Therefore, our findings suggest that nNOS could be essential for the proper functioning of the olfactory system.

Alternative modes of O2 activation in P450 and NOS enzymes are clarified by DFT modeling and resonance Raman spectroscopy

The functions of heme proteins are modulated by hydrogen bonds (H-bonds) directed at the heme-bound ligands by protein residues. When the gaseous ligands CO, NO, or O2 are bound, their activity is strongly influenced by H-bonds to their atoms. These H-bonds produce characteristic changes in the vibrational frequencies of the heme adduct, which can be monitored by resonance Raman spectroscopy and interpreted with density functional theory (DFT) computations. When the protein employs a cysteinate proximal ligand, bound O2 becomes particularly reactive, the course of the reaction being controlled by H-bonding and proton delivery. In this work, DFT modeling is used to examine the effects of H-bonding to either the terminal (Ot) or proximate (Op) atom of methylthiolate-Fe(II)porphine-O2, as well as to the thiolate S atom. H-bonds to Op produce a positive linear correlation between ν(Fe - O) and ν(O - O), because they increase the sp2 character of Op, weakening both the Fe - O and O - O bonds. H-bonds to Ot produce a negative correlation, because they increase Fe backbonding, strengthening the Fe - O but weakening the O - O bond. Available experimental data accommodate well to the computed pattern. In particular, this correspondence supports the interpretation of cytochrome P450 data by Kincaid and Sligar [M. Gregory, P.J. Mak, S.G. Sligar, J.R. Kincaid, Angew. Chem. Int. Ed. 125 (2013) 5450-5453], involving steering between hydroxylation and lyase reaction channels by differential H-bonds. Similar channeling between the first and second steps of the nitric oxide synthase reaction is likely.

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Reaction Intermediates of Nitric Oxide Synthase from Deinococcus radiodurans as Revealed by Pulse Radiolysis: Evidence for Intramolecular Electron Transfer from Biopterin to FeII-O2 Complex

Nitric oxide synthase (NOS) is a cytochrome P450-type mono-oxygenase that catalyzes the oxidation of l-arginine (Arg) to nitric oxide (NO) through a reaction intermediate N-hydroxy-l-arginine (NHA). The mechanism underlying the reaction catalyzed by NOS from Deinococcus radiodurans was investigated using pulse radiolysis. Radiolytically generated hydrated electrons reduced the heme iron of NOS within 2 μs. Subsequently, ferrous heme reacted with O2 to form a ferrous-dioxygen intermediate with a second-order rate constant of 2.8 × 108 M-1 s-1. In the tetrahydrofolate (H4F)-bound enzyme, the ferrous-dioxygen intermediate was found to decay an another intermediate with a first-order rate constant of 2.2 × 103 s-1. The spectrum of the intermediate featured an absorption maximum at 440 nm and an absorption minimum at 390 nm. In the absence of H4F, this step did not proceed, suggesting that H4F was reduced with the ferrous-dioxygen intermediate to form a second intermediate. The intermediate further converted to the original ferric form with a first-order rate constant of 4 s-1. A similar intermediate could be detected after pulse radiolysis in the presence of NHA, although the intermediate decayed more slowly (0.5 s-1). These data suggested that a common catalytically active intermediate involved in the substrate oxidation of both Arg and NHA may be formed during catalysis. In addition, we investigated the solvent isotope effects on the kinetics of the intermediate after pulse radiolysis. Our experiments revealed dramatic kinetic solvent isotope effects on the conversion of the intermediate to the ferric form, of 10.5 and 2.5 for Arg and NHA, respectively, whereas the faster phases were not affected. These data suggest that the proton transfer in DrNOS is the rate-limiting reaction of the intermediate with the substrates.

Emerging Roles of Nitric Oxide Synthase in Bacterial Physiology

Nitric oxide (NO) is a potent inhibitor of diverse cellular processes in bacteria. Therefore, it was surprising to discover that several bacterial species, primarily Gram-positive organisms, harboured a gene encoding nitric oxide synthase (NOS). Recent attempts to characterize bacterial NOS (bNOS) have resulted in the discovery of structural features that may allow it to function as a NO dioxygenase and produce nitrate in addition to NO. Consistent with this characterization, investigations into the biological function of bNOS have also emphasized a role for NOS-dependent nitrate and nitrite production in aerobic and microaerobic respiration. In this review, we aim to compare, contrast, and summarize the structure, biochemistry, and biological role of bNOS with mammalian NOS and discuss how recent advances in our understanding of bNOS have enabled efforts at designing inhibitors against it.

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).

Dissecting structural and electronic effects in inducible nitric oxide synthase

Nitric oxide synthases (NOSs) are haem-thiolate enzymes that catalyse the conversion of L-arginine (L-Arg) into NO and citrulline. Inducible NOS (iNOS) is responsible for delivery of NO in response to stressors during inflammation. The catalytic performance of iNOS is proposed to rely mainly on the haem midpoint potential and the ability of the substrate L-Arg to provide a hydrogen bond for oxygen activation (O-O scission). We present a study of native iNOS compared with iNOS-mesohaem, and investigate the formation of a low-spin ferric haem-aquo or -hydroxo species (P) in iNOS mutant W188H substituted with mesohaem. iNOS-mesohaem and W188H-mesohaem were stable and dimeric, and presented substrate-binding affinities comparable to those of their native counterparts. Single turnover reactions catalysed by iNOSoxy with L-Arg (first reaction step) or N-hydroxy-L-arginine (second reaction step) showed that mesohaem substitution triggered higher rates of Fe(II)O₂ conversion and altered other key kinetic parameters. We elucidated the first crystal structure of a NOS substituted with mesohaem and found essentially identical features compared with the structure of iNOS carrying native haem. This facilitated the dissection of structural and electronic effects. Mesohaem substitution substantially reduced the build-up of species P in W188H iNOS during catalysis, thus increasing its proficiency towards NO synthesis. The marked structural similarities of iNOSoxy containing native haem or mesohaem indicate that the kinetic behaviour observed in mesohaem-substituted iNOS is most heavily influenced by electronic effects rather than structural alterations.

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

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

Redox potential determines the reaction mechanism of HNO donors with Mn and Fe porphyrins: defining the better traps

Azanone ((1)HNO, nitroxyl) is a highly reactive molecule with interesting chemical and biological properties. Like nitric oxide (NO), its main biologically related targets are oxygen, thiols, and metalloproteins, particularly heme proteins. As HNO dimerizes with a rate constant between 10(6) and 10(7) M(-1) s(-1), reactive studies are performed using donors, which are compounds that spontaneously release HNO in solution. In the present work, we studied the reaction mechanism and kinetics of two azanone donors Angelís Salt and toluene sulfohydroxamic acid (TSHA) with eight different Mn porphyrins as trapping agents. These porphyrins differ in their total peripheral charge (positively or negatively charged) and in their Mn(III)/Mn(II) reduction potential, showing for each case positive (oxidizing) and negative (reducing) values. Our results show that the reduction potential determines the azanone donor reaction mechanism. While oxidizing porphyrins accelerate decomposition of the donor, reducing porphyrins react with free HNO. Our results also shed light into the donor decomposition mechanism using ab initio methods and provide a thorough analysis of which MnP are the best candidates for azanone trapping and quantification experiments.

An introduction to nitric oxide sensing and response in bacteria

Nitric oxide (NO) is a radical gas that has been intensively studied for its role as a bacteriostatic agent. NO reacts in complex ways with biological molecules, especially metal centers and other radicals, to generate other bioactive compounds that inhibit enzymes, oxidize macromolecules, and arrest bacterial growth. Bacteria encounter not only NO derived from the host during infection but also NO derived from other bacteria and inorganic sources. The transcriptional responses used by bacteria to respond to NO are diverse but usually involve an iron-containing transcription factor that binds NO and alters its affinity for either DNA or factors involved in transcription, leading to the production of enzymatic tolerance systems. Some of these systems, such as flavohemoglobin and flavorubredoxin, directly remove NO. Some do not but are still important for NO tolerance through other mechanisms. The targets of NO that are protected by these systems include many metabolic pathways such as the tricarboxylic acid cycle and branched chain amino acid synthesis. This chapter discusses these topics and others and serves as a general introduction to microbial NO biology.

Cryptochrome in sponges: a key molecule linking photoreception with phototransduction

Sponges (phylum: Porifera) react to external light or mechanical signals with contractile or metabolic reactions and are devoid of any nervous or muscular system. Furthermore, elements of a photoreception/phototransduction system exist in those animals. Recently, a cryptochrome-based photoreceptor system has been discovered in the demosponge. The assumption that in sponges the siliceous skeleton acts as a substitution for the lack of a nervous system and allows light signals to be transmitted through its glass fiber network is supported by the findings that the first spicules are efficient light waveguides and the second sponges have the enzymatic machinery for the generation of light. Now, we have identified/cloned in Suberites domuncula two additional potential molecules of the sponge cryptochrome photoreception system, the guanine nucleotide-binding protein β subunit, related to β-transducin, and the nitric oxide synthase (NOS)-interacting protein. Cryptochrome and NOSIP are light-inducible genes. The studies show that the NOS inhibitor L-NMMA impairs both morphogenesis and motility of the cells. Finally, we report that the function of primmorphs to produce reactive nitrogen species can be abolished by a NOS inhibitor. We propose that the sponge cryptochrome-based photoreception system, through which photon signals are converted into radicals, is coupled to the NOS apparatus.