Heparin-binding domains in vascular biology

Abstract

Heparin is a major anticoagulant with activity mediated primarily through its interaction with antithrombin (AT). Heparan sulfate (HS), structurally related to heparin, binds a wide range of proteins of different functionality, taking part in various physiological and pathological processes. The heparin-AT complex, the most well understood facet of anticoagulation, serves as a prototypical example of the important role of heparin/HS in vascular biology. Extensive studies have identified common structural features in heparin/HS-binding sites of proteins. These include the elucidation of consensus sequences in proteins, patterns of clusters of basic and nonbasic residues, and common spatial arrangements of basic amino acids in the heparin-binding sites. Although these studies have provided valuable information, heparin/HS-binding proteins differ widely in structure. The prediction of heparin/HS-binding proteins from sequence information is not currently possible, and elucidation of protein-binding sites requires the individual study of each glycosaminoglycan-protein complex. Thus, x-ray crystallography and site-directed mutagenesis experiments are among the most powerful tools, providing accurate structural information, facilitating the characterization of heparin-protein complexes. Heparin and structurally related heparan sulfate bind a large number of proteins, taking part in a wide range of biological processes, particularly ones involved in vascular biology. Heparin-binding domains share certain common structural features, but there is no absolute dependency on specific sequences or protein folds.

Figure 1

A, Major and minor disaccharide-repeating units in heparin and HS (X=H or SO₃⁻; Y=Ac, SO₃⁻, or H). B, Structure of AT-binding heparin pentasaccharide.

Figure 2

A, Crystal structure of the AT–pentasaccharide complex (from Protein Data Bank). The pentasaccharide is marked in red. The basic residues of AT involved in interaction with the heparin pentasaccharide are shown in B, C, and D. B, Lys 114, Lys125, and Arg 129. C, Arg 47, Arg 46, Lys 11, and Arg 13. D, Arg 24, Phe 121, and Phe 122.

Figure 3

A, In the intact vessel, HSPGs catalyze the inactivation of thrombin (T) and factor Xa by AT through formation of covalent complexes T-AT and Xa-AT. B, In the wounded vessel, the endothelium containing HSPGs with AT-binding sites is lost. Thus, T catalyzes fibrinogen (Fbng)→fibrin (Fbn) clot formation promoting coagulation. Inhibition of T by HC mediated by HSPGs also takes place in the wounded vessel, avoiding subluminal coagulation. C, Chemokines (Cx) are involved in the recruitment of lymphocytes that occur on inflammation. Cx form stable gradients through interaction with HSPG on the endothelium.