Protein S human

Thrombin converts protein C to activated protein C (APC). Protein S functions as a cofactor for APC and together they proteolytically cleave factor Va and VIIIa. This inhibits the activities of the prothrombinase and tenase complexes respectively and leads to a reduction in thrombin generation. The reduction in thrombin generation suppresses the coagulation cascade preventing additional clotting from occuring. Protein S is suggested to inhibit factors Va and Xa through protein-protein interactions. Protein S is also suggested to inhibit production of factor Xa by the tenase complex by competing for binding to phospholipids via its gamma-carboxyglutamic acid domain.

Protein S is thought to exert an anti-inflammatory effect in addition to its anticoagulant properties. The protein S-C4b-binding protein complex is believed to bind to phosphotidylserines on the cell membranes of apoptotic cells and interact with macrophages, signalling them to phagocytose the cell before it can rupture.