Hierarchical map of protein unfolding and refolding at thermal equilibrium revealed by wide-angle X-ray scattering

Hierarchical features of the thermal unfolding-refolding structural transition of hen egg white lysozyme (HEWL) have been studied in the temperature range from 13 to 84 degrees C by using high-resolution wide-angle X-ray scattering (WAXS) measurements at a third-generation synchrotron source. We have gathered high-statistic WAXS data of the reversible unfolding-refolding process of HEWL in the q range from approximately 0.05 to approximately 3 A(-1) [q = (4pi/lambda) sin(theta/2), where theta is the scattering angle and lambda the wavelength]. This measured q range corresponds to the spatial distance from approximately 2 to approximately 125 A, which covers all hierarchical structures of a small globular protein such as HEWL, namely, tertiary, domain, and secondary structures. Because of this, we have found that the pH dependence of the thermal structural transition of HEWL is well characterized by the various hierarchical levels and the transition concurrence among them. In this report, we present a new hierarchical map depiction of unfolding-refolding transitions. Using scattering with various ranges of q values, we determine the molar ratio of native-like protein structure defined by the data in each range, thus producing a map of the amount of native-like structure as a function of the hierarchical level or resolution. This map can visualize a detailed feature of the unfolding-refolding transition of a protein depending on various structural hierarchical levels; however, the exact meaning of the map will await sharpening by additional works.