Antithrombin, interaction with heparin

Answer Positively charged amino acid residues would be the best candidates to bind to the highly negatively charged groups on heparin. In fact, Lys residues of antithrombin III interact with heparin. 

Antithrombin III (AT-III), a single-chain glycoprotein of 58 kDa and 480 amino acids, is synthesized in the liver. It is a serine protease inhibitor, and acts as the most important inhibitor in the coagulation cascade to avoid blood clot formation. AT-III inhibits a wide spectram of serine proteases induding thrombin, factors IXa, Xa and XIa, kaUikrein, plasmin, urokinase, Cl-esterase, and trypsin. AT-III interacts with heparin by binding to specific sul-fated and non-sulfated monosaccharide units on heparin. The binding of AT-III to heparin enhances the inhibition of factors IXa, Xa, and thrombin. 

By using the reverse loading sequence thrombin could interact with heparin/antithrombin III complex or with heparin alone. The colour yield obtained was similar in this sequence to that obtained by only thrombin loading, indicating that none of the thrombin on heparin-PVA gel had been neutralized by the heparin/antithrombin III complex. 

Figure 5. Fluorescence anisotropy of F-D labelled heparin-antithrombin interaction. F-D-heparin (0.02 fluoresceins per uronic acid) at 0.1 mg/ml was incubated with different concentrations of antithrombin (open circles) or bovine serum albumin (solid diamonds) in 20 mM sodium phosphate buffer, pH 7.4.

Based on the mixed-phase method, ACE is introduced for studying the interaction of heparin with the serine protease inhibitors, antithrombin III (ATIII) and secretory leukocyte proteinase inhibitor (SLPI) (85). An etched capillary, to which heparin has been covalently immobilized, was used in this study. This modified capillary both afforded an improvement in the separation of heparin-binding proteins and required a lower quantity of loaded protein. 

Heparin binds to antithrombin III and induces a conformational change that accelerates the interaction of antithrombin III with the coagulation factors. Heparin also catalyzes the inhibition of thrombin by heparin cofactor II, a circulating inhibitor. Smaller amounts of heparin are needed to prevent the formation of free thrombin than are needed to inhibit the protease activity of clot-bound thrombin. Inhibition of free thrombin is the basis of low-dose prophylactic therapy. 

Figure 17.2 Schematic representation of the molecular weight distribution of unfractionated heparin (UH) and of low molecular weight heparin (LMWH). In the lower part of the figure, the polysaccharide chain of heparin, the pentasaccharide sequence, and the interaction between heparin, antithrombin (AT), thrombin, and factor Xa is represented. (Reproduced from Boneu B.Thrombosis Research 2000 100 V113-20, with permission from Elsevier Science.)...

The indirect thrombin inhibitors are so-named because their antithrombotic effect is exerted by their interaction with a separate protein, antithrombin. Unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), and the... 

Heparin acts indirectly at multiple points within the coagulation cascade, Its major anticoagulant effect is via interaction with its requisite co-factor, antithrombin III (AT). The heparin-AT complex inactivates factors IXa, Xa, and XIla, and binds thrombin at its active site to prevent the conversion of fibrinogen to fibrin (3). Heparin also prevents fibrin stabilization through the inhibition of fibrin stabilization factor. Heparin has no fibrinolytic activity and therefore is ineffective as a thrombolytic (4,5). 

Active clotting factors (lla, IXa, Xa, Xla, Xlla, Xllla) a-globulin that inhibits serine proteases, including several of the clotting factors, for example, thrombin (Factor II, Figure 20.5). In the absence of heparin, antithrombin III interacts with thrombin... 

Heparins inhibit factors Xa and lla through their interaction with antithrombin, a naturally occurring anticoagulant. Unfractionated heparin (UFH) is a parenteral agent that requires monitoring of the aPTT. Complications with UFH... 

Chemical synthesis of heparin fragments and analogues has been used for biological studies and to define structure-activity relationships [46]. For example, the heparin-antithrombin m interaction [47,48,49,50,51,52,53], which is responsible for the anticoagulant activity of heparin, has been studied in detail by using synthetic fragments. The interaction of heparin with FGF-1, FGF-2, platelets [54,55,56,57,58,59,60,61,62,63,64], and the Herpes Simplex virus, has also been studied by using synthetic specimens. 

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