The effect of zinc oxide nanoparticles on the structure of the periplasmic domain of the Vibrio cholerae ToxR protein.

Proteins adsorbed on nanoparticles (NPs) are being used as biosensors and in drug delivery. However, our understanding of the effect of NPs on the structure of proteins is still in a nascent state. In this work we report the unfolding behavior of the periplasmic domain of the ToxR protein (ToxRp) of Vibrio cholerae on zinc oxide (ZnO) nanoparticles with a diameter of 2.5 nm. This protein plays a crucial role in regulating the expression of several virulence factors in the pathogenesis of cholera. Thermodynamic analysis of the equilibrium of unfolding, induced both by urea and by guanidine hydrochloride (GdnHCl), and measured by fluorescence spectroscopy, revealed a two-state process. NPs increased the susceptibility of the protein to denaturation. The midpoints of transitions for the free and the NP-bound ToxRp in the presence of GdnHCl were 1.5 and 0.5 m respectively, whereas for urea denaturation, the values were 3.3 and 2.4 m, respectively. Far-UV CD spectra showed a significant change in the protein conformation upon binding to ZnO NPs, which was characterized by a substantial decrease in the α-helical content of the free protein. Isothermal titration calorimetry, used to quantify the thermodynamics of binding of ToxRp with ZnO NPs, showed an exothermic binding isotherm (ΔH = -9.8 kcal·mol(-1) and ΔS = -5.17 cal·mol(-1)·K(-1)).