New mechanistic insights of nanoplastics synergistic cadmium induced overactivation of trypsin: Joint analysis from protein multi-level conformational changes and computational modeling

Nanoplastics (NPs) are emerging global contaminants that can exacerbate the animal toxicity and cytotoxicity of cadmium (Cd). However, the mechanisms by which NPs influence the toxic effects of Cd on key functional proteins within the body remain unknown. In this study, trypsin, a protein that is prone to coexist with NPs in the digestive tract, was selected as the target protein. The effects and mechanisms of NPs on Cd-induced structural damage at multiple levels and alterations in the biological function of trypsin were investigated using multi-spectroscopy techniques, enzyme activity assays, and computational modeling. Results indicated that the Cd-induced decrease and red shift of the trypsin backbone peak were exacerbated by the presence of NPs, leading to more serve backbone loosening. Furthermore, compared to Cd, NPs@Cd caused a more pronounced reduction in the α-helix content of trypsin. These structural changes led to the opening of the trypsin pocket and the overactivation of the enzyme (NPs@Cd: 227.22%; Cd: 53.35%). Ultimately, the formation of a "protein corona" around NPs@Cd and the metal contact of Cd to the trypsin surface were identified as the mechanisms by which NPs enhanced the protein toxicity of Cd. This study elucidates, for the first time, the effects and underlying mechanisms of NPs on the toxicity of key functional proteins of Cd. These findings offer novel mechanistic insights and critical evidence essential for evaluating the risks associated with NPs.