The Escherichia coli K4 bacterium (05:K4:H4) synthesizes a capsule polysaccharide with a carbohydrate backbone identical to chondroitin formed of a GlcA (1->3) GalNAc (1->4)n backbone to which -fructofuranose units are linked to C-3 of D-glucuronic acid (GlcA) residues 1. The polysaccharide backbone obtained after hydrolytic removal of the fructose residues can be used as a model substrate for the C-5 epimerase involved in dermatan sulfate biosynthesis 2, and both the GlcA- and the GalNAc-transferases in chondroitin sulfate formation 3. Therefore, defructosylated K4 is useful to gain detailed insight into the mode of action of enzymes.