title: Time resolved calorimetry of photo-induced folding in horse heart cytochrome c at high pH.
abstract: Here the molar volume and enthalpy changes associated with the early events in the folding of ferrocytochrome c (Cc) at high pH have been examined using time resolved photoacoustic calorimetry (PAC). The data reveal an overall volume change of 1.3 ± 0.3 mL mol-1and an enthalpy change of 13 ± 7 kcal mol-1occurring subsequent to photodissociation of the unfolded CO bound Cc species in <∼20 ns. Two additional kinetic phases are observed that are associated with non-native His binding (ΔH and ΔV of 2 ± 4 kcal mol-1and -0.5 mL mol-1, τ ∼ 2.5 μs ) and Met binding (ΔH and ΔV -0.4 ± 2 kcal mol-1and -0.1 ± 0.1 mL mol-1, τ∼ 660 ns). Considering only protein conformational changes (excluding volume and enthalpies associated with heme ligation events) the initial conformational event exhibits a ΔH and ΔV of 6 ± 3 kcal mol-1and -3±0.1 mL mol-1, respectively, that are attributed to a small contraction of the unfolded protein. The corresponding enthalpy associated with both native and non-native ligand binding are found to be -5±4 kcal mol-1(Fe-Met) and +20 ± 4 kcal mol-1(Fe-His) with the change in volume for both phases being essential negligible. This would indicate that non-native ligand binding likely occurs from an already collapsed conformation.
authors.affiliation: Department of Chemistry, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33602 USA.
authors.forename: Tarah A
authors.initials: TA
authors.lastname: Word
authors.affiliation: Department of Chemistry, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33602 USA. Electronic address: rwlarsen@usf.edu.
authors.forename: Randy W
authors.initials: RW
authors.lastname: Larsen
mesh_terms: D000162:Acoustics; D000818:Animals; D002151:Calorimetry / Q000379:methods*; D002248:Carbon Monoxide / Q000378:metabolism; D045304:Cytochromes c / Q000737:chemistry*; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D009206:Myocardium / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D012997:Solvents / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D008024:Ligands; D012997:Solvents; D002248:Carbon Monoxide; D045304:Cytochromes c
keywords: Cytochrome c; Kinetics; Photoacoustic calorimetry; Protein folding