Trypanosoma cruzi trans-sialidase operates through a covalent sialyl-enzyme intermediate: tyrosine is the catalytic nucleophile.
Watts AG, Damager I, Amaya ML, Buschiazzo A, Alzari P, Frasch AC, Withers SG. Trypanosoma cruzi trans-sialidase operates through a covalent sialyl-enzyme intermediate: tyrosine is the catalytic nucleophile. J Am Chem Soc. 2003 Jun 25; 125(25):7532-3.