Time Resolved Thermodynamics of Fast Protein Folding in Cytochrome c

Abstract

One of the earliest studies of fast protein folding involved the photolysis of CO bound Cytochorme c in the presence of 4.5 M Gdn-HCl HCl, pH 7.0. These studies revealed fast coordination of non-native His and Met residues resulting in a ‘frustrated’ folding pathway resulting from non-native coordination. These early events occur on timescales of less than 10 μs. Similar fast folding can be initiated at lower Gdn-HCl at pHs above 9.5. Under these conditions the non-native coordination occurs with time constants of ∼ 300 ns and ∼ 3 μs at 22 dg C. Previous time resolved CD spectra suggest that 8% of the native like secondary structure forms in < 1 μs (Goldbeck et al., PNAS (1999), 96, 2782). Here, photoacoustic calorimetry (PAC) has been utilized to probe the thermodynamics associated with fast folding and non-native ligand coordination in CO-Cytochrome c ta pH 12.7 and 350 mM Gdn-HCl. At temperatures below 18 ° C the PAC signals indicate multi-phasic kinetics that can not be fit to simple exponential sums suggesting a distribution of conformations in the presence of CO. The integrated thermodynamics (integrated thermodynamics for processes occurring in < ∼ 20 μs) give an enthalpy change of 5±2 kcal/mol and a molar volume change of −0.5±0.4 mL/mol. Taking into account the enthalpy and volume changes associated with CO photo-release from the heme, an enthalpy change of - 12 kcal/mol and volume change of -6 mL/mol is obtained for the 8% folding as well as non-native His and Met coordination. These results will be discussed within the context of the protein funnel mechanism for fast folding in cytochorme c.