Semi-Synthetic Sialic Acid Probes for Challenging the Substrate Promiscuity of Enzymes in the Sialoconjugation Pathway

Abstract

A series of unusual sialic acid analogs were prepared using a semi-synthetic strategy. Truncation of natural N-acetylneuraminic acid was followed by diastereoselective carbon backbone reconstruction using Barbier-type carboligations as well as different functional group interconversions, which provided access to a variety of functional motifs in the terminal carbon backbone, including examples of saturated and unsaturated, linear and branched alkyl chains, partially deoxygenated sialic acids, sialic diacids and a first truncated legionaminic acid analog. The synthetic sialic acid probes were studied for nucleotide activation by the CMP-sialic acid synthetase from Neisseria meningitidis using a universal pH-shift assay for kinetic analysis. One-pot enzymatic nucleotide activation and sialyltransfer to lactose was performed using a selection of five probes together with an engineered α2,3-sialyltransferase from Photobacterium phosphoreum to furnish five new-to-nature analogs of the GM3 trisaccharide, which were finally utilized to test the substrate tolerance of two bacterial sialidases. The obtained set of sialic acid analogs and neo-sialocojugates provides interesting opportunities for further glycobiology studies. (Figure presented.).