Primary13C and β-Secondary2H KIEs for Trans-sialidase. A Snapshot of Nucleophilic Participation during Catalysis

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Abstract
Trypanosoma cruzi trans-sialidase catalyzes a novel reaction that involves the transfer of sialic acid between host and parasite glycoconjugates. In this paper, we report kinetic isotope effect studies on recombinant trans-sialidase. β-Dideuterium and primary 13C isotope effects were measured for a good substrate, sialyl-lactose, and a slow substrate, sialyl-galactose, in both acid-catalyzed solvolysis and enzymatic transfer reactions. The β-dideuterium isotope effect for sialyl-lactose in the acid hydrolysis reaction was 1.113 ± 0.012. The primary 13C isotope effects for hydrolysis of sialyl-lactose and sialyl-galactose were 1.016 ± 0.011 and 1.015 ± 0.008, respectively. In the enzymatic transfer reactions, the β-dideuterium and primary 13C effects for sialyl-galactose were 1.060 ± 0.008 and 1.032 ± 0.008, respectively. The isotope effects for hydrolysis describe a dissociative SN1-like mechanism, and these data are contrasted by the data for the enzyme-catalyzed reaction. The enzymatic deuterium isotope effects are lower by a factor of 2, but the primary carbon isotope effects are higher by a factor of 2. This pattern describes a mechanism involving nucleophilic participation in the rate-determining transition state.