Protein folding thermodynamics applied to the photocycle of the photoactive yellow protein

Two complementary aspects of the thermodynamics of the photoactive yellow protein (PYP), a new type of photoreceptor that has been isolated from Ectothiorhodospira halophila, have been investigated. First, the thermal denaturation of PYP at pH 3.4 has been examined by global analysis of the temperature-induced changes in the UV-VIS absorbance spectrum of this chromophoric protein. Subsequently, a thermodynamic model for protein (un)folding processes, incorporating heat capacity changes, has been applied to these data. The second aspect of PYP that has been studied is the temperature dependence of its photocycle kinetics, which have been reported to display an unexplained deviation from normal Arrhenius behavior. We have extended these measurements in two solvents with different hydrophobicities and have analyzed the number of rate constants needed to describe these data. Here we show that the resulting temperature dependence of the rate constants can be quantitatively explained by the application of a thermodynamic model which assumes that heat capacity changes are associated with the two transitions in the photocycle of PYP. This result is the first example of an enzyme catalytic cycle being described by a thermodynamic model including heat capacity changes. It is proposed that a strong link exists between the processes occurring during the photocycle of PYP and protein (un)folding processes. This permits a thermodynamic analysis of the light-induced, physiologically relevant, conformational changes occurring in this photoreceptor protein.