Yv. Griko et Vp. Kutyshenko, DIFFERENCES IN THE PROCESSES OF BETA-LACTOGLOBULIN COLD AND HEAT DENATURATIONS, Biophysical journal, 67(1), 1994, pp. 356-363
The changes in P-lactoglobulin upon cold and heat denaturation were st
udied by scanning calorimetry, CD, and NMR spectroscopy. It is shown t
hat, in the presence of urea, these processes of beta-lactoglobulin de
naturation below and above 308 K are accompanied by different structur
al and thermodynamic changes. Analysis of the NOE spectra of beta-lact
oglobulin shows that changes in the spin diffusion of beta-lactoglobul
in after disruption of the unique tertiary structure upon cold denatur
ation are much more substantial than those upon heat denaturation. In
cold denatured P-lactoglobulin, the network of residual interactions i
n hydrophobic and hydrophilic regions of the molecule is more extensiv
e than after heat denaturation. This suggests that upon cold- and heat
-induced unfolding, the molecule undergoes different structural rearra
ngements, passing through different denaturation intermediates. From t
his point of view, cold denaturation can be considered to be a two sta
ge process with a stable intermediate. A similar equilibrium intermedi
ate can be obtained at 35 degrees C in 6.0 M urea solution, where the
molecule has no tertiary structure. Cooling or heating of the solution
from this temperature leads to unfolding of the intermediate. However
, these processes differ in cooperativity, showing noncommensurate sig
moidal-like changes in efficiency of spin diffusion, ellipticity at 22
2 nm, and partial heat capacity. The disruption with cooling is accomp
anied by cooperative changes in heat capacity, whereas with heating th
e heat capacity changes only gradually. Considering the sigmoidal shap
e of the heat capacity change an extended heat absorption peak, we pro
pose that the intermediate state is stabilized by enthalpic interactio
ns.