Multifrequency calorimetry of the folding/unfolding transition of cytochrome c.
The folding-unfolding transition of Fe(III) cytochrome c has been studied with the new technique of multifrequency calorimetry. Multifrequency calorimetry is aimed at measuring directly the dynamics of the energetic events that take place during a thermally induced transition by measuring the frequency dispersion of the heat capacity. This is done by modulating the folding/unfolding equilibrium using a variable frequency, small oscillatory temperature perturbation (approximately 0.05-0.1 degrees C) centered at the equilibrium temperature of the system. Fe(III) cytochrome c at pH 4 undergoes a fully reversible folding/unfolding transition centered at 67.7 degrees C and characterized by an enthalpy change of 81 kcal/mol and heat capacity difference between unfolded and folded states of 0.9 kcal/K*mol. By measuring the temperature dependence of the frequency dispersion of the heat capacity in the frequency range of 0.1-1 Hz it has been possible to examine the time regime of the enthalpic events associated with the transition. The multifrequency calorimetry results indicate that approximately 85% of the excess heat capacity associated with the folding/unfolding transition relaxes with a single relaxation time of 326 +/- 68 ms at the midpoint of the transition region. This is the first time that the time regime in which heat is absorbed and released during protein folding/unfolding has been measured.
van Osdol
W W
WW
Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218.
Mayorga
O L
OL
Freire
E
E
D000818:Animals; D002151:Calorimetry / Q000295:instrumentation / Q000379:methods; D003574:Cytochrome c Group / Q000737:chemistry*; D006736:Horses; D008962:Models, Theoretical; D009206:Myocardium; D011487:Protein Conformation; D011489:Protein Denaturation
D003574:Cytochrome c Group