Structural Modelling of Platelet Activating Factor Acetyl Hydrolase in Leishmania Donovani, Trypanosoma Cruzi, and Trypanosoma Brucei: Implications on Therapeutics for Leishmaniasis, Chagas Disease, and Sleeping Sickness.

Purpose: Leishmaniasis, Chagas disease, and sleeping sickness are caused by protozoa Leishmania donovani, Trypanosoma cruzi, and Trypanosoma brucei, respectively. Platelet activating factor acetyl hydrolase (PAF-AH) is an inflammatory protein implicated in pathogenesis of these three infections, thereby making them attractive drug targets.Methods: PAF-AH sequences were retrieved from UniProt and aligned using Clustal Omega. Homologous models of parasitic proteins were built based on crystal structure of human PAF-AH and validated using PROCHECK server. Volumes of substrate-binding channel were calculated using the ProteinsPlus program. High throughput virtual screening using Glide program in Schrodinger was done with ZINC drug library against parasitic PAF-AH enzymes. Complexes with best hits were energy-minimized and subjected to 100 ns molecular dynamic simulation and analyzed.Results: PAF-AH enzyme sequences from protozoa Leishmania donovani, Trypanosoma cruzi, Trypanosoma brucei, and human have a minimum of 34% sequence similarity with each other. Corresponding structures show a globular conformation consisting of twisted beta-pleated sheets, flanked by alpha-helices on either side. Catalytic triad of serine-histidine-aspartate is conserved. Substrate-binding channel residues are conserved to an extent, with a lower channel volume in human as compared to target enzymes. Drug screening resulted in identification of three molecules that had better affinities than the substrate to the target enzymes. These molecules fulfill Lipinski's rules for drug likeness and also bind with less affinity to the human counterpart, thereby establishing a high selective index.Conclusion: Structures of PAF-AH from protozoan parasites and humans belong to the same family of enzymes and have a similar three-dimensional fold. However, they show subtle variations in residue composition, secondary structure composition, substrate-binding channel volume, and conformational stability. These differences result in certain specific molecules being potent inhibitors of the target enzymes while simultaneously having weaker binding to human homologue.