Probing Structure Changes of Molten Globule Domains of a Protein near Equilibrium Unfolding

U-Ser Jeng^1*

¹Scientific Research Division, NSRRC, Hsinchu, Taiwan

* Presenter:U-Ser Jeng, email:usjeng@nsrrc.org.tw

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-Vis absorption with integrated on-line size exclusion chromatography, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I1, I2, and E along the unfolding pathway. The I1-I2 transition, overlooked in all previous studies, involves mainly coordinated re-orientations across interconnected molten globule subdomains; which transition activates a critical pivot-domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change ca. 9.5 kcal/mol extracted based the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E-form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein. I will also report the status of the new 13A biological small- and wide-angle X-ray scattering beamline of the 3.0 GeV Taiwan Photon Source of the National Synchrotron Radiation Research Center.

Keywords: SAXS, protein unfolding, molten globular domains, Modified Ising model