Tryptophan as a molecular shovel in the glycosyl transfer activity of Trypanosoma cruzi trans-sialidase

Abstract

Molecular dynamics investigations into active site plasticity of Trypanosoma cruzi trans-sialidase, a protein implicated in Chagas disease, suggest that movement of the Trp(312) loop plays an important role in the enzyme's sialic acid transfer mechanism. The observed Trp(312) flexibility equates to a molecular shovel action, which leads to the expulsion of the donor aglycone leaving group from the catalytic site. These computational simulations provide detailed structural insights into sialyl transfer by the trans-sialidase and may aid the design of inhibitors effective against this neglected tropical disease.

Scheme 1

First step of sialyl transfer catalyzed by TcTS.

Figure 1

Flexibility of the Tyr¹¹⁹ and Trp³¹² residues of TcTS during the molecular dynamics simulation of the sialyl enzyme covalent intermediate complexed with lactose: Tyr¹¹⁹ switches from the stacked to a rotated position and Trp³¹² moves between stacked and open conformations.

Figure 2

Selected steps from along the MD trajectory of the sialylated-enzyme/lactose complex, III. (a) Initially Tyr¹¹⁹ is in the stacking position and the Trp³¹² loop is stacked (b) At 22 ns, the Tyr¹¹⁹ rotates away from the stack, followed by a Trp³¹² motion to shift the Trp³¹²/lactose portion of the stack (c) Full opening of the Trp³¹² loop enables lactose to vacate the active site by 32 ns.

Figure 3

Dynamics of sialylated TcTS/lactose (III): time series of (a) side-chain dihedral angle, χ₁, of Tyr¹¹⁹; (b) intercentroid distance between Tyr¹¹⁹ and Trp³¹² rings; and (c) distance between centers of mass of sialic acid and lactose.