Figure 8.

Molecular model for allosteric inhibition of TryS by Ebselen. (A) Amino acid sequence of TbTryS highlighting the N-terminal amidase domain (black box), the C-terminal synthetase domain (blue box) and the linker region (white bold letters on green background). The cysteine residues (red background: conserved in Tc- and Li-TryS, black background: conserved in TcTryS, grey background: unique to TbTryS) that were found modified by Ebselen (asterisk) and fulfils a role in amidase catalytic activity (dot) are highlighted. The region harbouring structural elements with highly-conserved residues (in white bold letters) relevant for the catalytic activity of the synthetase domain and forming a hydrophobic pocket (italics) in the vicinity of Cys277 is highlighted in magenta background. The strictly conserved serine residue acting as nucleophilic attack facilitator for conjugation of the polyamine moiety (SP or Gsp) to GSH is shown in yellow background. (B) Cartoon representation of L. major TryS 3D structure (PDB 2VOB) showing the conserved cysteine residues from the amidase (grey cartoon, Cys60) and the synthetase (blue cartoon, Cys277) domain that bind Ebselen covalently in TbTryS. The linker region is shown in green and the substrates bound to the synthetase active site are shown in sticks. (C) Location of the cysteine from the synthetase domain modified by Ebselen (Cys 277 in LiTryS and Cys270 in TbTryS). Hydrophobic residues (Phe330, Phe332 and Phe334) in the vicinity of Cys277 and forming part of structural elements harbouring several residues relevant for catalytic activity are coloured magenta. (D) Synthetase active site with bound substrates and strictly conserved residues relevant for catalysis are shown in magenta (Arg329, Asp331 and Asn347) and yellow (Ser352) coloured sticks. The network of hydrogen bonds and electrostatic interactions between these residues and with substrates is shown with dashed lines.