Effects of metal oxide nanoparticles on the structure and activity of lysozyme.

We investigated the effects of nanoparticles (NPs) on the structure and activity of hen egg-white lysozyme (HEWL) using CeO2 and ZnO NPs. Our results showed that CeO2 NPs triggered the transition of lysozyme secondary structure from α-helix to β-sheet. CeO2 NPs also induced the hydrophobic region of lysozyme to become exposed to the solvent. In contrast, the secondary structure content and hydrophobic region of lysozyme were only slightly changed in the case of ZnO NPs. In addition, the activity of the lysozyme was observed to decrease upon adsorption on CeO2 NPs, whereas the effect of ZnO NPs on activity was negligible. The glutaraldehyde crosslinking results indicated that the percentage of the dimeric form of lysozyme was greatly enhanced by the addition of both NPs. Furthermore, the adsorption capacity, degree of favorability of adsorption, and surface heterogeneity for CeO2 NPs were found to be greater than those on ZnO NPs. Given that CeO2 NPs exhibit a higher surface area/mass than ZnO NPs, the surface concentration of lysozyme on CeO2 NPs was lower than that on ZnO NPs. This result suggested that more direct interactions were involved between CeO2 NPs and lysozyme, thereby leading to a more significant effect. Moreover, higher surface curvatures may also cause destruction of lysozyme's structure and thus affect its activity. In addition, taking into account the surface properties and protein properties, the Toth adsorption model along with the generated site energy distribution was further used to exaplain the difference between the results (e.g., structure, stability, and activity) of lysozyme adsorption on CeO2 and ZnO NPs. The results reported here may aid in better understanding the beneficial or harmful impacts of nanoparticles on the biological systems.