Enzymatic synthesis of 7-deoxy-N-acetylneuraminic acid and 7-O-methyl-N-acetylneuraminic acid

Chemoenzymatic synthesis of sialosides containing C7-modified sialic acids and their application in sialidase substrate specificity studies

Modifications at the glycerol side chain of sialic acid in sialosides modulate their recognition by sialic acid-binding proteins and sialidases. However, limited work has been focused on the synthesis and functional studies of sialosides with C7-modified sialic acids. Here we report chemical synthesis of C4-modified ManNAc and mannose and their application as sialic acid precursors in a highly efficient one-pot three-enzyme system for chemoenzymatic synthesis of α2-3- and α2-6-linked sialyl para-nitrophenyl galactosides in which the C7-hydroxyl group in sialic acid (N-acetylneuraminic acid, Neu5Ac, or 2-keto-3-deoxynonulosonic acid, Kdn) was systematically substituted by -F, -OMe, -H, and -N3 groups. Substrate specificity study of bacterial and human sialidases using the obtained sialoside library containing C7-modified sialic acids showed that sialosides containing C7-deoxy Neu5Ac were selective substrates for all bacterial sialidases tested but not for human NEU2. The information obtained from sialidase substrate specificity can be used to guide the design of new inhibitors that are selective against bacterial sialidases.

Asymmetric transformations involving 1,2-dicarbonyl compounds as pronucleophiles

This article concentrates on the versatile nucleophilic reactivity of 1,2-dicarbonyl compounds in various asymmetric transformations. Although underexploited in comparison to their 1,3-dicarbonyl homologues, the presence of adjacent multiple reactive centres allows the selection of specific activation modes for enhancing the reactivity of these important ambident pronucleophiles. They can be involved in selective formation of C-C, C-O or C-N bonds leading to various optically active targets in the acyclic and cyclic series including three- to seven-membered ring systems. Recent contributions in the field of biochemical, organometallic and organic catalytic transformations as well as some relevant stoichiometric approaches are discussed from synthetic and mechanistic point of views highlighting some important stereochemical issues.

Chemoenzymatic synthesis of CMP-N-acetyl-7-fluoro-7-deoxy-neuraminic acid

7-Fluoro sialic acid was prepared and activated as cytidine monophosphate (CMP) ester. The synthesis started with d-glucose, which was efficiently converted into N-acetyl-4-fluoro-4-deoxy-d-mannosamine. Aldolase catalyzed transformation yielded the corresponding fluorinated sialic acid which was activated as CMP ester using three different synthetases in the presence as well as in the absence of pyrophosphatase which possesses inhibitory properties. Finally, conditions were optimized to perform a one-pot reaction starting from fluorinated mannosamine, which yielded the 7-fluoro-7-deoxy-CMP-sialic acid by incubation with three enzymes.

Aldolase-Catalyzed Synthesis of β-d-Galp-(1→9)-d-KDN: A Novel Acceptor for Sialyltransferases

[reaction: see text] beta-D-Galp-(1-->9)-D-KDN, a disaccharide component of the cell wall of Streptomyces sp. MB-8, was synthesized from beta-D-Galp-(1-->6)-D-Manp and pyruvate using a sialic acid aldolase. The obtained KDN-containing compound was a novel acceptor for bacterial sialyltransferases. Unusual alpha2,3- and alpha2,6-linked sialyltrisaccharides and a tetrasaccharide were synthesized using a one-pot two-enzyme system containing a Neisseria meningitidis CMP-sialic acid synthetase and a Pasteurella multocida sialyltransferase or a Photobacterium damsela alpha2,6-sialyltransferase.

Accessibility of N-acyl-D-mannosamines to N-acetyl-D-neuraminic acid aldolase

N-Acetyl-D-neuraminic acid (NeuNAc) aldolase is an important enzyme for the metabolic engineering of cell-surface NeuNAc using chemically modified D-mannosamines. To explore the optimal substrates for this application, eight N-acyl derivatives of D-mannosamine were prepared, and their accessibility to NeuNAc aldolase was quantitatively investigated. The N-propionyl-, N-butanoyl-, N-iso-butanoyl-, N-pivaloyl-, and N-phenylacetyl-D-mannosamines proved to be as good substrates as, or even better than, the natural N-acetyl-D-mannosamine, while the N-trifluoropropionyl and benzoyl derivatives were poor. It was proposed that the electronic effects might have a significant influence on the enzymatic aldol condensation reaction of D-mannosamine derivatives, with electron-deficient acyl groups having a negative impact. The results suggest that N-propionyl-, N-butanoyl-, N-iso-butanoyl-, and N-phenylacetyl-D-mannosamines may be employed to bioengineer NeuNAc on cells.