COVID-19 remains a global pandemic, necessitating the urgent development of more effective therapeutics. By combining molecular docking, molecular dynamics (MD), and fragment molecular orbital (FMO) calculations, the binding structure and properties with Mpro were predicted for Nelfinavir (NLF), which was identified as a candidate compound through drug repositioning targeting the Main Protease (Mpro) produced by the causative virus, SARS-CoV-2. For the four docking poses selected by scoring using FMO energy, 100 structures each from the MD trajectory were sampled, and FMO calculations were performed and ranked based on binding energy. Besides the interaction between NLF and each Mpro residue, the desolvation effect of the pocket affected the ranking order. Furthermore, we identified several residues important in ligand recognition, including Glu47, Asp48, Glu166, Asp187, and Gln189, all of which interacted strongly with NLF. Asn142 was mentioned as a residue with hydrogen bonds or CH/π interaction with NLF; however, it was considered a transient interacting residue because of its unstable structure. Moreover, the tert-butyl group of NLF had no interaction with Mpro. Identifying weak interactions provides candidates for substituting ligand functional groups and important suggestions for drug discovery using drug repositioning. Our approach provides a new guideline for structure-based drug design starting from a candidate compound whose complex crystal structure has not been obtained.
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