Chemoenzymatic Synthesis of Homogeneous Heparan Sulfate and Chondroitin Sulfate Chimeras.

Heparan sulfate (HS) and chondroitin sulfate (CS) are two structurally distinct natural polysaccharides. Here, we report the synthesis of a library of seven structurally homogeneous HS and CS chimeric dodecasaccharides (12-mers). The synthesis was accomplished using six HS biosynthetic enzymes and four CS biosynthetic enzymes. The chimeras contain a CS domain on the reducing end and a HS domain on the nonreducing end. The synthesized chimeras display anticoagulant activity as measured by both in vitro and ex vivo experiments. Furthermore, the anticoagulant activity of H/C 12-mer 5 is reversible by protamine, a U.S. Food and Drug Administration-approved polypeptide to neutralize anticoagulant drug heparin. Our findings demonstrate the synthesis of unnatural HS-CS chimeric oligosaccharides using natural biosynthetic enzymes, offering a new class of glycan molecules for biological research.

Deciphering the substrate recognition mechanisms of the heparan sulfate 3-O-sulfotransferase-3.

The sulfation at the 3-OH position of a glucosamine saccharide is a rare modification, but is critically important for the biological activities of heparan sulfate polysaccharides. Heparan sulfate 3-O-sulfotransferase (3-OST), the enzyme responsible for completing this modification, is present in seven different isoforms in humans. Individual isoforms display substrate selectivity to uniquely sulfated saccharide sequences present in heparan sulfate polysaccharides. Here, we report two ternary crystal structures of heparan sulfate 3-OST isoform 3 (3-OST-3) with PAP (3'-phosphoadenosine 5'-phosphate) and two octasaccharide substrates: non 6-O-sulfated octasaccharide (8-mer 1) and 6-O-sulfated octasaccharide (8-mer 3). The 8-mer 1 is a known favorable substrate for 3-OST-3, whereas the 8-mer 3 is an unfavorable one. Unlike the 8-mer 1, we discovered that the 8-mer 3 displays two binding orientations to the enzyme: productive binding and non-productive binding. Results from the enzyme activity studies demonstrate that 8-mer 3 can contribute to either substrate or product inhibition, possibly attributed to a non-productive binding mode. Our results suggest that heparan sulfate substrates interact with the 3-OST-3 enzyme in more than one orientation, which may regulate the activity of the enzyme. Our findings also suggest that different binding orientations between polysaccharides and their protein binding partners could influence biological outcomes.

Structural and substrate specificity analysis of 3-O-sulfotransferase isoform 5 to synthesize heparan sulfate.

Heparan sulfate 3-O-sulfotransferase (3-OST) transfers a sulfo group to the 3-OH position of a glucosamine saccharide unit to form 3-O-sulfated heparan sulfate. 3-O-sulfation is known to be critically important for bestowing anticoagulant activity and other biological functions of heparan sulfate. Here, we report two ternary crystal structures of 3-OST-5 with PAP (3'-phosphoadenosine 5'-phosphate) and two octasaccharide substrates. We also used 3-OST-5 to synthesize six 3-O-sulfated 8-mers. Results from the structural analysis of the six 3-O-sulfated 8-mers revealed the substrate specificity of 3-OST-5. The enzyme prefers to sulfate a 6-O-sulfo glucosamine saccharide that is surrounded by glucuronic acid over a 6-O-sulfo glucosamine saccharide that is surrounded by 2-O-sulfated iduronic acid. 3-OST-5 modified 8-mers display a broad range of anti-factor Xa activity, depending on the structure of the 8-mer. We also discovered that the substrate specificity of 3-OST-5 is not governed solely by the side chains from amino acid residues in the active site. The conformational flexibility of the 2-O-sulfated iduronic acid in the saccharide substrates also contributes to the substrate specificity. These findings advance our understanding for how to control the biosynthesis of 3-O-sulfated heparan sulfate with desired biological activities.

Chemoenzymatic synthesis of heparan sulfate and heparin oligosaccharides and NMR analysis: paving the way to a diverse library for glycobiologists.

Heparan sulfate (HS) is a member of the glycosaminoglycans (GAG) family that plays essential roles in biological processes from animal sources. Heparin, a highly sulfated form of HS, is widely used as anticoagulant drug worldwide. The high diversity and complexity of HS and heparin represent a roadblock for structural characterization and biological activity studies. Access to structurally defined oligosaccharides is critical for the successful development of HS and heparin structure-activity relationships. In this study, a library of 66 HS and heparin oligosaccharides covering different sulfation patterns and sizes was prepared through an efficient method of chemoenzymatic synthesis. A systematic nuclear magnetic resonance spectroscopy study was firstly undertaken for every oligosaccharide in the library. In addition to the availability of different oligosaccharides, this work also provides spectroscopic data helpful for characterizing more complicated polysaccharide structures providing a safeguard to ensure the quality of the drug heparin. This HS/heparin library will be useful for activity screening and facilitate future structure-activity relationship studies.

Expression of heparan sulfate sulfotransferases in Kluyveromyces lactis and preparation of 3'-phosphoadenosine-5'-phosphosulfate.

Heparan sulfate (HS) belongs to a major class of glycans that perform central physiological functions. Heparin is a specialized form of HS and is a clinically used anticoagulant drug. Heparin is a natural product isolated from pig intestine. There is a strong demand to replace natural heparin with a synthetic counterpart. Although a chemoenzymatic approach has been employed to prepare synthetic heparin, the scale of the synthesis is limited by the availability of sulfotransferases and the cofactor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Here, we present a novel method to produce secreted forms of sulfotransferases in the yeast cells, Kluyveromyces lactis. Five sulfotransferases including N-sulfotransferase, 2-O-sulfotransferase, 3-O-sulfotransferase 1 and 6-O-sulfotransferases 1 and 3 were expressed using this method. Unlike bacterial-expressed sulfotransferases, the yeast proteins can be directly used to modify polysaccharides without laborious purification. The yeast-expressed sulfotransferases also tend to have higher specific activity and thermostability. Furthermore, we demonstrated the possibility for the gram-scale synthesis of PAPS from adenosine 5'-triphosphate at only 1/5000th of the price purchased from a commercial source. Our results pave the way to conduct the enzymatic synthesis of heparin in large quantities.