Cyclooxygenase
Cyclooxygenases (COX) is the key enzyme at the beginning of prostaglandin from arachidonic acid, dihomogammalinolenic acid of ( DGLA ) or eicosapentaenoic acid (EPA). This step is inhibited by nonsteroidal anti-inflammatory drugs and is the rate-limiting step in prostaglandin synthesis. The COX therefore play a central role in the regulation of the inflammatory process.
Cyclooxygenases are inside the endoplasmic reticulum, localized within the nuclear envelope and in the Golgi apparatus and adhere the inner faces of the membranes of the cell compartments at. They occur in cells of animals since the early development of invertebrates, eg already in the cells of the coral, but not in unicellular organisms, plants or insects. Here, however, are related enzymes before from the parent family of pathogen - inducible oxygenase ( PIOXs ).
There are already very early in the evolution of cyclooxygenases two isoenzymes, cyclooxygenase -1 and cyclooxygenase -2, which differ in their gene locus, have a slightly different structure, occur in different cell types are differentially regulated, show a different substrate specificity and are pharmacologically differently influenced.
History of Exploration
Prostaglandins are known since the 1930s. In the 1970s, for the first time Cyclooxidasen were prepared from tissue homogenates of the seminal vesicles of cattle and sheep purified and identified as the enzymes of prostaglandin synthesis with Cyxlooxygenase and peroxidase activity. 1971 could be demonstrated that at that time already in use: NSAIDs inhibit cyclooxygenase activity. From 1972, the enzyme reactions has been speculated that there must be more than a cyclooxygenase due to different kinetics.
The protein structures of cyclooxygenase -1 and cyclooxygenase -2 were sequenced in the 1990s and enlightened in their tertiary and quaternary structure and led to the development of additional drugs affecting these enzymes.
The first cyclooxygenase -2 selective inhibitors in 1999 were on the market.
Furthermore, a paradigm shift in the Prostaglandinforschung place in the early 1990s instead, as it was recognized that the regulation of cyclooxygenase is the essential control point in the prostaglandin (previously you had thought that these were the phospholipases that form, and others arachidonic acid).
Structure
Cyclooxygenases are globular proteins with 600 amino acids. They have a Molar mass 67-72 kDa to 65 % identical in its amino acid sequence and have almost identical active sites. Add ever assembled to two dimers. With a hydrophobic region to float on / in the inner sides of the microsomal membranes such as the endoplasmic reticulum. These membrane- binding region forms a hydrophobic likewise, narrow opening in a blind-ended channel to the active site of the cyclooxygenase activity. This channel is narrower in the Cyclooxygenase -1 and Cyclooxygenase-2 in the ( by an exchange at position 523 from valine into isoleucine ).
In the inner part of the channel is located ( at position 385), a tyrosine, which is activated at a tyrosyl radical before the cyclooxygenases to perform their actual reaction. This is done by means of the peroxidase of the cyclooxygenases, which ( at the opposite side of the membrane of the endoplasmic reticulum enzyme) is another active site. Here bound Fe3 to a ferryl - oxo porphyrin radical ( Fe4 = O • ) is first oxidized by means of occurring in the endoplasmic reticulum oxidants heme, which is then an electron from the tyrosine -OH subtracting the center of the cyclooxygenase activity and thus the active radical tyrosyl -O • forms. (For more details see page 400 and 401 of ).
Function
Cyclooxygenases catalyze the conversion of arachidonic acid to prostaglandin H2, and the DGLA and EPA and to the corresponding precursors of the PG1 and PG3. This occurs in two steps in two different reaction centers of the enzyme:
The various other prostaglandins are eliminated from the resulting prostaglandin H2 then synthesized partly by spontaneous isomerization, partly with the help of various synthases or oxidases.
The cyclooxygenases are but in prostaglandin formation of the rate-determining step, they have such a central role in the regulation of the inflammatory process. They have a half-life of 1-2 minutes if they are exposed to arachidonic acid in a concentration leading to maximum utilization of the enzyme.
Sub-forms ( isoenzymes )
There are two sub- forms of cyclooxygenase which are described in more detail in the corresponding sub items. They have separated early in the evolution of invertebrate animals by gene duplication from each other and go their own evolutionary paths since then. Have 65% of the amino acid sequences common to catalyze the same enzymatic reaction, but can be found in the body differently distributed and regulated. An important difference between the cyclooxygenase-1 and cyclooxygenase-2 is the exchange at position 523 from valine to isoleucine, which makes the active site of cyclooxygenase-2 a bit larger and there can also oxidize substrates apart somewhat bulkier arachidonic acid. This is the example of endocannabinoids such as anandamide (see page 399 of ). Another difference is the multiple regulation of the transcription of cyclooxygenase -2, which is induced primarily by inflammation and other conditions of the cell activation.
Meanwhile was also another cyclooxygenase isoform - cyclooxygenase 3 - proclaimed. However, a proof or a more precise definition has not yet been given. However, to inhibit paracetamol COX -3. Meanwhile, it is assumed that it is merely a splice variant of COX -1 in COX -3.
COX inhibitors
- Because in an inhibition of cyclooxygenase more arachidonic acid is the lipoxygenase pathway are available, resulting in the formation of leukotrienes has the consequence that are anti- reinforcing and anaphylaxieverstärkend, inhibitors of cyclooxygenase can trigger an asthma attack.
- Acetylsalicylic acid (aspirin ) leads to a transacetylation at serine in position 530 in the active site of cyclooxygenase, which makes the enzyme inoperative until it is re- formed. Cyclooxygenase -1 is this 10-100 times more sensitive than cyclooxygenase -2.
- Competitively acting NSAIDs compete in Cyclooxygenasezentrum for the binding site for arachidonic acid. Ibuprofen binds this very fast and is also washed out quickly. Diclofenac or indomethacin have a trägeres binding behavior.
- Selective COX -2 inhibitors mainly inhibit the activity of cyclooxygenase -2 (for details see there).
- Analgesic / antipyretic substances such as paracetamol or dipyrone are important drugs in pain and fever without anti-inflammatory properties. The mechanism and the conditions of their rather weak inhibition of cyclooxygenases are still unexplored.
Treatment of inflammatory diseases
NSAIDs are currently used as drugs of first choice for the treatment of osteoarthritis, rheumatoid, systemic lupus erythematosus ( SLE) and other inflammatory diseases. The treatment is mostly palliative and does not alter the disease course. NSAIDs inhibit inflammation and reduce the pain.