The conformational properties of human plasma apolipoprotein C-II. A spectroscopic study

The solution properties of human plasma apolipoprotein C-II (apoC-II) have been studied by analytical ultracentrifugation, circular dichroic spectroscopy, and fluorescence spectroscopy. ApoC-II self-associates in solution, even though no rigorous thermodynamic analysis of the mode of self-association could be established. The reversible denaturation of apoC-II by guanidinium chloride (GdmCl) proceeded in a sequential fashion. Initial disruption of protein self-association by 0.3 M GdmCl was followed by cooperative unfolding of monomeric protein at higher GdmCl concentrations with a midpoint 1.1 M GdmCl. Based on tryptophan fluorescence quenching unfolded apoC-II was more permeable to penetration by small molecules than the self-associated protein. A very low free energy (ΔG(H₂O) = 2.8 kcal/mol) of denaturation was calculated from the GdmCl denaturation titration curve. Heating apoC-II to 55°C did not induce a reversible cooperative unfolding of the protein. Calculations, based on Chou-Fasman probability algorithms, reveal three sequential helical regions in apoC-II and one β sheet structure (residues 61-74). The locations of these regions are consistent with the known physiological functions of apoC-II.