Structural changes in apolipoproteins bound to nanoparticles

Nanoparticles are widely used in the pharmaceutical and food industries, but the consequences of exposure to the human body have not been thoroughly investigated. Apolipoprotein A-I (apoAI), the major protein in high-density lipoprotein (HDL), and other lipoproteins are found in the corona around many nanoparticles, but data on protein structural and functional effects are lacking. Here we investigate the structural consequences of the adsorption of apoAI, apolipoprotein B100 (apoB100), and HDL on polystyrene nanoparticles with different surface charges. The results of circular dichroism, fluorescence spectroscopy, and limited proteolysis experiments indicate effects on both secondary and tertiary structures. Plain and negatively charged nanoparticles induce helical structure in apoAI (negative net charge) whereas positively charged nanoparticles reduce the amount of helical structure. Plain and negatively charged particles induce a small blue shift in the tryptophan fluorescence spectrum, which is not noticed with the positively charged particles. Similar results are observed with reconstituted HDL. In apoB100, both secondary and tertiary structures are perturbed by all particles. To investigate the generality of the role of surface charge, parallel experiments were performed using human serum albumin (HSA, negative net charge) and lysozyme (positive net charge). Again, the secondary structure is most affected by nanoparticles carrying an opposite surface charge relative to the protein. Nanoparticles carrying the same net charge as the protein induce only minor structural changes in lysozyme whereas a moderate change is observed for HSA. Thus, surface charge is a critical parameter for predicting structural changes in adsorbed proteins, yet the effect is specific for each protein.