Thermodynamic and Kinetic Analysis of Unfolding of P23k Protein Isolated from Spinach Photosystem II

The unfolding of 23kD (P23k) protein isolated from spinach photosystem II particle was studied by high pressure and fluorescence spectroscopy. The thermal equilibrium study indicated that the protein could be totally unfolded by 180 or 160 MPa at 20 degreesC and 3 degreesC, respectively. The standard free energy and standard volume change of the protein for unfolding at 20 degreesC is 23.45 kJ/mol and - 150.3 ml/mol, respectively. Kinetics study indicated that at 20 degreesC the activation volume for unfolding, Delta V-u(double dagger), was negative ( - 66.2 ml/mol), meanwhile the activation volume for folding, Delta V-f(double dagger), was positive (84.1 ml/mol). The rate constants for folding and unfolding (K-0f, K-0u) were 1.87 s(-1) and 1.3 x 10(-4) s(-1), respectively, these results provide some clues to explain why the protein is so sensitive to pressure.