B. Ibarramolero et Jm. Sanchezruiz, A MODEL-INDEPENDENT, NONLINEAR EXTRAPOLATION PROCEDURE FOR THE CHARACTERIZATION OF PROTEIN-FOLDING ENERGETICS FROM SOLVENT-DENATURATION DATA, Biochemistry, 35(47), 1996, pp. 14689-14702
We have characterized the guanidine-induced denaturation of hen egg wh
ite lysozyme within the 30-75 degrees C temperature range on the basis
of equilibrium fluorescence measurements, unfolding assays, kinetic f
luorescence measurements, and differential scanning calorimetry. Analy
sis of the guanidine denaturation profiles according to the linear ext
rapolation method yields values for the denaturation Gibbs energy whic
h are about 15 kJ/mol lower than those derived from differential scann
ing calorimetry. Our results strongly suggest that this discrepancy is
not due to deviations from the two-state denaturation mechanism. We p
ropose a new method for the determination of denaturation Gibbs energi
es from solvent-denaturation data (the constant-Delta G extrapolation
procedure). It employs several solvent-denaturation profiles (obtained
at different temperatures) to generate the protein stability curve at
zero denaturant concentration within the -8 to 8 kJ/mol Delta G range
. The method is model-independent and provides a practical, nonlinear
alternative to the commonly employed linear extrapolation procedure. T
he application of the constant-Delta G method to our data suggests tha
t the guanidine-concentration dependence of the denaturation Gibbs ene
rgy is approximately linear over an extended concentration range but,
also, that strong deviations from linearity may occur at low guanidine
concentrations. We tentatively attribute these deviations to the abru
pt change of the contribution to protein stability that arises from pa
irwise charge-charge electrostatic interactions. This contribution may
be positive, negative, or close to zero, depending on the pH value an
d the charge distribution on the native protein surface [Yang, A.-S.,
& Honig, B. (1993) J. Mol. Biol. 231, 459-474], which may help to expl
ain why disparate effects have been found when studying protein denatu
ration at low guanidine concentrations. Kinetic nz values for lysozyme
denaturation depend on temperature, in a manner which appears consist
ent with Hammond behavior.