Open-Bundle Structure as the Unfolding Intermediate of Cytochrome <i>c</i>′ Revealed by Small Angle Neutron Scattering

Abstract

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 (Rg = 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, Rg = 18 Å for the Cyt c′ dimer) at neutral or mildly alkaline pD transited to a remarkably larger open-bundle structure at pD ~13 (Rg = 25 Å for the Cyt c′ monomer). The open-bundle structure was also supported by ab initio modeling.

Keywords

• protein unfolding
• cytochrome <i>c</i>′
• small-angle neutron scattering
• open-bundle structure