Mutagenesis of the Conserved Active-Site Tyrosine Changes a Retaining Sialidase into an Inverting Sialidase^†

Mutagenesis of the conserved tyrosine (Y370) of the Micromonospora viridifaciens sialidase changes the mechanism of catalysis from retention of anomeric configuration to an unprecedented inverting mechanism in which water efficiently functions as the nucleophile. Three mutants, Y370A, Y370D, and Y370G, were produced recombinantly in Escherichia coli, and all are catalytically active against the activated substrate 4-methylumbelliferyl α-d-N-acetylneuraminide. The Y370D mutant was also shown to catalyze the hydrolysis of natural substrate analogues such as 3‘-sialyllactose. A comparison of the pH-rate profiles for the wild-type and the Y370D mutant sialidase reveals no major differences, although with respect to the kinetic term k_cat/K_m, an ionized form of the aspartate-370 enzyme is catalytically compromised. For the wild-type enzyme, the value of the Brønsted parameter β_lg on k_cat is 0.02 ± 0.03, while for the Y370D mutant sialidase β_lg = −0.55 ± 0.03 for the substrates with bad leaving groups. Thus, for the wild-type enzyme, a nonchemical step(s) is rate-limiting, but for the tyrosine mutant cleavage of the glycosidic C−O bond is rate-determining. The Brønsted slopes derived for the kinetic parameter k_cat/K_m display a similar trend (β_lg −0.30 ± 0.04 and −0.74 ± 0.04 for the wild-type and Y370D, respectively). These results reveal that the tyrosine residue lowers the activation free energy for cleavage of 6‘-sialyllactose, a natural substrate analogue, by more than 24.9 kJ mol^-1. Evidence is presented that the mutant sialidases operate by a dissociative mechanism, and the wild-type enzyme operates by a concerted mechanism.