Mechanistic insights from substrate preference in unsaturated glucuronyl hydrolase

Natural and synthetic unsaturated glucuronides were tested as substrates for Clostridium perfringens unsaturated glucuronyl hydrolase to probe its mechanism and to guide inhibitor design. Of the natural substrates, a chondroitin disaccharide substrate with sulfation of the primary alcohol on carbon 6 of its N-acetylgalactosamine moiety was found to have the highest turnover number of any substrate reported for an unsaturated glucuronyl hydrolase, with kcat =112 s(-1) . Synthetic aryl glycoside substrates with electron-withdrawing aglycone substituents were cleaved more slowly than those with electron-donating substituents. Similarly, an unsaturated glucuronyl fluoride was found to be a particularly poor substrate, with kcat /Km =44 nM(-1) s(-1) -a very unusual result for a glycoside-cleaving enzyme. These results are consistent with a transition state with positive charge at carbon 5 and the endocyclic oxygen, as anticipated in the hydration mechanism proposed. However, several analogues designed to take advantage of strong enzyme binding to such a transition state showed little to no inhibition. This result suggests that further work is required to understand the true nature of the transition state stabilised by this enzyme.