Thromboxane synthesis
Eicosanoid synthesis
Thromboxane A synthase 1 (platelet, cytochrome P450, family 5, subfamily A), also known as TBXAS1, is a cytochrome P450 enzyme that, in humans, is encoded by the TBXAS1 gene. [5] [6] [7]
Thromboxane A synthase 1 (platelet, cytochrome P450, family 5, subfamily A), also known as TBXAS1, is a cytochrome P450 enzyme that, in humans, is encoded by the TBXAS1 gene. [5] [6] [7]
This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids, and other lipids. However, this protein is considered a member of the cytochrome P450 superfamily on the basis of sequence similarity rather than functional similarity. This endoplasmic reticulum membrane protein catalyzes the conversion of prostaglandin H2 to thromboxane A2, a potent vasoconstrictor and inducer of platelet aggregation, and also to 12-Hydroxyheptadecatrienoic acid (i.e. 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid or 12-HHT) an agonist of Leukotriene B4 receptors (i.e. BLT2 receptors) [8] and mediator of certain BLT2 receptor actions. [9] The enzyme plays a role in several pathophysiological processes including hemostasis, cardiovascular disease, and stroke. The gene expresses two transcript variants. [5]
Thromboxane synthase inhibitors are used as antiplatelet drugs. Ifetroban is a potent and selective thromboxane receptor antagonist. [10] Dipyridamole antagonizes this receptor too, but has various other mechanisms of antiplatelet activity as well. Picotamide has activity both as a thromboxane synthase inhibitor and as a thromboxane receptor antagonist. [11]
The human thromboxane A (TXA) synthase is a 60 kDa cytochrome P450 protein with 533 amino acids and a heme prosthetic group. This enzyme, anchored to the endoplasmic reticulum, is found in platelets, monocytes, and several other cell types. The NH2 terminus contains two hydrophobic segments whose secondary structure is believed to be helical. Evidence suggests that the peptides serve as a membrane anchor for the enzyme. [12] Moreover, the study of cDNA clones made possible by polymerase chain reaction techniques has further elucidated the TXA synthase's primary structure. Similar to other members in the cytochrome P450 family, TXA synthase has a heme group coordinated to the thiolate group of a cysteine residue, specifically cysteine 480. [13] Mutagenesis studies that made substitutions at that position resulted in loss of catalytic activity and minimal heme binding. Other residues that had similar results were W133, R478, N110, and R413. Located near the heme propionate groups or the distal face of the heme, these residues are also important for proper integration of heme into the apoprotein. [14] Unfortunately, researchers have found it difficult to obtain a crystal structure of TXA synthase due to the requirement of detergent treatment extraction from the membrane but they have utilized homology modeling to create a 3D structure. One model showed two domains, an alpha-helix-rich domain and a beta-sheet-rich domain. The heme was found to be sandwiched between helices I and L. [15]
Thromboxane A (TXA) is derived from the prostaglandin H2 (PGH2) molecule. PGH2 contains a relatively weak epidioxy bond, and a possible mechanism is known to involve homolytic cleavage of the epidioxide and a rearrangement to TXA. [16] A heme group in the active site of TXA synthase plays an important role in the mechanism. Stopped-flow kinetic studies with a substrate analog and recombinant TXA synthase revealed that substrate binding occurs in two steps. [14] First, there is a fast initial binding to the protein and then a subsequent ligation to the heme iron. In the first step of the mechanism, the heme iron coordinates to the C-9 endoperoxide oxygen. It participates in homolytic cleavage of the O-O bond in the endoperoxide, which represents the rate-limiting step, and undergoes a change in redox state from Fe(III) to Fe(IV). [17] A free oxygen radical forms at C-11, and this intermediate undergoes ring cleavage. With the free radical now at C-12, the iron heme then oxidizes this radical to a carbocation. [18] The molecule is now ready for intramolecular ring formation. The negatively charged oxygen attacks the carbonyl, and the electrons from one of the double bonds are drawn to the carbocation, thus closing the ring.
Maintaining a balance between prostacyclins and thromboxanes is important in the body, particularly because these two eicosanoids exert opposing effects. In catalyzing the synthesis of thromboxanes, TXA synthase is involved in a flux pathway that can modulate the amount of thromboxane produced. This control becomes an important factor in several processes, such as blood pressure regulation, clotting, and inflammatory responses. Dysregulation of TXA synthase and an imbalance in the prostacyclin-thromboxane ratio are thought to underlie many pathological conditions, such as pulmonary hypertension. [19] Because thromboxanes play a role in vasoconstriction and platelet aggregation, their dominance can disrupt vascular homeostasis and cause thrombotic vascular events. Furthermore, the importance of thromboxanes and their syntheses in vascular homeostasis is illustrated by findings that patients whose platelets were unresponsive to TXA displayed hemostatic defects and that a deficiency of platelet TXA production led to bleeding disorders. [20]
Furthermore, it has been found that the expression of TXA synthase may be of critical importance to the development and progression of cancer. An overall increase in TXA synthase expression has been observed in a variety of cancers, such as papillary thyroid carcinoma, prostate cancer, and renal cancer. Cancer cells are known for their limitless cellular replicative potential, and it has been hypothesized that changes in eicosanoid profile affect cancer growth. Research has led to the proposal that TXA synthase contributes to a range of tumor survival pathways, including growth, apoptosis inhibition, angiogenesis, and metastasis. [21]
The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derived enzymatically from the fatty acid arachidonic acid. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. They are a subclass of eicosanoids and of the prostanoid class of fatty acid derivatives.
Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, 20 carbon units in length. Eicosanoids are a sub-category of oxylipins, i.e. oxidized fatty acids of diverse carbon units in length, and are distinguished from other oxylipins by their overwhelming importance as cell signaling molecules. Eicosanoids function in diverse physiological systems and pathological processes such as: mounting or inhibiting inflammation, allergy, fever and other immune responses; regulating the abortion of pregnancy and normal childbirth; contributing to the perception of pain; regulating cell growth; controlling blood pressure; and modulating the regional flow of blood to tissues. In performing these roles, eicosanoids most often act as autocrine signaling agents to impact their cells of origin or as paracrine signaling agents to impact cells in the proximity of their cells of origin. Eicosanoids may also act as endocrine agents to control the function of distant cells.
Prostacyclin (also called prostaglandin I2 or PGI2) is a prostaglandin member of the eicosanoid family of lipid molecules. It inhibits platelet activation and is also an effective vasodilator.
Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring.
Prostanoids are a subclass of eicosanoids consisting of the prostaglandins, the thromboxanes, and the prostacyclins
The thromboxane receptor (TP) also known as the prostanoid TP receptor is a protein that in humans is encoded by the TBXA2R gene, The thromboxane receptor is one among the five classes of prostanoid receptors and was the first eicosanoid receptor cloned. The TP receptor derives its name from its preferred endogenous ligand thromboxane A2.
Most of the eicosanoid receptors are integral membrane protein G protein-coupled receptors (GPCRs) that bind and respond to eicosanoid signaling molecules. Eicosanoids are rapidly metabolized to inactive products and therefore are short-lived. Accordingly, the eicosanoid-receptor interaction is typically limited to a local interaction: cells, upon stimulation, metabolize arachidonic acid to an eicosanoid which then binds cognate receptors on either its parent cell or on nearby cells to trigger functional responses within a restricted tissue area, e.g. an inflammatory response to an invading pathogen. In some cases, however, the synthesized eicosanoid travels through the blood to trigger systemic or coordinated tissue responses, e.g. prostaglandin (PG) E2 released locally travels to the hypothalamus to trigger a febrile reaction. An example of a non-GPCR receptor that binds many eicosanoids is the PPAR-γ nuclear receptor.
Prostaglandin receptors or prostanoid receptors represent a sub-class of cell surface membrane receptors that are regarded as the primary receptors for one or more of the classical, naturally occurring prostanoids viz., prostaglandin D2,, PGE2, PGF2alpha, prostacyclin (PGI2), thromboxane A2 (TXA2), and PGH2. They are named based on the prostanoid to which they preferentially bind and respond, e.g. the receptor responsive to PGI2 at lower concentrations than any other prostanoid is named the Prostacyclin receptor (IP). One exception to this rule is the receptor for thromboxane A2 (TP) which binds and responds to PGH2 and TXA2 equally well.
Prostaglandin-I synthase also known as prostaglandin I2 (prostacyclin) synthase (PTGIS) or CYP8A1 is an enzyme involved in prostanoid biosynthesis that in humans is encoded by the PTGIS gene. This enzyme belongs to the family of cytochrome P450 isomerases.
Thromboxane A2 (TXA2) is a type of thromboxane that is produced by activated platelets during hemostasis and has prothrombotic properties: it stimulates activation of new platelets as well as increases platelet aggregation. This is achieved by activating the thromboxane receptor, which results in platelet-shape change, inside-out activation of integrins, and degranulation. Circulating fibrinogen binds these receptors on adjacent platelets, further strengthening the clot. Thromboxane A2 is also a known vasoconstrictor and is especially important during tissue injury and inflammation. It is also regarded as responsible for Prinzmetal's angina.
Prostaglandin H2 is a type of prostaglandin and a precursor for many other biologically significant molecules. It is synthesized from arachidonic acid in a reaction catalyzed by a cyclooxygenase enzyme. The conversion from Arachidonic acid to Prostaglandin H2 is a two step process. First, COX-1 catalyzes the addition of two free oxygens to form the 1,2-Dioxane bridge and a peroxide functional group to form Prostaglandin G2. Second, COX-2 reduces the peroxide functional group to a Secondary alcohol, forming Prostaglandin H2. Other peroxidases like Hydroquinone have been observed to reduce PGG2 to PGH2. PGH2 is unstable at room temperature, with a half life of 90-100 seconds, so it is often converted into a different prostaglandin.
G-protein coupled receptor 31 also known as 12-(S)-HETE receptor is a protein that in humans is encoded by the GPR31 gene. The human gene is located on chromosome 6q27 and encodes a G-protein coupled receptor protein composed of 319 amino acids.
Protease-activated receptor 4 (PAR-4), also known as coagulation factor II (thrombin) receptor-like 3, is a protein that in humans is encoded by the F2RL3 gene.
Probable G-protein coupled receptor 135 is a protein that in humans is encoded by the GPR135 gene.
Cytochrome P450 3A43 is a protein that in humans is encoded by the CYP3A43 gene.
Cytochrome P450 4F12 is a protein that in humans is encoded by the CYP4F12 gene.
CYP4X1 is a protein which in humans is encoded by the CYP4X1 gene.
Proadifen (SKF-525A) is a non-selective inhibitor of cytochrome P450 enzymes, preventing some types of drug metabolism. It is also an inhibitor of neuronal nitric oxide synthase (NOS), CYP-dependent arachidonate metabolism, transmembrane calcium influx, and platelet thromboxane synthesis. Further documented effects include the blockade of ATP-sensitive inward rectifier potassium channel 8 (KIR6.1), and stimulation of endothelial cell prostacyclin production.
Thromboregulation is the series of mechanisms in how a primary clot is regulated. These mechanisms include, competitive inhibition or negative feedback. It includes primary hemostasis, which is the process of how blood platelets adhere to the endothelium of an injured blood vessel. Platelet aggregation is fundamental to repair vascular damage and the initiation of the blood thrombus formation. The elimination of clots is also part of thromboregulation. Failure in platelet clot regulation may cause hemorrhage or thrombosis. Substances called thromboregulators control every part of these events.
12-Hydroxyheptadecatrenoic acid is a 17 carbon metabolite of the 20 carbon polyunsaturated fatty acid, arachidonic acid. It was first detected and structurally defined by P. Wlodawer, Bengt I. Samuelsson, and M. Hamberg as a product of arachidonic acid metabolism made by microsomes isolated from sheep seminal vesicle glands and by intact human platelets. 12-HHT is less ambiguously termed 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid to indicate the S stereoisomerism of its 12-hydroxyl residue and the Z, E, and E cis-trans isomerism of its three double bonds. The metabolite was for many years thought to be merely a biologically inactive byproduct of prostaglandin synthesis. More recent studies, however, have attached potentially important activity to it.
