PTGS2

Prostaglandin-endoperoxide synthase 2, also known as cyclooxygenase-2 or simply COX-2, is an is_associated_with::enzyme that in humans is encoded by the PTGS2 is_associated_with::gene.

History
COX-2 was discovered in 1991 by the Daniel Simmons laboratory at Brigham Young University.

Structure
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 is_associated_with::hydrophobic is_associated_with::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 is_associated_with::cyclooxygenase and the is_associated_with::peroxidase active sites are located in the catalytic domain, which accounts for approximately 80% of the protein. The catalytic domain is is_associated_with::homologous to mammalian peroxidases such as is_associated_with::myeloperoxidase.



It has been found that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (E-cat) and an allosteric monomer (E-allo). is_associated_with::Heme binds only to the is_associated_with::peroxidase site of E-cat while substrates, as well as certain inhibitors (e.g. is_associated_with::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. is_associated_with::naproxen) preferentially bind to the COX site of E-allo. AA 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 is_associated_with::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)

Function


Prostaglandin endoperoxide H synthase, COX 2, converts arachidonic acid (AA) to prostaglandin endoperoxide H2. PGHSs are targets for is_associated_with::NSAIDs and COX-2 specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each is_associated_with::monomer of the enzyme has a is_associated_with::peroxidase and a COX is_associated_with::active site. The COX enzymes catalyze the conversion of is_associated_with::arachidonic acid to is_associated_with::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 is_associated_with::PGH2 in the peroxidase active site. The synthesized PGH2 is converted to prostaglandins (is_associated_with::PGD2, is_associated_with::PGE2, PGF2R), is_associated_with::prostacyclin (PGI2), or is_associated_with::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.



Mechanism
The conversion of arachidonic acid to PGG2 can be shown as a series of is_associated_with::radical reactions analogous to polyunsaturated is_associated_with::fatty acid is_associated_with::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 is_associated_with::endoperoxide. The is_associated_with::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 is_associated_with::kinetic isotope effect is observed for the abstraction of the 13-pro (S )-hydrogen; 2) carbon-centered radicals are trapped during is_associated_with::catalysis; 3) small amounts of is_associated_with::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 is_associated_with::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 is_associated_with::arachidonic acid oxygenation.



Clinical significance
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. is_associated_with::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 is_associated_with::PGE2 and is_associated_with::PGI2 in the is_associated_with::stomach, which may contribute to is_associated_with::gastric ulceration. Since COX-2 is generally expressed only in cells where is_associated_with::prostaglandins are upregulated (e.g., during inflammation), drug-candidates that selectively inhibit COX-2 are thought to show fewer side-effects.

is_associated_with::Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit is_associated_with::prostaglandin production by cyclooxygenases (COX) 1 and 2. is_associated_with::NSAIDs selective for inhibition of COX-2 are less likely than traditional drugs to cause gastrointenstinal adverse effects, but could cause is_associated_with::cardiovascular events, such as is_associated_with::heart failure, is_associated_with::myocardial infarction, and is_associated_with::stroke. Studies with human is_associated_with::pharmacology and is_associated_with::genetics, genetically manipulated is_associated_with::rodents, and other animal models and randomized trials indicate that this is due to suppression of COX-2-dependent cardioprotective prostaglandins, is_associated_with::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 is_associated_with::gastric cancer. In addition, a connection was found between is_associated_with::Helicobacter pylori infection and the presence of the 5939C allele.

Interactions
PTGS2 has been shown to interact with is_associated_with::Caveolin 1.