Tiktopulo E. I., Loseva O. I., Dobritsa A. P., Potekhin S. A.
Institute of Protein Research, 142292, Puschino, Moscow Region, Russia
Heat denaturation of CryIIIa 
-endotoxin
Bacillus thuringiensis subsp. tenebrionis and its fragments
at acid pH was studied using differential scanning microcalorimetry. The
analysis of the obtained calorimetric data showed that at heating rates
from 0.125 to 2 K/min the denaturation of the preparations is inequilibrium.
This means that the stability of 
-endotoxin
(the apparent temperature of denaturation Tm) is determined
by kinetic parameters rather than by thermodynamic ones.
It was demonstrated that denaturation is a one-stage kinetic process.
The enthalpy of the of the conformation of 
-endotoxin
in an activated state and its native process and its activation energy
were measured as temperature functions. The data obtained allow us to confirm
that there is a slight difference in compactness conformation (hydration
extent). The comparison of the thermodynamic data for intact 
-endotoxin
and the 55kDa fragment revealed that the molecule transition to an activated
state does not result in any changes in the conformation of three N-terminal
-helices. Complete removal of
the N terminal domain of 
-endotoxin
and 40 amino acids in 
-structural
domain III brings about an irreversible loss of molecular compactness of
the thertiry structure. Thus, during protein self-assembly the nucleation
core determining protein stability is probably located in the C-terminal
part of the molecule and does not envolve its three initial 
-helices
of the N-terminal domain.
Functional significance of peculiarities of structure arrangement of
a 
-endotoxin molecule are discussed.