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Figure 2. Free energy model for protein folding and unfolding. Stabilization of the native state can be achieved by
destabilization of the denatured state (D') or a more stabilized native state (N'). The free energy barrier (TS') may also
be affected
The difference in the free energy (ΔG) of different states as measured from the reversible
transition from the native to the denatured state is small [18]. Externally forced conforma‐
tional changes in the protein structure lead to a substantial decrease in its stability. The de‐
naturation process causes conformational destabilization by exposing hydrophobic residues
to the solvent, normally deeply buried in the interior of a folded protein. The burial of non
polar surfaces and the hydrophobic force is considered as the main driving force for protein
folding and stability [19] as proteins become thermally more stable upon decreasing hydra‐
tion levels [20].
Evidence from proteins produced by hyperthermophil microorganisms, which are very
thermostable and resistant to chemical denaturation, indicates that this resistance comes
from lower protein flexibility and higher protein rigidity [21]. Thermostable proteins tend to
be very rigid at mesophilic temperatures (10-45 0 C), but allow for greater flexibility at high
temperatures, which is essential for their function in their thermophilic environment. It is as‐
sumed that intrinsic stability due to increased protein rigidity is important for thermal stabi‐
lization, since thermal motion decreases rigidity and enhances flexibility.
3.1. Assisted folding under stress conditions
Molecular responses to abiotic stress are complex and highly dependent on the level and du‐
ration of stress and on the tissue and organ that is affected. Sensing of environmental
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