Prostaglandin-endoperoxide synthase 2
|Prostaglandin-endoperoxide synthase 2 (prostaglandin G/H synthase and cyclooxygenase)|
PDB rendering based on 6COX
|External IDs||ChEMBL: GeneCards:|
|RNA expression pattern|
Prostaglandin-endoperoxide synthase 2, also known as cyclooxygenase-2 or simply COX-2, is an enzyme that in humans is encoded by the PTGS2 gene.. It is involved in the conversion of arachidonic acid to prostaglandin H2, an important precursor of prostacyclin and thromboxane A2, among others.
COX-2 exists as a homodimer, each monomer with a molecular mass of about 70 kDa. The tertiary and quaternary structures of COX-1 and COX-2 enzymes are almost identical. Each subunit has three different structural domains: a short N-terminal epidermal growth factor (EGF) domain; an α-helical membrane-binding moiety; and a C-terminal catalytic domain. COX enzymes are monotopic membrane proteins; the membrane-binding domain consists of a series of amphipathic α helices with several hydrophobic amino acids exposed to a membrane monolayer. COX-1 and COX-2 are bifunctional enzymes that carry out two consecutive chemical reactions in spatially distinct but mechanistically coupled active sites. Both the cyclooxygenase and the peroxidase active sites are located in the catalytic domain, which accounts for approximately 80% of the protein. The catalytic domain is homologous to mammalian peroxidases such as myeloperoxidase.
It has been found that human COX-2 functions as a conformational heterodimer having a catalytic monomer (E-cat) and an allosteric monomer (E-allo). Heme binds only to the peroxidase site of E-cat while substrates, as well as certain inhibitors (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-substrate fatty acid (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the 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 of huPGHS-2, binds only E-allo in palmitic acid/murine PGHS-2 co-crystals. Non-substrate FAs can potentiate or attenuate COX inhibitors depending on the fatty 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 COX inhibitors.(Figure 1)
Prostaglandin endoperoxide H synthase, COX 2, converts arachidonic acid (AA) to prostaglandin endoperoxide H2. PGHSs are targets for NSAIDs and COX-2 specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer of the enzyme has a peroxidase and a COX active site. The COX enzymes catalyze the conversion of arachidonic acid to prostaglandins in a two steps. First, hydrogen is abstracted from carbon 13 of arachidonic acid, and then two molecules of oxygen are added by the COX-2, giving PGG2. Second, PGG2 is reduced to PGH2 in the peroxidase active site. The synthesized PGH2 is converted to prostaglandins (PGD2, PGE2, PGF2α), prostacyclin (PGI2), or thromboxane A2 by tissue-specific isomerases.(Figure 2)
Both the peroxidase and the cyclooxygenase activities are inactivated during catalysis by mechanism-based, first-order processes, which means that PGHS-2 peroxidase or cyclooxygenase activities fall to zero within 1–2 minutes, even in the presence of sufficient substrates.
The conversion of arachidonic acid to PGG2 can be shown as a series of radical reactions analogous to polyunsaturated fatty acid autoxidation (Figure 3). The 13-pro(S) -hydrogen is abstracted and dioxygen traps the pentadienyl 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 the endoperoxide. The allylic radical generated is trapped by dioxygen at carbon 15 to form the 15-(S) -peroxyl radical; this radical is then reduced to PGG2 . This is supported by the following evidence: 1) a significant kinetic isotope effect is observed for the abstraction of the 13-pro (S )-hydrogen; 2) carbon-centered radicals are trapped during catalysis; 3) small amounts of oxidation products are formed due to the oxygen trapping of an allylic radical intermediate at positions 13 and 15.
Another mechanism shown in Figure 4 in which the 13-pro (S )-hydrogen is deprotonated and the carbanion is oxidized to a radical 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. 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. 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.
Cyclooxygenase-2 (COX-2, prostaglandin H synthase-2, PGHS-2) is unexpressed under normal conditions in most cells, but elevated levels are found during inflammation. COX-1 (prostaglandin H2 synthase 1) is constitutively expressed in many tissues and is the predominant form in gastric mucosa and in the kidneys. Inhibition of COX-1 reduces the basal production of cytoprotective PGE2 and PGI2 in the stomach, which may contribute to gastric ulceration. Since COX-2 is generally expressed only in cells where prostaglandins are upregulated (e.g., during inflammation), drug-candidates that selectively inhibit COX-2 were suspected to show fewer side-effects  but proved to substantially increase risk for cardiovascular events such as heart attack and stroke.The penn Group found that this was due to the for several reasons low-dose aspirin protects against heart attacks and strokes by blocking COX-1 from forming a prostaglandin called thromboxane A2 in sticks platelets together promotes clotting so inhibiting this helps prevent heart disease . On the other hand, COX-2 is the more important source of prostaglandins, particularly one called prostacyclin, which is found in blood vessel lining and this relaxes or unsticks platelets so in Celebrex because it selectively blocks this mechanism you get an increase risk MI.More than 10 years later, it is now clear what the COX inhibitors do in the body. Eight placebo-controlled, randomized trials, performed to find new uses of these drugs, showed that they posed a cardiovascular hazard, similar in magnitude to that resulting from being a smoker or a diabetic, notes FitzGerald. "Despite this, controversy has continued about how all this came about, until now."
Arguments against the proposed mechanism were threefold. First, it was proposed that COX-2 didn't exist under normal circumstances in the blood-vessel lining and that PGI-M (prostacyclin in humans, as reflected by its major metabolite in urine), came from some other source. The kidneys were suggested as the source by some researchers. Second, even if blood-vessel prostacyclin was blocked, other protective mechanisms, especially formation of nitric oxide (NO) would take over. And third, although NSAIDs elevate blood pressure, it was proposed that this observation was unrelated to COX-2 and treating high blood pressure would deal with the problem.
FitzGerald's group has now "closed the loop" with its earlier clinical studies and answered these questions in a paper just published in Science Translational Medicine. In it, they confirm that COX-2 is expressed in cells lining blood vessels and that selectively removing it predisposes mice to blood clotting and high blood pressure. These mice, just like humans taking COX-2 inhibitors, also see a fall in PGI-M. What's more, the Penn group discovered that COX-2 in lining cells controls the expression of eNOS, the enzyme that makes NO in the body. "So, rather than replacing the missing prostacyclin, as others have proposed, NO is lost and amplifies the effects of COX-2 inhibition on the cardiovascular system," says FitzGerald.
Indeed, the lost NO may not be the only step that magnifies the effects of losing prostacyclin. In a second paper, published in April 2012, in the Proceedings of the National Academy of Sciences, FitzGerald's group shows that arachidonic acid, the fat broken down by COX-2 to make prostacyclin, can be shunted down another pathway to make a new series of dangerous fats called leukotrienes when COX-2 is disrupted.
Clinical studies have shown that those most at risk from COX-2 inhibitors are patients who already have heart disease. However, the Penn group now suggests broader implications. Here, the group resolves one aspect of the controversy, showing that COX-2 disruption causes hardening of the arteries in mice. This result is provocative because randomized trials of Vioxx and Celebrex in patients at low risk of heart disease detected an increase in heart attacks after patients had been taking the drugs for more than a year. These current Penn studies raise the disturbing prospect that heart-healthy patients taking NSAIDs for prolonged periods might be gradually increasing their risk of heart attacks and strokes by progressively hardening their arteries.
"However, it's not all bad news," says FitzGerald. This risk of hardening of the arteries was diminished in mice by reducing leukotriene formation, via blocking a critical protein called the 5-lipoxygenase activating protein, or FLAP. Inhibitors of FLAP are already in trials in humans to see if they work in asthma. Perhaps, FitzGerald concludes, they can now find an additional use -- protecting the heart from NSAIDs.  Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin production by cyclooxygenases (COX) 1 and 2. NSAIDs selective for inhibition of COX-2 are less likely than traditional drugs to cause gastrointestinal adverse effects, but could cause cardiovascular events, such as heart failure, myocardial infarction, and stroke. Studies with human pharmacology and genetics, genetically manipulated rodents, and other animal models and randomized trials indicate that this is due to suppression of COX-2-dependent cardioprotective prostaglandins, prostacyclin in particular.
The expression of COX-2 is upregulated in many cancers. The overexpression of COX-2 along with increased angiogenesis and GLUT-1 expression is significantly associated with gallbladder carcinomas. Furthermore the product of COX-2, PGH2 is converted by prostaglandin E2 synthase into PGE2, which in turn can stimulate cancer progression. Consequently inhibiting COX-2 may have benefit in the prevention and treatment of these types of cancer.
The mutant allele PTGS2 5939C carriers among the Han Chinese population have been shown to have a higher risk of gastric cancer. In addition, a connection was found between Helicobacter pylori infection and the presence of the 5939C allele.
- Hla T, Neilson K (August 1992). "Human cyclooxygenase-2 cDNA". Proc. Natl. Acad. Sci. U.S.A. 89 (16): 7384–8. doi:10.1073/pnas.89.16.7384. PMC 49714. PMID 1380156.
- Xie WL, Chipman JG, Robertson DL, Erikson RL, Simmons DL (April 1991). "Expression of a mitogen-responsive gene encoding prostaglandin synthase is regulated by mRNA splicing". Proc. Natl. Acad. Sci. U.S.A. 88 (7): 2692–6. doi:10.1073/pnas.88.7.2692. PMC 51304. PMID 1849272. [better source needed]
- Picot D, Loll PJ, Garavito RM (January 1994). "The X-ray crystal structure of the membrane protein prostaglandin H2 synthase-1". Nature 367 (6460): 243–9. doi:10.1038/367243a0. PMID 8121489.
- Kurumbail RG, Kiefer JR, Marnett LJ (December 2001). "Cyclooxygenase enzymes: catalysis and inhibition". Curr. Opin. Struct. Biol. 11 (6): 752–60. doi:10.1016/S0959-440X(01)00277-9. PMID 11751058.
- Dong L, Vecchio AJ, Sharma NP, Jurban BJ, Malkowski MG, Smith WL (May 2011). "Human cyclooxygenase-2 is a sequence homodimer that functions as a conformational heterodimer". J. Biol. Chem. 286 (21): 19035–46. doi:10.1074/jbc.M111.231969. PMC 3099718. PMID 21467029.
- O'Banion MK (1999). "Cyclooxygenase-2: molecular biology, pharmacology, and neurobiology". Crit Rev Neurobiol 13 (1): 45–82. PMID 10223523.
- Smith WL, Garavito RM, DeWitt DL (December 1996). "Prostaglandin endoperoxide H synthases (cyclooxygenases)-1 and -2". J. Biol. Chem. 271 (52): 33157–60. doi:10.1074/jbc.271.52.33157. PMID 8969167.
- Wu G, Wei C, Kulmacz RJ, Osawa Y, Tsai AL (April 1999). "A mechanistic study of self-inactivation of the peroxidase activity in prostaglandin H synthase-1". J. Biol. Chem. 274 (14): 9231–7. doi:10.1074/jbc.274.14.9231. PMID 10092596.
- Callan OH, So OY, Swinney DC (February 1996). "The kinetic factors that determine the affinity and selectivity for slow binding inhibition of human prostaglandin H synthase 1 and 2 by indomethacin and flurbiprofen". J. Biol. Chem. 271 (7): 3548–54. doi:10.1074/jbc.271.7.3548. PMID 8631960.
- PDB 3OLT
- Porter NA (1986). "Mechanisms for the autoxidation of polyunsaturated lipids". Accounts of Chemical Research 19 (9): 262–8. doi:10.1021/ar00129a001.
- Mason RP, Kalyanaraman B, Tainer BE, Eling TE (June 1980). "A carbon-centered free radical intermediate in the prostaglandin synthetase oxidation of arachidonic acid. Spin trapping and oxygen uptake studies". J. Biol. Chem. 255 (11): 5019–22. PMID 6246094.
- Hecker M, Ullrich V, Fischer C, Meese CO (November 1987). "Identification of novel arachidonic acid metabolites formed by prostaglandin H synthase". Eur. J. Biochem. 169 (1): 113–23. doi:10.1111/j.1432-1033.1987.tb13587.x. PMID 3119336.
- Xiao G, Tsai AL, Palmer G, Boyar WC, Marshall PJ, Kulmacz RJ (February 1997). "Analysis of hydroperoxide-induced tyrosyl radicals and lipoxygenase activity in aspirin-treated human prostaglandin H synthase-2". Biochemistry 36 (7): 1836–45. doi:10.1021/bi962476u. PMID 9048568.
- Kwok PY, Muellner FW, Fried J (June 1987). "Enzymatic conversions of 10,10-difluoroarachidonic acid with PGH synthase and soybean lipoxygenase". Journal of the American Chemical Society 109 (12): 3692–3698. doi:10.1021/ja00246a028.
- Dean AM, Dean FM (May 1999). "Carbocations in the synthesis of prostaglandins by the cyclooxygenase of PGH synthase? A radical departure!". Protein Sci. 8 (5): 1087–98. doi:10.1110/ps.8.5.1087. PMC 2144324. PMID 10338019.
- Hamberg M, Samuelsson B (November 1967). "On the mechanism of the biosynthesis of prostaglandins E-1 and F-1-alpha". J. Biol. Chem. 242 (22): 5336–43. PMID 6070851.
- Wolfe MM (December 2004). "Rofecoxib, Merck, and the FDA". N. Engl. J. Med. 351 (27): 2875–8; author reply 2875–8. doi:10.1056/NEJM200412303512719. PMID 15625749.
- Wang D, Patel VV, Ricciotti E, Zhou R, Levin MD, Gao E, Yu Z, Ferrari VA, Lu MM, Xu J, Zhang H, Hui Y, Cheng Y, Petrenko N, Yu Y, FitzGerald GA (May 2009). "Cardiomyocyte cyclooxygenase-2 influences cardiac rhythm and function". Proc. Natl. Acad. Sci. U.S.A. 106 (18): 7548–52. doi:10.1073/pnas.0805806106. PMC 2670242. PMID 19376970.
- PDB 3PGH
- Legan M (August 2010). "Cyclooxygenase-2, p53 and glucose transporter-1 as predictors of malignancy in the development of gallbladder carcinomas". Bosn J Basic Med Sci 10 (3): 192–6. PMID 20846124.
- EntrezGene 5743
- Menter DG, Schilsky RL, DuBois RN (March 2010). "Cyclooxygenase-2 and cancer treatment: understanding the risk should be worth the reward". Clin. Cancer Res. 16 (5): 1384–90. doi:10.1158/1078-0432.CCR-09-0788. PMID 20179228.
- Li Y, He W, Liu T, Zhang Q (December 2010). "A new cyclo-oxygenase-2 gene variant in the Han Chinese population is associated with an increased risk of gastric carcinoma". Mol Diagn Ther 14 (6): 351–5. doi:10.2165/11586400-000000000-00000. PMID 21275453.
- Liou JY, Deng WG, Gilroy DW, Shyue SK, Wu KK (September 2001). "Colocalization and interaction of cyclooxygenase-2 with caveolin-1 in human fibroblasts". J. Biol. Chem. 276 (37): 34975–82. doi:10.1074/jbc.M105946200. PMID 11432874.
- Richards JA, Petrel TA, Brueggemeier RW (February 2002). "Signaling pathways regulating aromatase and cyclooxygenases in normal and malignant breast cells". J. Steroid Biochem. Mol. Biol. 80 (2): 203–12. doi:10.1016/S0960-0760(01)00187-X. PMID 11897504.
- Koki AT, Khan NK, Woerner BM, Seibert K, Harmon JL, Dannenberg AJ, Soslow RA, Masferrer JL (January 2002). "Characterization of cyclooxygenase-2 (COX-2) during tumorigenesis in human epithelial cancers: evidence for potential clinical utility of COX-2 inhibitors in epithelial cancers". Prostaglandins Leukot. Essent. Fatty Acids 66 (1): 13–8. doi:10.1054/plef.2001.0335. PMID 12051953.
- Saukkonen K, Rintahaka J, Sivula A, Buskens CJ, Van Rees BP, Rio MC, Haglund C, Van Lanschot JJ, Offerhaus GJ, Ristimaki A (October 2003). "Cyclooxygenase-2 and gastric carcinogenesis". APMIS 111 (10): 915–25. doi:10.1034/j.1600-0463.2003.1111001.x. PMID 14616542.
- Sinicrope FA, Gill S (2004). "Role of cyclooxygenase-2 in colorectal cancer". Cancer Metastasis Rev. 23 (1–2): 63–75. doi:10.1023/A:1025863029529. PMID 15000150.
- Jain S, Khuri FR, Shin DM (2004). "Prevention of head and neck cancer: current status and future prospects". Curr Probl Cancer 28 (5): 265–86. doi:10.1016/j.currproblcancer.2004.05.003. PMID 15375804.
- Saba N, Jain S, Khuri F (2004). "Chemoprevention in lung cancer". Curr Probl Cancer 28 (5): 287–306. doi:10.1016/j.currproblcancer.2004.05.005. PMID 15375805.
- Cardillo I, Spugnini EP, Verdina A, Galati R, Citro G, Baldi A (October 2005). "Cox and mesothelioma: an overview". Histol. Histopathol. 20 (4): 1267–74. PMID 16136507.
- Brueggemeier RW, Díaz-Cruz ES (March 2006). "Relationship between aromatase and cyclooxygenases in breast cancer: potential for new therapeutic approaches". Minerva Endocrinol. 31 (1): 13–26. PMID 16498361.
- Fujimura T, Ohta T, Oyama K, Miyashita T, Miwa K (March 2006). "Role of cyclooxygenase-2 in the carcinogenesis of gastrointestinal tract cancers: a review and report of personal experience". World J. Gastroenterol. 12 (9): 1336–45. PMID 16552798.
- Bingham S, Beswick PJ, Blum DE, Gray NM, Chessell IP (October 2006). "The role of the cylooxygenase pathway in nociception and pain". Semin. Cell Dev. Biol. 17 (5): 544–54. doi:10.1016/j.semcdb.2006.09.001. PMID 17071117.
- Minghetti L, Pocchiari M (2007). "Cyclooxygenase-2, prostaglandin E2, and microglial activation in prion diseases". Int. Rev. Neurobiol. International Review of Neurobiology 82: 265–75. doi:10.1016/S0074-7742(07)82014-9. ISBN 9780123739896. PMID 17678966.