Glycoprotein IIb/IIIa
In biochemistry and medicine, glycoprotein IIb/IIIa is an integrin complex found on platelets. It is a transmembrane receptor for fibrinogen and von Willebrand factor, and aids platelet activation. The complex is formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation and endothelial adherence. Platelet activation by ADP leads to the aforementioned conformational change in platelet gpIIb/IIIa receptors that induces binding to fibrinogen. The gpIIb/IIIa receptor is a target of several drugs including abciximab, eptifibatide, and tirofiban.
gpIIb/IIIa complex formation
Once platelets are activated, granules secrete clotting mediators, including both ADP and TXA2. These then bind their respective receptors on platelet surfaces, in both an autocrine and paracrine fashion. The binding of these receptors result in a cascade of events resulting in an increase in intracellular calcium. Hence, this calcium increase triggers the calcium-dependent association of gpIIb and gpIIIa to form the activated membrane receptor complex gpIIb/IIIa, which is capable of binding fibrinogen, resulting in many platelets "sticking together" as they may connect to the same strands of fibrinogen, resulting in a clot. The coagulation cascade then follows to stabilize the clot, as thrombin converts the soluble fibrinogen into insoluble fibrin strands. These strands are then cross-linked by factor XIII to form a stabilized blood clot.Pathology
Defects in glycoprotein IIb/IIIa cause Glanzmann's thrombasthenia.Autoantibodies against IIb/IIIa can be produced in immune thrombocytopenic purpura.