A one-dimensional free energy surface does not account for two-probe folding kinetics of protein alpha(3)D.
We present fluorescence-detected measurements of the temperature-jump relaxation kinetics of the designed three-helix bundle protein alpha(3)D taken under solvent conditions identical to previous infrared-detected kinetics. The fluorescence-detected rate is similar to the IR-detected rate only at the lowest temperature where we could measure it (326 K). The fluorescence-detected rate decreases by a factor of 3 over the 326-344 K temperature range, whereas the IR-detected rate remains nearly constant over the same range. To investigate this probe dependence, we tested an extensive set of physically reasonable one-dimensional (1D) free energy surfaces by Langevin dynamics simulation. The simulations included coordinate- and temperature-dependent roughness, diffusion coefficients, and IR/fluorescence spectroscopic signatures. None of these can reproduce the IR and fluorescence data simultaneously, forcing us to the conclusion that a 1D free energy surface cannot accurately describe the folding of alpha(3)D. This supports the hypothesis that alpha(3)D has a multidimensional free energy surface conducive to downhill folding at 326 K, and that it is already an incipient downhill folder with probe-dependent kinetics near its melting point.
Liu
Feng
F
Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Illinois 61801, USA.
Dumont
Charles
C
Zhu
Yongjin
Y
DeGrado
William F
WF
Gai
Feng
F
Gruebele
Martin
M
D004058:Diffusion; D005453:Fluorescence; D007259:Infrared Rays; D007700:Kinetics; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry* / Q000378:metabolism; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics
D011506:Proteins; C119365:alpha3D protein, synthetic