STRUCTURAL CHARACTERIZATION OF THE MOLTEN GLOBULE AND NATIVE STATES OF APOMYOGLOBIN BY SOLUTION X-RAY-SCATTERING

Compactness and shape are two of the critical properties that describe the degree of protein folding. Solution X-ray scattering is an effect ive technique for measuring these properties quantitatively. Structura l characteristics of various conformational states of horse myoglobin were studied in terms of size and shape by solution X-ray scattering. The radius of gyration for native holomyoglobin was 17.5 Angstrom, whi le that of the apomyoglobin native state was 19.7 Angstrom. Correspond ing to the increase in the radius of gyration, the largest dimension o f the molecule also increased from 47.5 Angstrom to 62.5 Angstrom. Bot h states are globular in shape. The scattering profiles in the high an gle region suggest that the apomyoglobin native state has a distinct t ertiary structure, and that packing of alpha-helices in the apomyoglob in native state would be looser than that of holomyoglobin. These obse rvations indicate that the native state of apomyoglobin is expanded fr om that of holomyoglobin, and that the conformations of the two are no t identical. The radii of gyration for the acid-unfolded state and the denaturant-unfolded state were 30 Angstrom and 35 Angstrom, respectiv ely; Both unfolded states have chain-like conformations without any te rtiary structures. The radius of gyration and the largest dimension of the molten globule stabilized by trichloroacetate were 23.1 Angstrom and 72.5 Angstrom, respectively The molten globule is expanded from th e native state although it is globular, and is much more compact than the unfolded state. The bimodal distance distribution function and sca ttering profile at high-angle region suggest that the structure of the apomyoglobin molten globule contains a core comprising a cluster of m ultiple alpha-helices and flaring tail(s), which would be a common str uctural property of the compact denatured state appearing during the f olding process. The compactness of each conformational state is highly correlated with the extent of formation of the alpha-helix.