Proteoglycans (PGs) are an important class of glycoproteins widely distributed in mammals. They are involved in numerous biological events, including tumor progression, inflammation, and cellular communication. Generally, a PG is composed of a core protein and one or more glycosaminoglycan (GAG) polysaccharide chains. The GAG chain is covalently attached to the core protein via a serine residue in the consensus sequence -Ser-Gly-X-Gly- (X being any natural amino acid residue but proline) by a common tetrasaccharide linkage. Heparan sulfate proteoglycans (HSPGs), along with chondroitin sulfate proteoglycans (CSPGs) and keratan sulfate proteoglycans (KSPGs), are main subtypes of the PG family. Naturally existing HSPGs, due to complex post-translational modifications (PTMs) on the GAG chains, are highly heterogeneous. That makes direct isolation of homogeneous HSPGs from natural sources almost impossible. To date, preparing structurally defined HSPGs solely relies on formidable and tedious chemical synthesis.In this dissertation, two novel approaches have been investigated to expedite the synthesis of HSPGs. The convergent chemoenzymatic approach takes advantage of efficient enzymatic synthesis of heparan sulfate (HS) oligosaccharides and well-developed solid phase supported peptide synthesis (SPPS). By substituting the non-functional tetrasaccharide linkage, the GAG chain and peptide were conjugated through a flexible artificial linker to make a syndecan-1 mimetic, which mimics the natural structures of syndecan-1, an important member of HSPG family. The mimetic binds strongly to integrin αvβ3, a key cell-surface protein that plays an active role in tumor proliferation process. Furthermore, the mimetic compound is able to inhibit the migration of breast cancer cells MDA-MB-231. In the native form of PGs, the core protein and GAG chains are connected through a common tetrasaccharide linkage consisted of GlcA-β(1→3)-Gal-β(1→3)-Gal-β(1→4)-Xyl-β(1→O)-Ser to efficiently prepare native heparan sulfate glycopeptides and glycoproteins, enzymes involved in the PG linkage biosynthesis were investigated and developed as synthetic tools. Human β-1,4- galactosyltransferase 7 (β4GalT7) was used to catalyze the transfer of galactose units and synthesize galactose-xylose (Gal-Xyl) bearing PG glycopeptides. Human xylosyltransferase I (XT-I), the enzyme that initiates PG biosynthesis in nature, was then studied and applied towards the synthesis of PG linkage region.
Read
