Thromboxane-A synthase

Thromboxane A synthase 1 (platelet, cytochrome P450, family 5, subfamily A), also known as TBXAS1, is a is_associated_with::cytochrome P450 is_associated_with::enzyme that, in humans, is encoded by the TBXAS1 is_associated_with::gene.

Function
This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases that is_associated_with::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 is_associated_with::vasoconstrictor and inducer of is_associated_with::platelet aggregation and is_associated_with::12-Hydroxyheptadecatrienoic acid (i.e. 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid or 12-HHT) and agonist of is_associated_with::Leukotriene B4 receptors (i.e. BLT2 receptors) and mediator of certain BLT2 receptor]] actions. The enzyme plays a role in several pathophysiological processes including hemostasis, cardiovascular disease, and stroke. The gene expresses two transcript variants.

Inhibitors
are used as is_associated_with::antiplatelet drugs. is_associated_with::Ifetroban is a potent and selective thromboxane receptor antagonist. is_associated_with::Dipyridamole antagonizes this receptor too, but has various other mechanisms of antiplatelet activity as well. is_associated_with::Picotamide has activity both as a thromboxane synthase inhibitor and as a is_associated_with::thromboxane receptor antagonist.

Structure
The human is_associated_with::thromboxane A (TXA) synthase is a 60 kDa 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. 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. 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. 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.

Mechanism


is_associated_with::Thromboxane A (TXA) is derived from the is_associated_with::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. 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. 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). 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. 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.

Biological Significance
Maintaining a balance between prostacyclins and is_associated_with::thromboxanes is important in the body, particularly because these two is_associated_with::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 is_associated_with::pulmonary hypertension. Because thromboxanes play a role in is_associated_with::vasoconstriction and platelet aggregation, their dominance can disrupt vascular homeostasis and cause is_associated_with::thrombotic vascular events. Furthermore, the importance of thromboxanes and their synthases 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.

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 is_associated_with::papillary thyroid carcinoma, is_associated_with::prostate cancer, and is_associated_with::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, is_associated_with::apoptosis inhibition, is_associated_with::angiogenesis, and is_associated_with::metastasis.