Abstract
The interaction of an important antibiotic drug, ceftazidime, with an imperative globular protein, lysozyme (which also possesses antibacterial properties), was studied. The formation of a complex between ceftazidime and lysozyme was verified by ultraviolet-visible (UV-Vis) spectroscopy. Intrinsic fluorescence spectroscopy (FS) was primarily employed to obtain information about the binding mechanism. The fluorescence data were corrected for the inner filter effect (IFE) before the analysis because of the strong absorption of ceftazidime at the excitation wavelength and the emission range of lysozyme. The apparent fluorescence quenching parameters, which were obtained after the IFE correction, exhibited the weak affinity between lysozyme and ceftazidime. The interaction was due to the dynamic quenching with a binding ratio of 1:1. Ceftazidime exerted small effect on the secondary structure of lysozyme. The binding site of ceftazidime in lysozyme was determined by molecular docking. The large hydrophobic pocket of lysozyme was the most preferred binding site for ceftazidime. The interaction was, primarily, reinforced by hydrophobic forces and hydrogen bonding with a small participation of electrostatic forces. Molecular dynamics (MD) simulations were also performed to theoretically understand the behavior of the drug-protein complex. The stability of lysozyme changed slightly in the presence of ceftazidime. The pattern of the radius of gyration (R-g) in the absence and presence of ceftazidime also exhibited a slight change. There was a slight decrease in the residual fluctuation of residues 50-90, whereas a significant increase was observed in the residual fluctuations of residues 100-120. In the 100 ns simulation of the formation of lysozyme and ceftazidime, the disruption of hydrogen bonding occurred with the probability of forming three to six hydrogen bonds during most of the time evolutions of the simulation. The density-functional theory (DFT) calculations indicated that the stability of ceftazidime decreased after binding with lysozyme. (C) 2021 Elsevier B.V. All rights reserved.