Heparin pentasaccharide sequence

Effects of oligosaccharides on the biological functions of proteins (15 examples). The capacity for homophilic binding of the neural cell adhesion molecule (N-CAM, Section 12.5.3) is modulated by the extension of its polysialic chains. The binding of a highly specific heparin pentasaccharide sequence to the protein antithrombin III converts it to a potent anticoagulant (Section 17.3). 

Scheme 17. The structure of the specific heparin pentasaccharide sequence binding to AT.

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. 

This pentasaccharide sequence induces a conformational change in AT III which probably causes the complex to be more accessible to the active site of the proteases. The most relevant protease affected by the pentasaccharide 3 is factor Xa, but factor Xlla and plasma kallikrein activities can also be potentiated. Sequence 3 occurs in heparin as well as in various heparan sulfate proteoglycans of different origin including the vascular endothelium. 

It is obtained as fragments of commercial grade heparin which is produced by chemical or enzymatic depolymerisation. It contains less pentasaccharide sequences with a high affinity for antithrombin III. 

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.)...

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. 

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... 

Complex oligosaccharides as complex carbohydrates are classified into specific classes based on the presence and sequence of various multiple sugar moieties linked via functional groups at totally different positions. One of the specific functionalities of tetra-, penta-, and hexasaccha-rides is their water solubility, highly dependant on the presence of a number of 0-glycosylated and W-acetamido moieties of individual fragments. One well-known example of this classical functionality is heparin pentasaccharide. The physical properties of the most important selected complex oligosaccharides, such as tetra-, penta-, and hexasaccharides, are listed in O Table 8. 

Finally, we recall that oligosaccharide ligands are not systematically nonreducing terminal sequences. We have mentioned the conformational epitope of an a- 2—>8) sialic acid in Section 12.5.2. Another very well known example is the heparin pentasaccharide (Section 17.3.4). In this case we note that a succession of metabolic transformations has greatly altered a five-sugar sequence in a monotonous polysaccharide chain. 

Heparin has been used clinically for decades to prevent and treat thromboembolic disease and is isolated on an industrial scale from animal tissues, in particular pig intestinal mucosa. The conesponding physiological blood anticoagulant is presumably not heparin, but an HS species that is located on the surface of vascular endothelial cells. Both heparin and HS contain a specific pentasaccharide sequence that binds and activates the plasma proteinase inhibitor AT. This pentasaccharide sequence (4) is present only in a subfraction of heparin and HS preparations. It displays a characteristic structural feature, namely a 3-0-sulfated GlcN residue, that is only rarely seen in other portions of heparin/HS chains. 

The interactions of HS withFGFs " involve primarily the NS-domains (see Fig. 2) of the polysaccharide. Biochemical analyses of FGF2 interactions with heparin and HS oligosaccharides implicated a I-Ans-I-Ans-I2s pentasaccharide sequence (for symbols, see Fig. 1 reducing terminus to the right), notably lacking any 6-G-sulfatc groups, as the smallest and least sulfated structure capable... 

The completed polymers are modified uniformly There are clusters of sulfo groups with imusual stmctures in chondroitin from squid and shark cartilages and fucosylated chondroitin from echinoderms. Similar modifications are present less extensively in vertebrates. One of the best known modifications forms the unique pentasaccharide sequence shown in Fig. 4-13, which is essential to the anticoagulant activity of heparin. This sequence has been synthesized in the laboratory as have related longer heparin chains. A sequence about 17 residues in length containing an improved synthetic version of the imique pentasaccharide binds tightly to both thrombin and antithrombin (Chapter 12, Section This... 

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