Structural transformation of cytochrome c and apo cytochrome c induced by sulfonated polystyrene

The structural transformation of cytochrome c (cyt c) and its heme-free precursor, apo cyt c, induced by negatively charged sulfonated polystyrene (SPS) with different charge density (degree of sulfonation) and chain length was studied to understand the factors that influence the folding and unfolding of the protein. SPS forms stable transparent nanoparticles in aqueous solution. The hydrophobic association of the backbone chain and phenyl groups is balanced by the electrostatic repulsion of the sulfonate groups on the particle surface. The binding of cyt c to negatively charged SPS particles causes an extensive disruption of the native compact structure of cyt c: the cleavage of Fe-Met80 ligand, about 40% loss of the helical structure, and the disruption of the asymmetry environment of Trp59. On the other hand, SPS particle-bound apo cyt c undergoes a conformational change from the random coil to alpha-helical structure. The folding of apo cyt c in SPS particles was influenced by pH and ionic strength of the solution, SPS concentration, and the degree of sulfonation and chain length of SPS. The folding can reach more than 90% of the alpha-helix content of native cyt c in solution. Poly(sodium 4-styrenesulfonate) (PSS), which is 100% sulfonated polystyrene and cannot form hydrophobic cores in the solution, induces only two-thirds of the alpha-helix content compared with SPS. It appears that the electrostatic interaction between PSS/SPS and apo cyt c induces an early partially folded state of apo cyt c. The hydrophobic interaction between nonpolar residues in apo cyt c and the hydrophobic cores in SPS particles extends the alpha-helical structure of apo cyt c.