Heparin antithrombin-binding pentasaccharide

Olson ST, Bjork I, Sheffer R, Craig PA, Shore JD, Choay J. Role of the antithrombin-binding pentasaccharide in heparin acceleration of antithrombin-proteinase reactions. Resolution of the antithrombin conformational... 

Synthesis of the Antithrombin-Binding Pentasaccharide Sequence in Heparin (1984) [19, 20]... 

Fondaparinux is a chemically synthesized pentasaccharide that mimics the antithrombin-binding site of heparin and LMWH. Its molecular size (1728Da) is too small to bind to thrombin molecules while it is bound to antithrombin, Therefore, it is a pure anti-Xa inhibitor. It binds very little to platelets, proteins, or endothelium and is excreted in the urine, It does not form a complex with PF4 or other positively charged molecules. It is not neutralizable by protamine sulfate, Recent clinical trials have resulted in FDA approval for prophylaxis of deep vein thrombosis in orthopedic surgery, It has been shown to be effective and safe for the treatment of pulmonary embolism (20,21) and ACS (non-ST-elevation Ml) (OASIS 5—Michelangelo Trial) (17). 

Fondaparinnx is a sjmthetic pentasaccharide that mimics the site of heparin that binds to antithrombin III and inhibits factor Xa activity, which in tnm inhibits thrombin generation. It does not release tissne factor pathway inhibitor. It is nearly completely absorbed after snbcntaneous... 

Figure 4-13 A pentasaccharide segment of heparin which binds with high affinity to the serum protein antithrombin causing it to inhibit most of the serine protease enzymes participating in the blood coagulation process (see Chapter 12). See Lindahl et al. ...

Hydrophobic interaction may also play a minor role in heparin s interaction with antithrombin III (AT III), where hydrophobic interactions are reported between hydrophobic amino acid side chains in AT III and the A-acetyl group in the AT Ill-binding pentasaccharide sequence in porcine mucosal heparin (8). 

The antithrombin-binding heparin-like pentasaccharide glycoside (8) has been synthesized, and the corresponding free sugar sulphated at -6 of the reducing moiety has also been prepared (c.f.,... 

The repeating monosaccharide units of heparin are N-acetyl-o-glucosamine, D-glucuronic acid, D-glucosamine, and L-ioduronic acid bonded by a combination of a-1,4- and S-l,4-glycosidic bonds. Figure 25.6 shows a pentasaccharide unit of heparin that binds to and inhibits the enzymatic activity of antithrombin III. 

Linear Pentasaccharides.-The effect of different bulky substituents at C-6 of the nonreducing terminal unit of a glycoside of maltopentaoside on the rates of hydrolysis ty human salivary and pancreatic a-amylases has been studied. Several methyl ethers of the pentasaccharide unit of heparin responsible for antithrombin binding have been reported. ... 

A heparin-chain segment longer than its minimal binding site (the pentasaccharide 12) is necessary for enhancing the antithrombin-me-... 

The anticoagulant effect of UFH is mediated through a specific pentasaccharide sequence on the heparin molecule that binds to antithrombin, provoking a conformational change. The UFH-antithrombin complex is 100 to 1,000 times more potent as an anticoagulant than antithrombin alone. 

Low-molecular-weight fragments produced by chemical depolymerization and extraction of standard heparin consist of heterogeneous polysaccharide chains of molecular weight 2,000 to 9,000. The LMWH molecules contain the pentasaccharide sequence necessary for binding to antithrombin III but not the 18-saccharide sequence needed for binding to thrombin. Compared to standard heparin, LMWH has a 2- to 4-fold greater antifactor Xa activity than antithrombin activity. 

Fondaparinux sodium is the first of a new class of direct-acting antithrombin agents. The anticoagulant activity of heparin depends on the binding of ATIII to a critical pentasaccharide sequence within the heparin molecule. This results in a change in the conformation of AITII. This leads to the inhibitory effect of ATIII on factor Xa, which activates the conversion of prothrombin to thrombin. Fondaparinux is a synthetic pentasaccharide identical to the ATIII-binding site of heparin. 

The indirect thrombin inhibitors are so named because their antithrombotic effect is exerted by their interaction with antithrombin. Unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), and the synthetic pentasaccharide fondaparinux bind to antithrombin and enhance its inactivation of factor Xa. UFH and to a lesser extent LMWH also enhance antithrombin s inactivation of thrombin (Ha). 

UFH binds to exosite 2, located on antithrombin, forming a ternary complex. This ternary complex is necessary for the inhibition of thrombin by antithrombin (Fig. I A, left). Conversely to thrombin inhibition, inactivation of factor Xa does not require the formation of the ternary complex. UFH inhibits thrombin and factor Xa in the same proportion (the ratio of anti-Xa/lla activity equals I) (Fig, I A, right). The interaction of the heparins (UFH and LMWH) with antithrombin is mediated by a unique pentasaccharide sequence, which is present in approximately one-third of the UFH chains (2). 

Heparin is a glycosaminoglycan extracted from animal tissues (porcine mucosa, beef lung, etc.). It is a mixture of molecules having a mean molecular weight of 15,000 Da. A pentasaccharide sequence found in approximately one third of the molecules binds to antithrombin in mammalian blood, enhancing its inhibitory effects on the enzymes thrombin, factor Xa, factor Vila, and factor IXa. The reaction is reversible, heparin being released after the antithrombin molecule binds to the procoagulant enzymes. Heparin binds to platelets, platelet factor-4 (which neutralizes it), histidine-rich GP vWp and a number of other proteins. Its half-life is about one hour in the circulation (18). Antibodies to heparin... 

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