Synthesis and characterization of an anomeric sulfur analogue of CMP-sialic acid

alpha-2,3-Sialyltransferase catalyzes the transfer of sialic acid from CMP- sialic acid (1) to a lactose acceptor. An analogue of 1 was synthesized in which the anomeric oxygen atom was replaced with a sulfur atom (1S). The ke y step in the synthesis of 1S was a tetrazole-promoted coupling of a cytidi ne-5'-phosphoramidite with a glycosyl thiol of a protected sialic acid. Com pounds 1 and 1S were characterized for their activity in a sialyl transfer assay. The rate of solvolysis in aqueous buffer of analogue 1S was 50-fold slower than that of 1. Analogue 1S was found to be substrate for alpha-2,3- sialyltransferase. The K-m of 1S was just 3-fold higher than that of 1, whi le the k(cat) of 1S was 2 orders of magnitude lower compared to 1.