PTGS2 (COX-2), convertsarachidonic acid (AA) to prostaglandin endoperoxide H2. PTGSs are targets forNSAIDs and PTGS2 (COX-2) specific inhibitors called coxibs. PTGS-2 is a sequence homodimer. Eachmonomer of the enzyme has aperoxidase and a PTGS (COX)active site. The PTGS (COX) enzymes catalyze the conversion of AA toprostaglandins in two steps. First, hydrogen is abstracted from carbon 13 of arachidonic acid, and then two molecules of oxygen are added by the PTGS2 (COX-2), givingPGG2. Second, PGG2 is reduced toPGH2 in the peroxidase active site. The synthesized PGH2 is converted to prostaglandins (PGD2,PGE2,PGF2α),prostacyclin (PGI2), orthromboxane A2 by tissue-specific isomerases (Figure 2).[6]
While metabolizing arachidonic acid primarily to PGG2, COX-2 also converts this fatty acid to small amounts of a racemic mixture of15-hydroxyicosatetraenoic acids (i.e., 15-HETEs) composed of ~22% 15(R)-HETE and ~78% 15(S)-HETEstereoisomers as well as a small amount of 11(R)-HETE.[7] The two 15-HETE stereoisomers have intrinsic biological activities but, perhaps more importantly, can be further metabolized to a major class of agents, thelipoxins. Furthermore,aspirin-treated COX-2 metabolizes arachidonic acid almost exclusively to 15(R)-HETE which product can be further metabolized to epi-lipoxins.[8] The lipoxins and epi-lipoxins are potent anti-inflammatory agents and may contribute to the overall activities of the two COX's as well as to aspirin.[citation needed]
COX-2 is naturally inhibited bycalcitriol (the active form of vitamin D).[9][10]
Arachidonic acid bound to the PTGS2 (COX-2) enzyme.Polar interactions betweenarachidonic acid (cyan) andSer-530 andTyr-385 residues are shown with yellow dashed lines. The substrate is stabilized by hydrophobic interactions.[11] Mechanism of COX activation and catalysis. Ahydroperoxide oxidizes the heme to aferryl-oxo derivative that either isreduced in the first step of theperoxidase cycle or oxidizesTyrosine 385 to atyrosyl radical. The tyrosyl radical can then oxidize the 13-pro(S) hydrogen ofarachidonic acid to initiate the COX cycle.
Both the peroxidase and PTGS activities are inactivated during catalysis by mechanism-based, first-order processes, which means that PGHS-2 peroxidase or PTGS activities fall to zero within 1–2 minutes, even in the presence of sufficient substrates.[12][13][14]
The conversion of arachidonic acid to PGG2 can be shown as a series ofradical reactions analogous to polyunsaturatedfatty acidautoxidation.[15] The 13-pro(S) -hydrogen is abstracted and dioxygen traps thepentadienyl radical at carbon 11. The 11-peroxyl radical cyclizes at carbon 9 and the carbon-centered radical generated at C-8 cyclizes at carbon 12, generating theendoperoxide. Theallylic radical generated is trapped by dioxygen at carbon 15 to form the 15-(S) -peroxyl radical; this radical is then reduced toPGG2. This is supported by the following evidence: 1) a significantkinetic isotope effect is observed for the abstraction of the 13-pro (S)-hydrogen; 2) carbon-centered radicals are trapped duringcatalysis;[16] 3) small amounts ofoxidation products are formed due to the oxygen trapping of an allylic radical intermediate at positions 13 and 15.[17][18]
Another mechanism in which the 13-pro (S)-hydrogen isdeprotonated and thecarbanion isoxidized to aradical is theoretically possible. However, oxygenation of 10,10-difluoroarachidonic acid to 11-(S)-hydroxyeicosa-5,8,12,14-tetraenoic acid is not consistent with the generation of a carbanion intermediate because it would eliminate fluoride to form a conjugated diene.[19] The absence of endoperoxide-containing products derived from 10,10-difluoroarachidonic acid has been thought to indicate the importance of a C-10 carbocation in PGG2 synthesis.[20] However, the cationic mechanism requires that endoperoxide formation comes before the removal of the 13-pro (S)-hydrogen. This is not consistent with the results of the isotope experiments of arachidonic acid oxygenation.[21]
As shown, different ligands bind either the allosteric or the catalytic subunit. Allosteric subunit binds a non-substrate, activating FA (e.g., palmitic acid). The allosteric subunit with bound fatty acid activates the catalytic subunit by decreasing the Km for AA.[22]
PTGS2 (COX-2) exists as a homodimer, each monomer with a molecular mass of about 70 kDa. The tertiary and quaternary structures of PTGS1 (COX-1) and PTGS2 (COX-2) enzymes are almost identical. Each subunit has three different structural domains: a shortN-terminal epidermal growth factor (EGF) domain; anα-helical membrane-binding moiety; and aC-terminal catalytic domain. PTGS (COX, which can be confused with "cytochrome oxidase") enzymes aremonotopic membrane proteins; the membrane-binding domain consists of a series ofamphipathic α helices with severalhydrophobicamino acids exposed to a membrane monolayer. PTGS1 (COX-1) and PTGS2 (COX-2) are bifunctional enzymes that carry out two consecutive chemical reactions in spatially distinct but mechanisticallycoupled active sites. Both thecyclooxygenase and theperoxidase active sites are located in the catalytic domain, which accounts for approximately 80% of the protein. The catalytic domain ishomologous to mammalian peroxidases such asmyeloperoxidase.[23][24]
It has been found that human PTGS2 (COX-2) functions as a conformational heterodimer having a catalytic monomer (E-cat) and an allosteric monomer (E-allo).Heme binds only to theperoxidase site of E-cat while substrates, as well as certaininhibitors (e.g.celecoxib), bind the COX site of E-cat. E-cat is regulated by E-allo in a way dependent on what ligand is bound to E-allo.Substrate and non-substratefatty acids (FAs) and some PTGS (COX) inhibitors (e.g.naproxen) preferentially bind to the PTGS (COX) site of E-allo.Arachidonic acid can bind to E-cat and E-allo, but the affinity of AA for E-allo is 25 times that for Ecat. Palmitic acid, an efficacious stimulator ofhuPGHS-2, binds only E-allo in palmitic acid/murine PGHS-2 co-crystals. Non-substrate FAs can potentiate orattenuate PTGS (COX) inhibitors depending on thefatty acid and whether the inhibitor binds E-cat or E-allo. Studies suggest that the concentration and composition of the free fatty acid pool in the environment in which PGHS-2 functions in cells, also referred to as the FA tone, is a key factor regulating the activity of PGHS-2 and its response to PTGS (COX) inhibitors.[22]
NSAID (non-specific inhibitor of PTGS2 (COX-2))flurbiprofen (green) bound to PTGS2 (COX-2). Flurbiprofen is stabilized via hydrophobic interactions and polar interactions (Tyr-355 andArg-120).[25]
PTGS2 (COX-2) is unexpressed under normal conditions in most cells, but elevated levels are found duringinflammation. PTGS1 (COX-1) is constitutively expressed in many tissues and is the predominant form in gastric mucosa and in the kidneys.[26] Inhibition of PTGS1 (COX-1) reduces thebasal production of cytoprotectivePGE2 andPG12 in thestomach, which may contribute togastric ulceration. Since PTGS2 (COX-2) is generally expressed only in cells whereprostaglandins are upregulated (e.g., during inflammation), drug-candidates that selectively inhibit PTGS2 (COX-2) were suspected to show fewerside-effects[24] but proved to substantially increase risk for cardiovascular events such as heart attack and stroke. Two different mechanisms may explain contradictory effects. Low-dose aspirin protects against heart attacks and strokes by blocking PTGS1 (COX-1) from forming a prostaglandin called thromboxane A2. It sticks platelets together and promotes clotting; inhibiting this helps prevent heart disease. On the other hand, PTGS2 (COX-2) is a more important source of prostaglandins, particularly prostacyclin which is found in blood vessel lining. Prostacyclin relaxes or unsticks platelets, soselective COX-2 inhibitors (coxibs) increase risk of cardiovascular events due to clotting.[27]
The expression of PTGS2 (COX-2) is upregulated in many cancers. The overexpression of PTGS2 (COX-2) along with increased angiogenesis and SLC2A1 (GLUT-1) expression is significantly associated with gallbladder carcinomas.[29] Furthermore, the product of PTGS2 (COX-2),PGH2 is converted byprostaglandin E2 synthase intoPGE2, which in turn can stimulate cancer progression. Consequently, inhibiting PTGS2 (COX-2) may have benefit in the prevention and treatment of these types of cancer.[30][31]
COX-2 expression was found in human idiopathic epiretinal membranes.[32] Cyclooxygenases blocking bylornoxicam in acute stage of inflammation reduced the frequency of membrane formation by 43% in thedispase model ofPVR and by 31% in theconcanavalin one.Lornoxicam not only normalized the expression of cyclooxygenases in both models of PVR, but also neutralized the changes of theretina and thechoroid thickness caused by the injection of pro-inflammatory agents. These facts underline the importance of cyclooxygenases and prostaglandins in the development of PVR.[33]
PTGS2 gene upregulation has also been linked with multiple stages of human reproduction. Presence of gene is found in thechorionic plate, in theamnion epithelium,syncytiotrophoblasts, villous fibroblasts, chorionictrophoblasts,amniotic trophoblasts, as well as thebasal plate of the placenta, in thedecidual cells andextravillous cytotrophoblasts. During the process ofchorioamnionitis/deciduitis, the upregulation of PTGS2 in theamnion and choriodecidua is one of three limited effects of inflammation in theuterus. Increased expression of the PTGS2 gene in thefetal membranes is connected to the presence of inflammation, causing uterine prostaglandin gene expression and immunolocalization ofprostaglandin pathway proteins in chorionic trophoblast cells and adjacent decidua, or choriodecidua. PTGS2 is linked with the inflammatory system and has been observed in inflammatoryleukocytes. It has been noted that there is a positive correlation with PTGS2 expression in the amnion during spontaneous labour and was discovered to have increased expression with gestational age following the presence of labour with no change observed in amnion and choriodecidua during either preterm or term labour. Additionally,oxytocin stimulates the expression of PTGS2 inmyometrial cells.[34]
The mutant allele PTGS2 5939C carriers among the Han Chinese population have been shown to have a higher risk ofgastric cancer. In addition, a connection was found betweenHelicobacter pylori infection and the presence of the 5939C allele.[35]
During an ischemic stroke, the deprivation of oxygen and glucose triggers a cascade of inflammatory responses, leading to increased COX-2 expression, particularly in neurons, glial cells, and endothelial cells.[36] This upregulation contributes to the production of pro-inflammatory prostaglandins such as PGE2, which exacerbates neuronal damage by promoting excitotoxicity,oxidative stress, and apoptosis.[37] Additionally, COX-2-derived prostaglandins can impair the integrity of the BBB, allowing peripheral immune cells and inflammatory mediators to infiltrate the brain, further worsening cerebral injury.
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^Tikhonovich MV, Erdiakov AK, Gavrilova SA (August 2018). "Nonsteroid anti-inflammatory therapy suppresses the development of proliferative vitreoretinopathy more effectively than a steroid one".International Ophthalmology.38 (4):1365–1378.doi:10.1007/s10792-017-0594-3.PMID28639085.S2CID4017540.
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