A role of methionine100 in facilitating PYPM-decay process in the photocycle of photoactive yellow protein

PYP (photoactive yellow protein) is a photoreceptor protein, which is activated upon photo-isomerization of the p-coumaric acid chromophore and is inactivated as the chromophore is thermally back-isomerized within a second (in PYPM-to-PyPdark conversion). Here we have examined the mechanism of the rapid thermal isomerization. by analyzing mutant PYPs of Met100, which was previously shown to play a major role in facilitating the reaction [Devanathan, S. et al. (1998) Biochemistry 37, 11563-11568]. The mutation to Lys, Leu, Ala, or Glu decelerated the dark state recovery by one to three orders of magnitude. By evaluating temperature-dependence of the kinetics, it was found that the retardation resulted unequivocally from elevations of activation enthalpy (DeltaHdouble dagger) but not the other parameters such as activation entropy or heat capacity changes. Another effect exerted by the mutations was an up-shift of the apparent pK(a). of the chromophore [the pK(a) of a titratable group (X) that controls the pK(a) of the chromophore] in the PYPM-decay process. The pK(a) up-shift and the DeltaHdouble dagger elevation show an approximately linear correlation. We, therefore, postulate that the role of Met100 is to reduce the energy barrier of the PYPM-decay process by an indirect interaction through X and that the process is thereby facilitated.