Volume 10, Issue 4, Pages 757-768 (October 2002) The Crystal Structure and Mode of Action of Trans-Sialidase, a Key Enzyme in Trypanosoma cruzi Pathogenesis  Alejandro Buschiazzo, Marı́a F. Amaya, Marı́a L. Cremona, Alberto C. Frasch, Pedro M. Alzari  Molecular Cell  Volume 10, Issue 4, Pages 757-768 (October 2002) DOI: 10.1016/S1097-2765(02)00680-9

Figure 1 Comparison of TcTS and TrSA (A) Superposition of Cα coordinates of the two trypanosomal enzymes. Their overall structures are highly similar, with an rmsd of 0.7 Å for 618 equivalent Cα positions. The N and C termini are labeled, and the ligands (DANA and lactose, shown in stick representation) indicate the location of the catalytic center. (B) Molecular surface of the active site cleft of T. cruzi trans-sialidase, color-coded according to the electrostatic potential. Residues Tyr119, Pro283, and Trp312 are represented with a transparent molecular surface. (C) Similar view of the cleft in T. rangeli sialidase. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 2 The TcTS Sialic Acid Binding Site (A) Triclinic TcTS crystals before and after soaking with 10 mM DANA. (B) Stereoview showing the different orientations of DANA in the active site clefts of TrSA (in green) and TcTS (in red; a single conformation of the glycerol moiety is shown for clarity). The conformation of Tyr119 seen in unliganded TcTS is shown in white (see also Figure 6A). (C) Electron density (2Fo-Fc) map of DANA in the catalytic pocket contoured at 1.2 σ. (D) Schematic diagram showing protein-ligand hydrogen bonding interactions. A second alternative conformation for the glycerol group of DANA is depicted in green. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 3 Substrate-Triggered Activation Switch (A) Electron density (2Fo-Fc) map of the TcTS-DANA complex (contoured at 1.5 σ) showing the two alternate conformations for Tyr342. (B) Relative positions of the tyrosine residue in all crystallographically independent models of TcTS after least-squares superposition of their catalytic β propeller domains. The orientation of Tyr342 in the unliganded structures is shown in green, the double conformations observed in all triclinic TcTS-ligand complexes are shown in blue and red. The minimal and maximal distances between Tyr342-OH and Glu230-Oϵ2 are also indicated. (C) Using the same color code, the histogram shows the mean values and the rmsd of the OH-Oϵ2 distance among all the structures. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 4 Lactose Binding to TcTS (A) Complete BIAcore sensorgrams showing the interaction between the inactive mutant Asp59Asn and sialic acid (continuous line). Lactose (10 mM) was injected after equilibrating the protein in the presence (continuous line) or absence (dashed line) of sialic acid. (b) Detailed view of the interaction between TcTS and lactose, demonstrating that the acceptor substrate only binds to TcTS when sialic acid is present. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 5 TcTS-Lactose Interactions (A) Electron density (2Fo-Fc) map, contoured at 1 σ, of the lactose binding site in the triclinic crystal form. The loop containing Gly145/Gly146 from a neighbor molecule in the crystal is also shown. (B) Electron density (2Fo-Fc) map, contoured at 1 σ, showing the lactose and DANA molecules in the ternary complex. (C) Schematic diagram showing protein-carbohydrate hydrogen bonding interactions. (D) Stacking interactions of the lactose moiety with the aromatic rings of Trp312 and Tyr119 in the ternary complex. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 6 General Mechanism of Trans-Sialidase (A) Composite figure showing the positions of DANA and lactose in the TcTS catalytic cleft. The dual conformations of residues involved in substrate-induced affinity (Tyr119) and enzyme activation (Tyr342) are represented. The position of Tyr119 in unliganded TcTS is shown with a transparent representation. (B) Scheme of the proposed events in the transglycosylation mechanism. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)

Figure 6 General Mechanism of Trans-Sialidase (A) Composite figure showing the positions of DANA and lactose in the TcTS catalytic cleft. The dual conformations of residues involved in substrate-induced affinity (Tyr119) and enzyme activation (Tyr342) are represented. The position of Tyr119 in unliganded TcTS is shown with a transparent representation. (B) Scheme of the proposed events in the transglycosylation mechanism. Molecular Cell 2002 10, 757-768DOI: (10.1016/S1097-2765(02)00680-9)