Proton Transfer Facilitated by Ligand Binding. An Energetic Analysis of the Catalytic Mechanism of Trypanosoma cruzi Trans-Sialidase

Trans-sialidase is a crucial enzyme for the infection of Trypanosoma cruzi, the protozoa responsible for Chagas' disease in humans. This enzyme catalyzes the transfer of sialic acids from mammalian host cells to parasitic cell surfaces in order to mask the infection from the host's immune system. It represents a promising target for the development of therapeutics to treat the disease and has been subject of extensive structural studies. Elaborate experiments suggested formation of a long-lived covalent intermediate in the catalytic mechanism and identified a Tyr/Glu pair as an unusual catalytic couple. This requires that the tyrosine hydroxyl proton is transferred to the carboxylate group of glutamate before the nucleophilic attack. Since the solution pK(a)s of tyrosine and glutamate are very different, this transfer can only be accomplished if the reaction environment selectively stabilizes the product state. We compute the free energy profile for the proton transfer in different environments, and our results indicate that it can take place in the active site of trans-sialidase, but only after substrate binding. By means of the energy decomposition method, we explain the influence that the active site residues exert on the reaction and how the pattern is changed when the substrate is present. This study represents an initial step that can shed light on our understanding of the catalytic mechanism of this reaction.