UC Davis

Sialic acids (Sias) are common terminal monosaccharides on the cell surface glycoconjugates of vertebrates and some higher invertebrates. Being the outermost sugar residues, Sias are involved in many molecular recognition events including viral and bacterial infections, cell-cell interaction, immune regulation, inflammation, cancer metastasis, and others.Among the various post-glycosylation modifications of Sias, O-acetylation is the most-common. O-Acetylation of Sia has striking effects on its recognition by viruses, lectins, sialidases and plays an important role in biological and pathological processes. Among all the O-acetyl modifications of Sias, 7-O-acetylation of sialoglycans is among the ones that are the most difficult to study. In addition to the sensitivity of the O-acetyl group towards both pH changes and esterase, it can migrate to hydroxyls at neighboring C-8 and C-9, resulting in a mixture of O-acetylated sialoglycans. 7-O-Acetylation of sialoglycans regulates many biological phenomena, specifically microbe-host interactions, and regulation of immune responses. We have designed and developed a chemical biology approach to partially overcome this challenge by replacing the oxygen atom linked to the C7 in the 7-O-acetylated sialic acid by a nitrogen to form an amide in the chemically and biologically stable 7-acetamido-7-deoxy-N-acetylenuraminic acid (Neu5Ac7NAc). The resulting Neu5Ac7NAc-containing sialoglycans as the stable mimics of Neu5,7Ac2-containing sialosides are synthesized using a highly efficient one-pot multienzyme (OPME) chemoenzymatic method. A novel chemoenzymatic synthon strategy is developed to construct a comprehensive library of a2–3- and a2–6-linked sialosides containing 7-N- or 7,9-di-N-acetyl sialic acid. Mannose derivatives containing multiple azido groups such as diazido-mannose (Man2,4diN3) and triazido-mannose (Man2,4,6triN3) are chemically synthesized from inexpensive galactose in multiple steps. These synthesized mannose derivatives were used as synthons for sialosides using bacterial sialoside biosynthetic enzymes which showed remarkable substrate promiscuity. They are successfully used in one-pot multienzyme (OPME) sialylation systems for highly efficient synthesis of sialosides containing azido moieties. Conversion of the azido groups in the sialic acid to N-acetyl groups generates the desired sialosides. Sialidase assays using these compounds showed that the N-acetyl group at C-5 is required for the catalytic activity of all the sialidases tested while sialosides with N-acetyl at C-7 in the sialic acid were poor or unsuitable substrates for these sialidases. Legionaminic acid, Leg5,7Ac2, is a bacterial nonulosonic acid reported as the component of lipopolysaccharides (LPS) and capsular polysaccharides (CPS) of pathogenic bacteria such as Campylobacter jejuni, Enterobacter cloacae, Acinetobacter baumannii and Cronobacter turicensis. Its presence has been linked to bacteria virulence in humans. One efficient strategy to investigate its role and to fight bacterial infections is to develop synthetic oligosaccharides which mimics the LPS or CPS. We have chemically synthesized the six carbon precursors of Leg5,7Ac2 and Leg5,7diN3 derivatives from commercially available D-fucose. 6-Deoxy Man2,4diN3 is used in an efficient one-pot multi-enzyme (OPME) chemoenzymatic methods for synthesizing a library of a2-3/6-linked glycans containing Leg5,7diN3. The Leg5,7diN3 in glycosides was chemically converted in one step to Leg5,7Ac2. An extensive library of glycosides containing Leg5,7Ac2 and underlying glycans may present novel targets for enabling study of its role in bacterial pathogenesis and physiology. In summary, during my doctoral research, I have worked on design and synthesis of stable mimics of unstable 7-O-acetyl sialic acids and 7-O-acetylated sialoglycans to explore their biology. I have also designed and synthesized glycans containing bacterial nonulosonic acids and derivatives to develop important probes and potential anti-bacterial therapeutics.