Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.4.21.69 (APC)
 16,337 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The assembly and function of the prothrombinase complex on the bovine and human platelet membrane is mediated through binding interactions in which factor Va bound to the platelet surface forms at least part of the "receptor" for factor Xa in a 1:1 stoichiometric complex. A model depicting these binding interactions is shown in Fig. 12. Data from our laboratory indicate that the prothrombinase catalyst assembles in an analogous manner on the surface of monocytes, lymphocytes, neutrophils, and well-defined phospholipid vesicles employed in model systems. The 74,000-Da subunit of factor Va, component E, which mediates the binding of factor Va to either bovine platelets, human monocytes, or phospholipid vesicles, is shown binding to the cell membrane through its putative "receptor." The 94,000-Da subunit of factor Va, component D, is associated with the membrane surface through its metal ion-dependent interaction with component E. Factor Va forms at least part of the receptor that mediates the binding of factor Xa to an appropriate membrane surface, because component E has been shown to contribute significantly to the interaction of factor Xa with either the platelet, monocyte, or vesicle membrane surface. Our data do not preclude the possibility that component D contributes to the binding of factor Xa and the function of the prothrombinase complex. Component D appears to be important for several reasons. Cleavage of component D by activated protein C results in the complete loss of factor Va cofactor activity. An interaction between factor Xa and component D is implied from the observation that factor Xa protects factor Va from activated protein C inactivation. Furthermore, the binding of factor Xa to platelet-bound factor Va results in the time-dependent cleavage of components D and D'. Because component D is not required absolutely for prothrombinase complex assembly, we would speculate that it may be important in mediating prothrombin binding (depicted as a three-domain molecule) and increasing the catalytic efficiency of the enzymatic complex.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Platelet factor Xa receptor. 133 8

Activated protein C has been derivatized with the active site-directed fluorophore 2-(dimethylamino)-6-naphthalenesulfonylglutamylglycylarginyl chloromethyl ketone (2,6-DEGR-APC). Covalently modified activated protein C has been used to investigate the binding interactions of the protein to factors V and Va in the presence of phospholipid vesicles. The fluorescence polarization of the 6-dimethylaminonaphthalene-2-sulfonyl moiety increased saturably with increasing phospholipid concentrations in the presence or absence of factor V or Va. Differences in the limiting polarization values indicated distinguishable differences in the interactions between 2,6-DEGR-APC and phospholipid in the presence of factor V or Va. The dissociation constant calculated for the 2,6-DEGR-APC/phospholipid interaction (7.3 X 10(-8) M) was not significantly altered by factor V but was decreased to 7 X 10(-9) M in the presence of factor Va. The interaction between 2,6-DEGR-APC and factor V or Va was characterized by a 1:1 stoichiometry. The binding of 2,6-DEGR-APC to factor V or Va in the presence of phospholipid could be reduced in a competitive manner by diisopropylphosphofluoridate-treated activated protein C. An analysis of the displacement curves indicated that the binding of 2,6-DEGR-APC was indistinguishable from the binding of diisopropylphosphofluoridate-treated activated protein C. The interaction between 2,6-DEGR-APC and phospholipid-bound factor Va was further examined using the isolated subunits of factor Va. Fluorescence polarization changes observed with component E of Va (light chain) closely corresponded with the changes observed with factor Va, whereas isolated component D (heavy chain) had little influence on the binding of 2,6-DEGR-APC to phospholipid vesicles. The data presented are consistent with the interpretation that component E of factor Va contains a binding site for activated protein C.
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PMID:The binding of activated protein C to factors V and Va. 375 31

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.
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PMID:Proteolytic alterations of factor Va bound to platelets. 684 22