Fig Protein foldingunfolding equilibrium

Because of the huge number of atoms in protein molecules, the heat capacities of both forms are extremely large. Consequently, the ACp term is important in both the enthalpy and entropy changes of the reaction. The standard free energy change of reaction (11.9) is expressed by:

AG U = AHU ,ref + AC P (T - Tref ) - T[ASU,ref + AC p ln(T / Tref )] (11.11)

where the reference state is at 298 K. The fraction of the protein in the unfolded state is given by:

Typical values of the reference thermodynamic properties in at are: AH U,ref = 50 kcal / mole; ASU,ref = 0.1kcal / mole - K; AC P = 1.5 kcal / mole - K The reaction is endothermic because breaking the bonds holding the folded state together requires energy. The entropy change is positive because the folded state is more highly-organized than the unfolded state. The heat capacity difference is large because of the large number of atoms in the protein molecule. It is positive because the unfolded state has many more degrees of freedom (in vibration and rotation) than the tighter folded state. From the combination of Eqs (11.10)- (11.12), Fig. 11.7 shows the effect of ACP on the variation of the unfolded fraction with temperature. The significant feature of these plots is the increasing steepness of the transition between states with increasing ACP. The large values of ACP causes the change of form to resemble a phase change of a single-component system, which is a step function at the transition temperature. For this reason, the folded ® unfolded transformation is often referred to as "melting", despite the fact that the system has two components (the protein and water).

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