Abstract
A thermodynamic study of ordered complexes of cytochrome c fragments. Kinetic and equilibrium measurements and comparison with the folding of the intact protein.
Highlights
The temperature dependence of the kinetic and equi- be formed, regenerating a continuous amino acid selibrium properties of t w o noncovalently bound ferric quence (8,9)
The intermediate in the equilibrium thermal unfolding of complex (1-25)H. (23-104) exhibits tryptophan fluorescence quenching, a slight decrease in the helical content, loss of ligation of methionine 80, and an increase in Soret absorbance. This similarity in equilibrium thermal unfolding further underscores the intimate relationship between the structure of this productive complex and the native protein (10)
The thermodynamic parameters obtained for the 695 nm transition of (1-25)H. (23-104) are comparable to those reported for the native protein (14,15).the standard Gibbs energy change for the complex (0.7 kcalemol-I)is smaller than that of the native protein (1.2 kcal-mol")
Summary
The temperature dependence of the kinetic and equi- be formed, regenerating a continuous amino acid selibrium properties of t w o noncovalently bound ferric quence (8,9). These steps arecomparable with the reversalof the f i s t and second order kinetic processes, respectively, observed in complementation The kinetic intermediate complexmay resemble the speciesformed in thefirst step of the thermal transition of the complex. Heme iron (7, 11).The following first order kinetic phase involvesligation of methionine 80 and anincrease in the redox potential to the level reducible with ascorbate (10, 12). (1-25)H (39-104), complexesutilizingporphyrin fragmentsrather than heme fragments)exhibit a very similar second order reaction step while fxst order rate processes are not observed (7, 11, 25)H*(39-104), resembling the intermediate com- 12). Productive and nonproductive complexes are kiplex, exhibits thermodynamic behavior characteristics netically differentiated on the basis of the presence or absence, of small globular proteins. In the case of the active respectively, of a step toform a native-like heme environment
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