Due to genetic mutations or acquired conditions, some patients are more prone to thrombi than others. Protein C is a vitamin K-dependent protein synthesized in the liver. It is part of the coagulation cascade (also known as factor XIV). The role of activated protein C (APC) is to inactivate factor Va and factor VIIIa. As we know from the coagulation cascade, factor VIIIa is part of the intrinsic pathway and leads to clot formation. Factor V is part of the common pathway, and when activated, it also leads to a fibrin clot. Protein C is thus a regulatory protein that 'turns off' factors VIIIa and Va. Protein C is activated when thrombin binds to thrombomodulin (an endothelial cell surface membrane protein). So Protein C can be thought of as an 'off switch' activated once a clot is made in a blood vessel.
Protein S is another vitamin K-dependent coagulation protein. Its role is to augment the activity of APC. Protein S is thus a cofactor of protein C.
If the proteins that C and S bind with are dysfunctional, the body will be missing an important regulator of coagulation. If the right mutation is present, the 'off switch' cannot be activated. The result will be excess coagulation, a state in which the coagulation will go 'unchecked' and potentially lead to a thrombus.
