The cyclooxygenases (COXs), also known as Prostaglandin H synthases or synthases prostaglandin endoperoxide, oxygenases are enzymes belonging to fatty acid myeloperoxidase superfamily and are found in all vertebrates.
Cyclooxygenases are bifunctional enzymes, as they have two different catalytic activities: a cyclooxygenase activity and a peroxidase activity, which allow them to catalyze bis -oxygenation and the reduction of arachidonic acid to form prostaglandin.
Reaction catalyzed by cyclooxygenase enzymes (Source: Pancrat via Wikimedia Commons)
They have not been found in plants, insects or unicellular organisms, but in vertebrate cells these enzymes are located mainly in the membrane of the endoplasmic reticulum, with reports of their presence in the nuclear envelope, lipid bodies, mitochondria, filamentous structures, vesicles, etc.
The first detections of the products synthesized by cyclooxygenases were made in seminal fluids, which is why it was initially thought that they were substances produced in the prostate, which is why they were called “prostaglandins”.
Today it is known that prostaglandins are synthesized in all the tissues of vertebrate animals and even in organisms that do not have prostate glands, and that the different isomers of these molecules have different functions in different physiological and pathological processes such as fever, sensitivity to pain or algesia, inflammation, thrombosis, mitogenesis, vasodilation and vasoconstriction, ovulation. kidney function, etc.
Types
The existence of two types of cyclooxygenases has been reported among vertebrate animals. The first to be discovered and purified is known as COX-1 or simply COX, and it was purified for the first time in 1976 from the seminal vesicles of sheep and cows.
The second cyclooxygenase discovered among eukaryotes was COX-2 in 1991. To date, all vertebrate animals, including cartilaginous fish, bony fish, birds and mammals, have been shown to possess two genes that code for enzymes. COX.
One of them, COX-1, codes for cyclooxygenase 1, which is constitutive, while the COX-2 gene codes for inducible cyclooxygenase 2.
Characteristics of both genes and their enzyme products
The COX-1 and COX-2 enzymes are quite similar, which is understood to be 60-65% similarity between their amino acid sequences.
The orthologous COX-1 genes (genes in different species that have the same origin) in all species of vertebrate animals produce COX-1 proteins that share up to 95% of the identity of their amino acid sequences, which is also true for the orthologs of COX-2, the products of which share 70-90% identity.
Cnidarians and sea squirts also have two COX genes, but these are different from those of other animals, so some authors hypothesize that these genes could have arisen in independent duplication events from the same common ancestor.
COX-1
The COX-1 gene weighs approximately 22 kb and is constitutively expressed to encode the COX-1 protein, which has more or less 600 amino acid residues before being processed, since it has a hydrophobic signal peptide after removal of which yields a protein of approximately 576 amino acids.
This protein is mainly found in the endoplasmic reticulum and its general structure is in the form of a homodimer, that is, two identical polypeptide chains that associate to form the active protein.
COX-2
The COX -2 gene, on the other hand, weighs about 8 kb and its expression is induced by cytokines, growth factors, and other substances. It codes for the COX-2 enzyme which has, including the signal peptide, 604 amino acid residues and 581 after processing.
This enzyme is also homodimeric and is found between the endoplasmic reticulum and the nuclear envelope.
Molecular structure of cyclooxygenase type 2 (COX-2) (Source: Cytochrome c at English Wikipedia via Wikimedia Commons)
From the analysis of their structures, it has been determined that the enzymes COX-1 and COX-2 possess at their N-terminal end and in the site adjacent to the signal peptide, a unique “module” of epidermal growth factor (EGF, of the English Epidermal Growth Factor).
In this module there are highly conserved disulfide bonds or bridges, which function as a "dimerization domain" between the two polypeptides of each homodimeric enzyme.
Proteins also have amphipathic helices that facilitate anchoring to one of the layers of the membrane. In addition, the catalytic domain of both has two active sites, one with cyclooxygenase activity and the other with peroxidase activity.
Both enzymes are highly conserved proteins, with little significant differences between different species with respect to dimerization and membrane binding mechanisms, as well as some characteristics of their catalytic domains.
The COX proteins additionally have glycosylation sites that are essential for their function and that are absolutely conserved.
Reaction
Cyclooxygenase 1 and 2 enzymes are responsible for catalyzing the first two steps of prostaglandin biosynthesis, which begin with the conversion of arachidonic acid into prostaglandin precursors known as hydroperoxy-endoperoxide PGG2.
For these enzymes to perform their functions, they must first be activated through a process dependent on their peroxidase activity. In other words, its main activity depends on the reduction of a peroxide substrate (mediated by the active site peroxidase) for the oxidation of iron associated with the heme group that serves as a cofactor to occur.
Oxidation of the heme group causes the formation of a tyrosyl radical at the cyclooxygenase active site, which activates the enzyme and promotes the initiation of the cyclooxygenase reaction. This activation reaction can only occur once, as the tyrosyl radical is regenerated during the last reaction in the pathway.
Inhibitors
Cyclooxygenases are involved in the synthesis of prostaglandins, which are hormones with functions in the protection of the intestinal mucosa, in the aggregation of platelets and in the regulation of kidney function, in addition to participating in the processes of inflammation, pain and fever.
Given that these enzymes are key to the production of these hormones, especially those that have to do with inflammatory processes, numerous pharmacological studies have focused on the inhibition of cyclooxygenases.
Molecular structure of cyclooxygenase 1 bound to ibuprofen (Source: Fvasconcellos 5 May 2007 via Wikimedia Commons)
Thus, it has been shown that the mechanism of action of many non-steroidal anti-inflammatory drugs has to do with the irreversible or reversible (inhibitory) acetylation of the cyclooxygenase active site on these enzymes.
These drugs include piroxicam, ibuprofen, aspirin, flurbiprofen, diclofenac, naproxen, and others.
References
- Botting, RM (2006). Inhibitors of cyclooxygenases: mechanisms, selectivity and uses. Journal of physiology and pharmacology, 57, 113.
- Chandrasekharan, NV, & Simmons, DL (2004). The cyclooxygenases. Genome biology, 5 (9), 241.
- Fitzpatrick, FA (2004). Cyclooxygenase enzymes: regulation and function. Current pharmaceutical design, 10 (6), 577-588.
- Kundu, N., Smyth, MJ, Samsel, L., & Fulton, AM (2002). Cyclooxygenase inhibitors block cell growth, increase ceramide and inhibit cell cycle. Breast cancer research and treatment, 76 (1), 57-64.
- Rouzer, CA, & Marnett, LJ (2009). Cyclooxygenases: structural and functional insights. Journal of lipid research, 50 (Supplement), S29-S34.
- Vane, JR, Bakhle, YS, & Botting, RM (1998). CYCLOOXYGENASES 1 AND 2. Annual review of pharmacology and toxicology, 38 (1), 97-120.