A UNIQUE ROLE FOR ACIDIC PHOSPHOLIPIDS IN THE TISSUE FACTOR PATHWAY OF BLOOD COAGULATION (PROTHROMBIN FRAGMENT 1).
Item
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Title
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A UNIQUE ROLE FOR ACIDIC PHOSPHOLIPIDS IN THE TISSUE FACTOR PATHWAY OF BLOOD COAGULATION (PROTHROMBIN FRAGMENT 1).
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Identifier
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AAI8629690
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identifier
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8629690
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Creator
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FORMAN, STEVEN DAVID.
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Contributor
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Yale Nemerson
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Date
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1986
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Biochemistry
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Abstract
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Negatively-charged phospholipids enhance the activity of several procoagulant reactions, including the activation of factor X by the membrane-localized complex of tissue factor-factor VIIa; a suggested mechanism is that binding of the gammacarboxyglutamic acid-containing zymogens to negatively-charged membranes increases their effective concentration as substrates. Prothrombin fragment 1, the 156 residue amino-terminal peptide from prothrombin, and factor X compete for binding sites on negatively-charged phospholipid surfaces. This phenomenon was exploited to determine how phosphatidylserine modulates the tissue factor pathway. In a tissue factor system containing 30% phosphatidylserine, prothrombin fragment 1 displaced phospholipid-bound factor X, increasing the fluid-phase factor X concentration and the rate of factor X activation nearly 4-fold. The increased reaction velocities correlated completely with the increases in the fluid-phase factor X concentration; therefore, fluid-phase factor X regulates the tissue factor pathway. In the phosphatidylserine system the Km (calculated using the fluid-phase factor X concentration) was 41 nM and 63 nM in the presence and absence of prothrombin fragment 1, respectively. In contrast, the Km in a neutral phosphatidycholine system (to which neither factor X nor prothrombin fragment 1 binds) was 877 nM and 791 nM in the presence and absence of prothrombin fragment 1, respectively. Thus, phosphatidylserine accelerates the tissue factor pathway not by concentrating substrate but by a direct effect on the catalytic complex, increasing the association of fluid-phase substrate. Binding of factor X to the phospholipid surface creates new pathways to encounter the catalytic complex: binding to the membrane surface with subsequent surface diffusion. Simulations indicate that surface diffusion of membrane-bound substrate should dominate the rate of substrate-enzyme encounters, yet membrane-bound substrate did not influence reaction rates. This finding combined with the fact that negatively-charged phospholipids inhibit factor X activation by three other proteases (which all attack the same peptide bond as that attacked by tissue factor-factor VIIa) leads to the conclusion that phospholipid-binding protects factor X from activation. Thus, contrary to the prevailing view in which binding of substrates to phospholipids enhances the rates of procoagulant reactions; phospholipid-binding of factor X inhibits its activation.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
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Program
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Biomedical Sciences
