A clotting scheme for 1984

Two processes govern the generation of thrombin: proteolytic proenzyme activation and heterogeneous biocatalysis. The main reaction sequence of proenzyme activation is F VII----F X----F II. And there is a reinforcement loop constituted by the reactions F VII----F IX----F X. All activations in this central triangle of blood coagulation require a protein cofactor and phospholipid. The protein cofactors from the plasma (F V and F VIII) have to be activated by thrombin in order to function. This means that thrombin acts in a positive feedback loop. This is the more important because thrombin will cause platelets that stick to collagen to present their procoagulant phospholipids at the outside of their plasma membrane. The two most probable sites of action of heparin are: inhibition of thrombin and frustration of the feedback loops and/or inhibition of factor Xa.