Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The blood coagulation protein factor Va forms the receptor for the serine protease factor Xa on the platelet surface. This membrane-bound complex of factor Va and factor Xa plus Ca2+ comprises the prothrombinase complex, the enzyme that catalyzes the proteolytic conversion of prothrombin to the clotting enzyme thrombin. Factor Va is a two-subunit protein composed of component D (Mr = 94,000) and component E (Mr = 74,000); subunit interaction is Ca2+ dependent. Factor Va bound to platelets consists of three peptides: component D, component E, and component D'(Mr = 90,000) which appears as the result of a platelet-associated protease cleavage of component D. The present studies were undertaken to determine which peptide(s) mediates the binding of factor Va to the platelet membrane surface and which peptide(s) serves as the binding site for factor Xa. These interactions were assessed by direct measurements of radiolabeled factor Va and factor Xa binding to platelets as well as autoradiographic visualization of the factor Va peptides associated with the platelet. Experiments were performed to determine the interaction of components D and E with platelets under reaction conditions in which components D and E were present as either the intact, functional two-subunit protein or as nonfunctional discrete peptides dissociated by the addition of Na2EDTA. The results suggest that component E mediates the binding of factor Va to the platelet and also serves as the binding site for the interaction of factor Xa with platelet-bound factor Va.
 ...
PMID:Prothrombinase complex assembly on the platelet surface is mediated through the 74,000-dalton component of factor Va. 657 81

The coagulation protein Factor Va forms the receptor for the serine protease Factor Xa at the platelet surface. This membrane-bound complex of Factor Va and Factor Xa plus calcium constitutes the enzymatic complex prothrombinase, which effects the conversion of prothrombin to the clotting enzyme, thrombin. Studies were undertaken to investigate the proteolytic events accompanying the inactivation of platelet-bound Factor Va by activated protein C as well as the ability of Factor Xa to protect Factor Va from activated protein C inactivation. During the course of these studies, observations were made which indicated that Factor Va was also cleaved by both a platelet-associated protease, as well as Factor Xa. When Factor Va was incubated with washed platelets, electrophoresis and autoradiography of solubilized platelet pellets indicated that three Factor Va peptides were associated with the platelet: component D (Mr = 94,000), component E (Mr = 74,000), and a 90,000-dalton peptide (component D') which appeared with time as the result of a platelet-associated protease cleavage of component D. The Factor Va peptides bound to platelets were proteolytically inactivated by activated protein C, resulting in five peptide products, all of which remained associated with the platelet-membrane surface. Factor Va was protected from activated protein C proteolysis by complex formation with Factor Xa or active site-blocked Factor Xa. However, active Factor Xa cleaved platelet-bound Factor Va to peptide products which also remained associated with the platelet. Whereas activated protein C rapidly cleaved components D and D' with secondary cleavages occurring in component E, Factor Xa rapidly cleaved component E with secondary cleavages occurring in components D and D'. The Factor Xa-cleaved Factor Va is catalytically functional. To determine whether cleavage was necessary for function, prothrombin conversion reaction mixtures were monitored for thrombin formation and Factor Va cleavage with time in a defined phospholipid vesicle model system. The results indicated that Factor Xa cleavage of Factor Va is not essential for Factor Va activity but may promote its ability to function in the prothrombinase complex.
 ...
PMID:Proteolytic alterations of factor Va bound to platelets. 684 22

Monoclonal antibodies have been used to probe for structural transitions which potentially occur during the activation of bovine Factor V by thrombin. One of these antibodies (alpha 2D) is reactive with an epitope on the NH2-terminal segment of Factor V (i.e. the Mr = 94,000 component D), while a second antibody (alpha 2E) recognizes an epitope on the carboxyl-terminal segment of Factor V (the Mr = 74,000 component E). Neither antibody reacts with native unactivated Factor V. The alpha 2D-reactive epitope requires at least two proteolytic events for expression, and the alpha 2E-reactive epitope is expressed following the initial cleavage catalyzed by thrombin. In addition, the alpha 2E-reactive epitope is expressed upon the addition of chelators to Factor V. However, the alpha 2D epitope is not influenced by the removal of calcium ions. The interactions of these antibodies with Factor V and Factor V-derived peptides suggest that conformational changes occur in both the NH2-terminal and carboxyl-terminal regions of Factor V concomitant with activation by thrombin, which give rise to the antibody recognition sites.
 ...
PMID:Monoclonal antibodies selective for activated Factor V. Immunochemical probes for structural transitions occurring during the thrombin-catalyzed activation of the procofactor. 685 34