Histone deacetylases (HDACs) are representative targets for the natural and synthetic chemicals used to transform cells to confer antitumor properties. In the current study, curcumin and hydroxamate-derivative PCI-34058-bound HDAC1 were subjected to atomistic simulation. The results support the view that fitting of curcumin and PCI-34058 within the HDAC1 pocket depends on extensive interactions between the aromatic moieties of the inhibitors and the extensive network of aromatic amino acid side chains lining the pocket of HDAC1. The interaction forces a local perturbation of the coiled structures connecting the pocket residues resulting in ligand-induced tightening of the pocket. In addition to the competitive occupancy of the histone-acetyl-lysine binding pocket by the inhibitors, interference with the in-pocket aspartate-histidine (ASP-HIS) charge relay system was also observed in inhibitor-bound HDAC1 systems. In conclusion, curcumin and PCI-34058-mediated ligand-dependent HDAC1 tunnel closure interferes negatively with the ASP-HIS charge relay system in HDAC1. Future design of HDAC inhibitors may benefit from optimizing competitive interaction with the ligand site and interference with the charge relay system.