Editing Nitric oxide synthase (section)

== Structure ==

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 [[protein dimer|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.<ref name="pmid9493011">{{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 }}</ref>

All three [[isoform]]s (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 O<sub>2</sub> and <small>L</small>-arginine to [[nitric oxide|NO]] and <small>L</small>-citrulline. The oxygenase domain of each NOS isoform also contains an BH<sub>4</sub> prosthetic group, which is required for the efficient generation of NO. Unlike other enzymes where BH<sub>4</sub> is used as a source of reducing equivalents and is recycled by [[dihydrobiopterin reductase]] ({{EC number|1.5.1.33}}), BH<sub>4</sub> activates heme-bound O<sub>2</sub> 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 "Ca<sup>2+</sup> sensitive" but it is now known that they are present in many different [[cell (biology)|cell]] types and that expression is regulated under specific physiological conditions.

In NOS1 and NOS3, physiological concentrations of Ca<sup>2+</sup> in cells regulate the binding of calmodulin to the "latch domains", thereby initiating electron transfer from the [[Flavin group|flavins]] to the [[heme]] moieties. In contrast, calmodulin remains tightly bound to the inducible and Ca<sup>2+</sup>-insensitive isoform (iNOS or NOS2) even at a low intracellular Ca<sup>2+</sup> 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.<ref>{{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}}</ref> 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 <small>L</small>-arginine in some cell types. This may be particularly important after the [[Enzyme induction and inhibition|induction]] of NOS2.