Blood clotting involves a multitude of
proteins that act in concert in response to
vascular injury to produce the procoagulant
enzyme alpha-thrombin, which in turn is responsible for the generation of the
fibrin plug. However, while generation of the
fibrin plug is required for the arrest of excessive
bleeding, unregulated clotting will result in the occlusion of the blood vessels and
thrombosis. Thus, the regulation of the delicate balance between the procoagulant and
anticoagulant mechanisms is of extreme importance for survival. While the majority of
proteins involved in blood coagulation circulate as inactive
zymogens that require proteolytic activation in order to function, approximately 1% of the circulating
factor VII molecules are active.
Factor VIIa, possess a
serine protease active site, has poor catalytic activity, and is not inhibited by the circulating stoichiometric
protease inhibitors. Following injury to the vasculature and subsequent exposure of the integral
membrane glycoprotein,
tissue factor (TF), the circulating
factor VIIa molecules can bind to the exposed TF forming the extrinsic
tenase complex (TF/
factor VIIa) and initiate the blood coagulation process. Formation of the TF/
factor VIIa complex increases the catalytic efficiency of the
enzyme by four orders of magnitude when compared with
factor VIIa alone. This cell-associated enzymatic complex initiates a series of enzymatic reactions, leading to the generation of
alpha-thrombin and ultimately to the formation of the
fibrin plug. The procoagulant enzymatic complexes (i.e.,
prothrombinase,
intrinsic tenase, and extrinsic
tenase) are similar in structure and composed of an
enzyme, a cofactor, and the substrate associated on a cell surface in the presence of divalent
metal ions. While the activity of the extrinsic
tenase complex is limited by the availability (exposure) of its cell-associated cofactor (TF) it is remarkable that the activities of both the
prothrombinase complex (
factor Va/
factor Xa) as well as the
intrinsic tenase complex (
factor VIIIa/
factor IXa) are limited by the presence of the two soluble, nonenzymatic cofactors,
factor Va and
factor VIIIa.
Factor Va and
factor VIIIa, which are very similar in structure and function, are required for
prothrombinase and
intrinsic tenase activities, respectively, because both cofactors express a dual function in their respective complexes, acting as an
enzyme receptor and catalytic effector on the cell surface. The cofactors derive from inactive plasma precursors by regulatory proteolytic events that involve
alpha-thrombin. In general,
bleeding tendencies are usually associated with defects in the activation of one of the
zymogens or the cofactors of the procoagulant complexes. However, the activity of all of the complexes is also limited by the availability of an adequate membrane surface provided by endothelial cells, platelets, and monocytes. The cell surface provides a site for the recruitment of the appropriate
proteins and allows for fast and efficient clot formation. In the absence of an appropriate membrane surface, the procoagulant complexes have limited catalytic efficiency. Thus, timely exposure of the adequate membrane surface is an additional step in the regulation of
alpha-thrombin formation.
alpha-Thrombin participates in its own down-regulation by binding to the endothelial cell receptor
thrombomodulin, initiating the
protein C pathway, which in turn leads to the formation of activated
protein C (APC). APC is required for efficient neutralization of
factor Va cofactor activity, which results in the inactivation of the
prothrombin-activating complex. This inactivation can only occur in the presence of the appropriate membrane surface. Thus, while following
alpha-thrombin activation,
factor VIIIa is rapidly and spontaneously inactivated by dissociation of the A2 domain from the rest of the cofactor, APC is required for down-regulation of
alpha-thrombin formation by
prothrombinase. (ABSTRACT