CHAPTER 8: Chemoenzymatic Synthesis of Low-Molecular-Weight Heparin and Heparan Sulfate

Heparin and low-molecular-weight heparins are specialized forms of the heparan sulfate glycosaminoglycan that are used therapeutically as anticoagulant drugs. These anionic polysaccharides and their oligosaccharide derivatives are generally prepared by extraction from animal tissues, purified, and then enzymatically or chemically treated before being used as biochemical reagents or drugs. Based on our understanding of glycosaminoglycan biosynthesis, a number of biosynthetic enzymes have been identified, cloned, and expressed in Escherichia coli. Using these enzymes, two strategies for the chemoenzymatic synthesis have been devised. In the first, a polysaccharide backbone, prepared through fermentation, is chemoenzymatically modified to yield a mixture of heparin molecules similar to the natural product. The second strategy involves the enzyme-catalyzed iterative elongation of an acceptor glycan with nucleotide sugar donors that are then chemoenzymatically modified to form single pure heparin or heparan sulfate oligosaccharides. These products of chemoenzymatic synthesis are useful for developing structure–activity relationships and as potential new therapeutic entities. The general approaches used and future prospects for chemoenzymatic glycosaminoglycan synthesis are discussed.