An in vitro investigation of the inhibitory mechanism of β-galactosidase by cinnamaldehyde alone and in combination with carvacrol and thymol

Background Some antibacterial agents exert their antimicrobial action by targeting the cytoplasmic macromolecules , such as proteins or nucleic acids , to disturb the properties of macromolecules that may deeply influence their biological activities and functions. Cinnamaldehyde (CIN) is a natural antibacterial ingredient found in the bark and leaves of cinnamon trees . Methods The inhibitory mechanism of a typical enzyme , β-galactosidase by CIN was investigated by UV–visible, fluorescence, 3-D spectroscopy, circular dichroism , atomic force microscopy and molecular modeling studies. Results CIN decreased the activity of β-galactosidase by competitive inhibition through a multiphase kinetic process. 3-D spectroscopy and circular dichroism showed that the binding of CIN to β-galactosidase resulted in changes in micro-environment of tryptophan and tyrosine residues, and conformation of β-galactosidase. The molecular recognition was also analyzed through modeling which indicated that CIN was inserted into the active site pocket of β-galactosidase and interacted with amino acid residues, such as Met502, Trp568, Phe601 and Trp999. Atomic force microscopy showed that a serious destabilization of the native conformation of β-galactosidase occurred after binding with CIN, e.g., morphological changes and increased dimensions of the β-galactosidase molecule. Moreover, it was found that the combinations of CIN, carvacrol and thymol exposure displayed synergistic effects on the inhibition of β-galactosidase. General significance This study exhibits a comprehensively understanding about the action mechanism of CIN that affects the conformation and activity of β-galactosidase in biochemical processes and provides some new insights into the possible intracellular targeting behaviors of CIN at a molecular level.