Interaction mechanism of a natural medicine product helicid with a typical digestive enzyme trypsin

The interaction mechanism of a kind of natural drug helicid with typical digestive enzyme trypsin was investigated using multi-spectroscopic techniques and molecular modeling method. The fluorescence quenching experiments showed that helicid quenched trypsin fluorescence via a combined quenching mechanism of both dynamic and static quenching processes because of the formation of the helicid–trypsin complex. Thermodynamic analyses suggested that the binding of helicid to trypsin was entropy-driven, and that hydrophobic interactions are the main force to stabilize the complex. Förster energy transfer theory applied demonstrated that energy transfer occurred within the complex. Ultraviolet-visible spectroscopy, synchronous fluorescence spectroscopy, three-dimension fluorescence spectroscopy together with circular dichroism spectroscopy indicated that the conformation of trypsin changed induced by helicid with the loosening of the polypeptide backbone of trypsin and its partial β-sheet and random coil structures being transformed into an α-helix structure and that helicid was closer to tryptophan residues than to tyrosine residues within the complex. Molecular simulation results explained the above experimental results very well and further implied that besides hydrophobic interactions, hydrogen bonds also helped stabilize the helicid–trypsin complex. This study is helpful to the understanding of the biochemical process of helicid in the human body and could provide useful information for the studies on the structure–bioactivity relationship of natural compounds, as well as the design and optimization of the drugs for the treatments of trypsin overactivity-related diseases.