2000 Volume 12 Issue 64 Pages 121-127
Heparin and heparan sulphate (HS) are characterised as a group of linear and highly sulphated polysaccharides of glycosaminoglycan (GAG) type. Heparin is produced mainly by connective tissue mast cells, whereas heparan sulfate is synthesized virtually by most cell types. Recently, interest is increasing in this group of polysaccharides due largely to its multiple biological functions and potential association with disease (1). In fact, heparin and HS have been implicated in modulating various biological processes, such as blood clotting, cell adhesion, growth factor signalling, and viral infection. The biological functions of heparin and HS largely depend on interactions of the negatively charged polysaccharide chain with a variety of proteins, such as proteases, protease inhibitors, growth factors, extracellular matrix components and viral proteins (2). In some cases, a single domain, generally consisting of less than 10 monosaccharide units but with a defined saccharide structure, is required for specific interaction with a selected protein. Moreover, the polysaccharide chain may form complexes with two or more proteins, identical or different. To induce biological response, these saccharide domains must be organized in a proper way.
The enormous structural diversity of heparin/HS is created by a complex biosynthetic pathway (3). The biosynthesis of the polysaccharide chain is initiated by the formation of a precursor polymer composed of repeating disaccharide units of alternating D-glucuronic acid and N-acetyl-D-glucosamine, [-GlcA-GlcNAc-]n. While the chain is elongating, the polymer is modified by a series of sequential reactions including N-deacetylation/N-sulphation of GlcNAc, C5-epimerization of GlcA to L-iduronic acid (IdoA) and O-sulphation of IdoA at position C2 and of GlcN residues at position C6. In addition, to a lesser extent, O-sulphation of GlcA at position C2 and of GlcN at position C3 may also occur and appear to be biologically important. Most of the enzymes involved in HS biosynthesis have been characterised in molecular detail. Interestingly, some of these enzymes exist in several genetic isoforms with distinct substrate specificities (2, 4, 5).
This thesis has been conducted to address two specific questions regarding the biosynthetic pathway and the structureactivity relationship of heparin/HS. Firstly, how 2-O-sulfation of GlcA and IdoA residues is accomplished in the biosynthetic process of heparin/HS. Secondly, how different saccharide domains must be organized in order to induce biological response.
