nitric oxide synthase

Arginine-derived NO synthesis has been identified in mammals, fish, birds, invertebrates, and bacteria.{{cite book |vauthors=Liu Q, Gross SS | title = Nitric Oxide Part A: Sources and Detection of NO; NO Synthase | chapter = Binding sites of nitric oxide synthases | series = Methods in Enzymology | volume = 268 | pages = 311–24 | year = 1996 | pmid = 8782597 | doi = 10.1016/S0076-6879(96)68033-1 | isbn = 9780121821692 }}

Best studied are mammals, where three distinct genes encode NOS isozymes: neuronal (nNOS or NOS-1), cytokine-inducible (iNOS or NOS-2) and endothelial (eNOS or NOS-3). iNOS and nNOS are soluble and found predominantly in the cytosol, while eNOS is membrane associated. Evidence has been found for NO signaling in plants, but plant genomes are devoid of homologs to the superfamily which generates NO in other kingdoms.{{cite journal |vauthors=Knowles RG, Moncada S | title = Nitric oxide synthases in mammals | journal = Biochem. J. | volume = 298 | issue = 2| pages = 249–58 |date=March 1994 | doi = 10.1042/bj2980249 | pmid = 7510950 | pmc = 1137932 }}

Image:NOSreaction.svg

Nitric oxide synthases produce NO by catalysing a five-electron oxidation of a guanidino nitrogen of L-arginine (L-Arg). Oxidation of L-Arg to L-citrulline occurs via two successive monooxygenation reactions producing N^ω-hydroxy-L-arginine (NOHLA) as an intermediate. 2 mol of O₂ and 1.5 mol of NADPH are consumed per mole of NO formed.

In mammals, the endothelial isoform is the primary signal generator in the control of vascular tone, insulin secretion, and airway tone, is involved in regulation of cardiac function and angiogenesis (growth of new blood vessels). NO produced by eNOS has been shown to be a vasodilator identical to the endothelium-derived relaxing factor produced in response to shear from increased blood flow in arteries. This dilates blood vessels by relaxing smooth muscle in their linings. eNOS is the primary controller of smooth muscle tone. NO activates guanylate cyclase, which induces smooth muscle relaxation by:

• Increased intracellular cGMP, which inhibits calcium entry into the cell, and decreases intracellular calcium concentrations
• Activation of K⁺ channels, which leads to hyperpolarization and relaxation
• Stimulates a cGMP-dependent protein kinase that activates myosin light chain phosphatase, the enzyme that dephosphorylates myosin light chains, which leads to smooth muscle relaxation.

eNOS plays a critical role in embryonic heart development and morphogenesis of coronary arteries and cardiac valves.{{cite journal |vauthors=Liu Y, Feng Q | title = NOing the heart: role of nitric oxide synthase-3 in heart development | journal = Differentiation | volume = 84 | issue = 1 | pages = 54–61 |date=July 2012 | pmid = 22579300 | doi = 10.1016/j.diff.2012.04.004 }}

The neuronal isoform is involved in the development of nervous system. It functions as a retrograde neurotransmitter important in long term potentiation and hence is likely to be important in memory and learning. nNOS has many other physiological functions, including regulation of cardiac function and peristalsis and sexual arousal in males and females. An alternatively spliced form of nNOS is a major muscle protein that produces signals in response to calcium release from the SR. nNOS in the heart protects against cardiac arrhythmia induced by myocardial infarction.{{cite journal |vauthors=Burger DE, Lu X, Lei M, Xiang FL, Hammoud L, Jiang M, Wang H, Jones DL, Sims SM, Feng Q | title = Neuronal nitric oxide synthase protects against myocardial infarction-induced ventricular arrhythmia and mortality in mice | journal = Circulation | volume = 120 | issue = 14 | pages = 1345–54 |date=October 2009 | pmid = 19770398 | doi = 10.1161/CIRCULATIONAHA.108.846402 | doi-access = free }}

The primary receiver for NO produced by eNOS and nNOS is soluble guanylate cyclase, but many secondary targets have been identified. S-nitrosylation appears to be an important mode of action.

The inducible isoform iNOS produces large amounts of NO as a defense mechanism. It is synthesized by many cell types in response to cytokines and is an important factor in the response of the body to attack by parasites, bacterial infection, and tumor growth. It is also the cause of septic shock and may play a role in many diseases with an autoimmune etiology.

NOS signaling is involved in development and in fertilization in vertebrates. It has been implicated in transitions between vegetative and reproductive states in invertebrates, and in differentiation leading to spore formation in slime molds. NO produced by bacterial NOS is protective against oxidative damage.

NOS activity has also been correlated with major depressive episodes (MDEs) in the context of major depressive disorder, in a large case-control treatment study published in mid-2021. 460 patients with a current major depressive episode were compared to 895 healthy patients, and by measuring L-citrulline/L-arginine ratio before and after 3–6 months of antidepressant treatment, results indicate that patients in a major depressive episode have significantly lower NOS activity compared to healthy patients, whilst treatment with antidepressants significantly elevated NOS activity levels in patients in a major depressive episode.{{cite journal|url=https://www.cambridge.org/core/journals/psychological-medicine/article/abs/nitric-oxide-synthase-activity-in-major-depressive-episodes-before-and-after-antidepressant-treatment-results-of-a-large-casecontrol-treatment-study/55DB96AEB6B05D0E9E792A4D97FD0A6C#|author1=E. Loeb|author2=K. El Asmar|journal=Psychological Medicine|author3=S. Trabado|author4=F. Gressier|author5=R. Colle|author6=A. Rigal|author7=S. Martin|author8=C. Verstuyft|author9=B. Fève|author10=P. Chanson|author11=L. Becquemont|author12=E. Corruble|title=Nitric Oxide Synthase activity in major depressive episodes before and after antidepressant treatment: Results of a large case-control treatment study|date=January 2022|accessdate=26 December 2021|volume=52|issue=1|pages=80–89|doi=10.1017/S0033291720001749|pmid=32524920|s2cid=219587961|url-access=subscription}}

Different members of the NOS family are encoded by separate genes.{{cite journal |vauthors=Taylor BS, Kim YM, Wang Q, Shapiro RA, Billiar TR, Geller DA | title = Nitric oxide down-regulates hepatocyte-inducible nitric oxide synthase gene expression | journal = Arch Surg | volume = 132 | issue = 11 | pages = 1177–83 |date=November 1997 | pmid = 9366709 | doi = 10.1001/archsurg.1997.01430350027005}} There are three known isoforms in mammals, two are constitutive (cNOS) and the third is inducible (iNOS).{{cite journal | author = Stuehr DJ | title = Mammalian nitric oxide synthases | journal = Biochim. Biophys. Acta | volume = 1411 | issue = 2–3 | pages = 217–30 |date=May 1999 | pmid = 10320659 | doi = 10.1016/S0005-2728(99)00016-X| doi-access = }} Cloning of NOS enzymes indicates that cNOS include both brain constitutive (NOS1) and endothelial constitutive (NOS3); the third is the inducible (NOS2) gene. Recently, NOS activity has been demonstrated in several bacterial species, including the notorious pathogens Bacillus anthracis and Staphylococcus aureus.{{cite journal |vauthors=Gusarov I, Starodubtseva M, Wang ZQ, McQuade L, Lippard SJ, Stuehr DJ, Nudler E | title = Bacterial Nitric-oxide Synthases Operate without a Dedicated Redox Partner | journal = J. Biol. Chem. | volume = 283 | issue = 19 | pages = 13140–7 |date=May 2008 | pmid = 18316370 | pmc = 2442334 | doi = 10.1074/jbc.M710178200 | doi-access = free }}

The different forms of NO synthase have been classified as follows:

Neuronal NOS (nNOS) produces NO in nervous tissue in both the central and peripheral nervous systems. Its functions include:{{cite journal|last1=Förstermann|first1=Ulrich|last2=Sessa|first2=William|title=Nitric oxide synthases: regulation and function|journal=European Heart Journal|date=Apr 2012|volume=33|issue=7|pages=829–837|pmc=3345541|doi=10.1093/eurheartj/ehr304|pmid=21890489}}

• Synaptic plasticity in the central nervous system (CNS)
• Smooth muscle relaxation
• Central regulation of blood pressure
• Vasodilatation via peripheral nitrergic nerves

Neuronal NOS also performs a role in cell communication and is associated with plasma membranes. nNOS action can be inhibited by NPA (N-propyl-L-arginine). This form of the enzyme is specifically inhibited by 7-nitroindazole.{{cite journal |vauthors=Southan GJ, Szabó C | title = Selective pharmacological inhibition of distinct nitric oxide synthase isoforms | journal = Biochem. Pharmacol. | volume = 51 | issue = 4 | pages = 383–94 |date=February 1996 | pmid = 8619882 | doi = 10.1016/0006-2952(95)02099-3 }}

The subcellular localisation of nNOS in skeletal muscle is mediated by anchoring of nNOS to dystrophin. nNOS contains an additional N-terminal domain, the PDZ domain.{{cite journal |vauthors=Ponting CP, Phillips C | title = DHR domains in syntrophins, neuronal NO synthases and other intracellular proteins | journal = Trends Biochem. Sci. | volume = 20 | issue = 3 | pages = 102–3 |date=March 1995 | pmid = 7535955 | doi = 10.1016/S0968-0004(00)88973-2 }}

The gene coding for nNOS is located on Chromosome 12.{{cite journal |vauthors=Knowles RG, Moncada S | title = Nitric oxide synthases in mammals | journal = Biochem. J. | volume = 298 | issue = 2| pages = 249–58 |date=March 1994 | pmid = 7510950 | pmc = 1137932 | doi = 10.1042/bj2980249}}

As opposed to the critical calcium-dependent regulation of constitutive NOS enzymes (nNOS and eNOS), iNOS has been described as calcium-insensitive, likely due to its tight non-covalent interaction with calmodulin (CaM) and Ca²⁺. The gene coding for iNOS is located on Chromosome 17. While evidence for ‘baseline’ iNOS expression has been elusive, IRF1 and NF-κB-dependent activation of the inducible NOS promoter supports an inflammation mediated stimulation of this transcript. iNOS produces large quantities of NO upon stimulation, such as by proinflammatory cytokines (e.g. Interleukin-1, Tumor necrosis factor alpha and Interferon gamma).{{cite journal |vauthors=Green SJ, Scheller LF, Marletta MA, Seguin MC, Klotz FW, Slayter M, Nelson BJ, Nacy CA | title = Nitric oxide: cytokine-regulation of nitric oxide in host resistance to intracellular pathogens | journal = Immunol. Lett. | volume = 43 | issue = 1–2 | pages = 87–94 |date=December 1994 | pmid = 7537721 | doi = 10.1016/0165-2478(94)00158-8| hdl = 2027.42/31140 | url = https://deepblue.lib.umich.edu/bitstream/2027.42/31140/1/0000037.pdf | hdl-access = free }}

Induction of the high-output iNOS usually occurs in an oxidative environment, and thus high levels of NO have the opportunity to react with superoxide leading to peroxynitrite formation and cell toxicity. These properties may define the roles of iNOS in host immunity, enabling its participation in anti-microbial and anti-tumor activities as part of the oxidative burst of macrophages.{{cite journal |vauthors=Mungrue IN, Husain M, Stewart DJ | title = The role of NOS in heart failure: lessons from murine genetic models | journal = Heart Fail Rev | volume = 7 | issue = 4 | pages = 407–22 |date=October 2002 | pmid = 12379825 | doi = 10.1023/a:1020762401408| s2cid = 26600958 }}

It has been suggested that pathologic generation of nitric oxide through increased iNOS production may decrease tubal ciliary beats and smooth muscle contractions and thus affect embryo transport, which may consequently result in ectopic pregnancy.{{cite journal |vauthors=Al-Azemi M, Refaat B, Amer S, Ola B, Chapman N, Ledger W | title = The expression of inducible nitric oxide synthase in the human fallopian tube during the menstrual cycle and in ectopic pregnancy | journal = Fertil. Steril. | volume = 94 | issue = 3 | pages = 833–40 |date=August 2010 | pmid = 19482272 | doi = 10.1016/j.fertnstert.2009.04.020 }}

{{Main|Endothelial NOS}}

Endothelial NOS (eNOS), also known as nitric oxide synthase 3 (NOS3), generates NO in blood vessels and is involved with regulating vascular function. The gene coding for eNOS is located on Chromosome 7. A constitutive Ca²⁺ dependent NOS provides a basal release of NO. eNOS localizes to caveolae, a plasma membrane domain primarily composed of the protein caveolin 1, and to the Golgi apparatus. These two eNOS populations are distinct, but are both necessary for proper NO production and cell health.{{cite journal |vauthors = Maulik SJ, Junyi Z, Aneesh TV, Yamuna K | title = A DNA-based fluorescent probe maps NOS3 activity with subcellular spatial resolution

| journal = Nat. Chem. Biol. | issue = 6 | pages = 660–6 | date=March 2020 | volume = 16

| pmid = 32152543 | doi = 10.1038/s41589-020-0491-3| s2cid = 212642840

}} eNOS localization to endothelial membranes is mediated by cotranslational N-terminal myristoylation and post-translational palmitoylation.{{cite journal |vauthors=Liu J, Hughes TE, Sessa WC | title = The First 35 Amino Acids and Fatty Acylation Sites Determine the Molecular Targeting of Endothelial Nitric Oxide Synthase into the Golgi Region of Cells: A Green Fluorescent Protein Study | journal = J. Cell Biol. | volume = 137 | issue = 7 | pages = 1525–35 |date=June 1997 | pmid = 9199168 | pmc = 2137822 | doi = 10.1083/jcb.137.7.1525 }} As an essential co-factor for nitric oxide synthase, tetrahydrobiopterin (BH4) supplementation has shown beneficial results for the treatment of endothelial dysfunction in animal experiments and clinical trials, although the tendency of BH4 to become oxidized to BH2 remains a problem.{{cite journal | vauthors = Yuyun MF, Ng LL, Ng GA | title=Endothelial dysfunction, endothelial nitric oxide bioavailability, tetrahydrobiopterin, and 5-methyltetrahydrofolate in cardiovascular disease. Where are we with therapy? | journal= Microvascular Research | volume=119 | pages=7–12 | year=2018 | doi= 10.1016/j.mvr.2018.03.012 | pmid=29596860}}

The enzymes exist as homodimers. In eukaryotes, each monomer consisting of two major regions: an N-terminal oxygenase domain, which belongs to the class of heme-thiolate proteins, and a multi-domain C-terminal reductase, which is homologous to NADPH:cytochrome P450 reductase ({{EC number|1.6.2.4}}) and other flavoproteins. The FMN binding domain is homologous to flavodoxins, and the two domain fragment containing the FAD and NADPH binding sites is homologous to flavodoxin-NADPH reductases. The interdomain linker between the oxygenase and reductase domains contains a calmodulin-binding sequence. The oxygenase domain is a unique extended beta sheet cage with binding sites for heme and pterin.

NOSs can be dimeric, calmodulin-dependent or calmodulin-containing cytochrome p450-like hemoprotein that combines reductase and oxygenase catalytic domains in one dimer, bear both flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), and carry out a 5`-electron oxidation of non-aromatic amino acid arginine with the aid of tetrahydrobiopterin.{{cite journal |vauthors=Chinje EC, Stratford IJ | title = Role of nitric oxide in growth of solid tumours: a balancing act | journal = Essays Biochem. | volume = 32 | pages = 61–72 | year = 1997 | pmid = 9493011 }}

All three isoforms (each of which is presumed to function as a homodimer during activation) share a carboxyl-terminal reductase domain homologous to the cytochrome P450 reductase. They also share an amino-terminal oxygenase domain containing a heme prosthetic group, which is linked in the middle of the protein to a calmodulin-binding domain. Binding of calmodulin appears to act as a "molecular switch" to enable electron flow from flavin prosthetic groups in the reductase domain to heme. This facilitates the conversion of O₂ and L-arginine to NO and L-citrulline. The oxygenase domain of each NOS isoform also contains an BH₄ prosthetic group, which is required for the efficient generation of NO. Unlike other enzymes where BH₄ is used as a source of reducing equivalents and is recycled by dihydrobiopterin reductase ({{EC number|1.5.1.33}}), BH₄ activates heme-bound O₂ by donating a single electron, which is then recaptured to enable nitric oxide release.

The first nitric oxide synthase to be identified was found in neuronal tissue (NOS1 or nNOS); the endothelial NOS (eNOS or NOS3) was the third to be identified. They were originally classified as "constitutively expressed" and "Ca²⁺ sensitive" but it is now known that they are present in many different cell types and that expression is regulated under specific physiological conditions.

In NOS1 and NOS3, physiological concentrations of Ca²⁺ in cells regulate the binding of calmodulin to the "latch domains", thereby initiating electron transfer from the flavins to the heme moieties. In contrast, calmodulin remains tightly bound to the inducible and Ca²⁺-insensitive isoform (iNOS or NOS2) even at a low intracellular Ca²⁺ activity, acting essentially as a subunit of this isoform.

Nitric oxide may itself regulate NOS expression and activity. Specifically, NO has been shown to play an important negative feedback regulatory role on NOS3, and therefore vascular endothelial cell function.{{Cite journal|last1=Kopincová|first1=Jana|last2=Púzserová|first2=Angelika|last3=Bernátová|first3=Iveta|date=2011-06-01|title=Biochemical aspects of nitric oxide synthase feedback regulation by nitric oxide|journal=Interdisciplinary Toxicology|language=en|volume=4|issue=2|pages=63–8|doi=10.2478/v10102-011-0012-z|issn=1337-9569|pmc=3131676|pmid=21753901}} This process, known formally as S-nitrosation (and referred to by many in the field as S-nitrosylation), has been shown to reversibly inhibit NOS3 activity in vascular endothelial cells. This process may be important because it is regulated by cellular redox conditions and may thereby provide a mechanism for the association between "oxidative stress" and endothelial dysfunction. In addition to NOS3, both NOS1 and NOS2 have been found to be S-nitrosated, but the evidence for dynamic regulation of those NOS isoforms by this process is less complete{{Citation needed|date=December 2014}}. In addition, both NOS1 and NOS2 have been shown to form ferrous-nitrosyl complexes in their heme prosthetic groups that may act partially to self-inactivate these enzymes under certain conditions{{Citation needed|date=December 2014}}. The rate-limiting step for the production of nitric oxide may well be the availability of L-arginine in some cell types. This may be particularly important after the induction of NOS2.

Ronopterin (VAS-203), also known as 4-amino-tetrahydrobiopterin (4-ABH₄), an analogue of BH₄ (a cofactor of NOS), is an NOS inhibitor that is under development as a neuroprotective agent for the treatment of traumatic brain injury.[http://adisinsight.springer.com/drugs/800023224] Other NOS inhibitors that have been or are being researched for possible clinical use include cindunistat, A-84643, ONO-1714, L-NOARG, NCX-456, VAS-2381, GW-273629, NXN-462, CKD-712, KD-7040, and guanidinoethyldisulfide, [https://pubchem.ncbi.nlm.nih.gov/compound/2733516 TFPI] among others.

NOS is found in many groups of eukaryotes (metazoans, fungi, algae, plants), mostly sharing the fused oxygenase-CAM-flavin-FNR structure of mammalian NOS. There can, however, be important changes to the structure. For example, a number of algal NOS has a C-terminal globin domain, which could confer additional functionality.{{cite journal |last1=Chatelain |first1=P |last2=Astier |first2=J |last3=Wendehenne |first3=D |last4=Rosnoblet |first4=C |last5=Jeandroz |first5=S |title=Identification of Partner Proteins of the Algae Klebsormidium nitens NO Synthases: Toward a Better Understanding of NO Signaling in Eukaryotic Photosynthetic Organisms. |journal=Frontiers in Plant Science |date=2021 |volume=12 |pages=797451 |doi=10.3389/fpls.2021.797451 |doi-access=free |pmid=35003186|pmc=8728061 }}

Bacterial NOS (bNOS) has been shown to protect bacteria against oxidative stress, diverse antibiotics, and host immune response. bNOS plays a key role in the transcription of superoxide dismutase (SodA). Bacteria late in the log phase who do not possess bNOS fail to upregulate SodA, which disables the defenses against harmful oxidative stress. Initially, bNOS may have been present to prepare the cell for stressful conditions but now seems to help shield the bacteria against conventional antimicrobials. As a clinical application, a bNOS inhibitor could be produced to decrease the load of Gram positive bacteria.{{cite journal |vauthors=Gusarov I, Nudler E | title = NO-mediated cytoprotection: Instant adaptation to oxidative stress in bacteria | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 102 | issue = 39 | pages = 13855–60 |date=September 2005 | pmid = 16172391 | pmc = 1236549 | doi = 10.1073/pnas.0504307102 | bibcode = 2005PNAS..10213855G | doi-access = free }}{{cite journal |vauthors=Gusarov I, Shatalin K, Starodubtseva M, Nudler E | title = Endogenous Nitric Oxide Protects Bacteria Against a Wide Spectrum of Antibiotics | journal = Science | volume = 325 | issue = 5946 | pages = 1380–4 |date=September 2009 | pmid = 19745150 | pmc = 2929644 | doi = 10.1126/science.1175439 | bibcode = 2009Sci...325.1380G }}

They seem to have a similar function in archaea.{{cite journal |last1=Orsini |first1=SS |last2=James |first2=KL |last3=Reyes |first3=DJ |last4=Couto-Rodriguez |first4=RL |last5=Gulko |first5=MK |last6=Witte |first6=A |last7=Carroll |first7=RK |last8=Rice |first8=KC |title=Bacterial-like nitric oxide synthase in the haloalkaliphilic archaeon Natronomonas pharaonis. |journal=MicrobiologyOpen |date=November 2020 |volume=9 |issue=11 |pages=e1124 |doi=10.1002/mbo3.1124 |pmid=33306280|pmc=7658456 }}

They exist in a wide variety of bacteria, including Gram negative ones (the aforementioned Sorangium cellulosum is Gram negative). The fused [Fe-S] NOS is by no means rare among bacteria, being seen across Cyanobacteria and Proteobacteria.{{cite journal |last1=Santolini |first1=J |title=What does "NO-Synthase" stand for ? |journal=Frontiers in Bioscience (Landmark Edition) |date=1 January 2019 |volume=24 |issue=1 |pages=133–171 |doi=10.2741/4711 |pmid=30468649}}

• Biological functions of nitric oxide

{{notelist}}

{{Reflist|2}}

• {{MeshName|Nitric+oxide+synthase}}
• [http://nobelprize.org/nobel_prizes/medicine/laureates/1998/ The Nobel Prize in Physiology or Medicine 1998]
• University of Edinburgh, School of Chemistry - [https://web.archive.org/web/20130212143027/http://homepages.ed.ac.uk/sd01/nospage.htm NO Synthase]
• [http://www.proteopedia.org/wiki/index.php/Nitric_oxide_synthase/ Nitric Oxide Synthase in Proteopedia]

{{Neurotransmitter metabolism enzymes}}

{{Dioxygenases}}

{{Enzymes}}

{{Nitric oxide signaling}}

{{Portal bar|Biology|border=no}}

{{DEFAULTSORT:Nitric Oxide Synthase}}

Category:EC 1.14.13

Category:Heme-thiolate proteins