UDP-6-deoxy-6-fluoro-α-d-galactose binds to two different galactosyltransferases, but neither can effectively catalyze transfer of the modified galactose to the appropriate acceptor

The effect of substitution of the HO-6 of d-galactose with fluorine on the ability of α-(1→3)-galactosyltransferase (EC 2.4.1.151) and β-(1→4)-galactosyltransferase (EC 2.4.1.22) to catalyze its transfer from UDP to an appropriate acceptor was determined. HPLC analyses indicated that each transferase properly catalyzed formation of the expected product [β-d-Gal-(1→4)-d-GlcNAc] for the β-(1→4)-galactosyltransferase and α-d-Gal-(1→3)-β-d-Gal-(1→4)-d-GlcNAc for the α-(1→3)-d-galactosyltransferase] when UDP-α-d-Gal was the substrate. When UDP-6-deoxy-6-fluoro-α-d-galactose (6) was used in conjunction with each transferase, no product indicative of transfer of 6-deoxy-6-fluoro-d-galactose to its respective acceptor sugar was identified. 6-Deoxy-6-fluoro-d-galactose (3) was obtained by hydrolysis of methyl 6-deoxy-6-fluoro-α-d-galactopyranoside, synthesized by the selective fluorination of methyl α-d-galactopyranoside with diethylaminosulfur trifluoride (DAST), with aqueous trifluoroacetic acid. Acetylation of 3 gave crystalline 1,2,3,4-tetra-O-acetyl-6-deoxy-6-fluoro-β-d-galactopyranose, which was converted to the corresponding 1-α-phosphate and used for the synthesis of 6.