The mechanisms and kinetics of initiation of blood coagulation by the extrinsic tenase complex

Abstract

The system of hemostasis includes coagulation of blood plasma and formation of platelet aggregate. Plasma clotting is a cascade of proteolytic reactions, triggered by the contact of blood plasma with any tissue except the normal vessel endothelium. During the contact an enzymatic complex is formed of the soluble blood plasma protein, factor VIIa, and a membrane-anchored protein, tissue factor. This complex is called extrinsic tenase; it is the key initiator of blood coagulation. The main substrates of extrinsic tenase are blood plasma factors X and IX. During the reaction they undergo proteolytic cleavage and become active serine proteases, factors Xa and IXa, respectively. Factor Xa in complex with its cofactor factor Va catalyzes formation of the key coagulation enzyme, thrombin, which leads to fibrin polymerization and plasma gelation. Although all of the proteins that participate in this process have been known for a long time, several questions remain unanswered. As an example, what is the role of the reaction surface on which the complex is formed, what is the role of membrane-bound multimeres of factor X (Xa), and in what way does the activation of the factor VII proceed? Here, we review recent theoretical and experimental works focused on the biophysical mechanisms of extrinsic tenase functioning and discuss some of these problems.