Open-Bundle Structure as the Unfolding Intermediate of Cytochrome c ' Revealed by Small Angle Neutron Scattering.

The dynamic structure changes, including the unfolding, dimerization, and transition from the compact to the open-bundle unfolding intermediate structure of Cyt c ', were detected by a small-angle neutron scattering experiment (SANS). The structure of Cyt c ' was changed into an unstructured random coil at pD = 1.7 (R g = 25 Å for the Cyt c ' monomer). The four-α-helix bundle structure of Cyt c ' at neutral pH was transitioned to an open-bundle structure (at pD ~13), which is given by a numerical partial scattering function analysis as a joint-clubs model consisting of four clubs (α-helices) connected by short loops. The compactly folded structure of Cyt c ' (radius of gyration, R g = 18 Å for the Cyt c ' dimer) at neutral or mildly alkaline pD transited to a remarkably larger open-bundle structure at pD ~13 (R g = 25 Å for the Cyt c ' monomer). The open-bundle structure was also supported by ab initio modeling.