Photoequilibrium in the primary steps of the photoreceptors phytochrome A and photoactive yellow protein

The photochromic equilibria between the ground state and the first intermediate of the native photoreceptors phytochrome A (phyA) of oat and photoactive yellow protein (PYP) of Ectothiorhodospira halophila have been studied by laser-induced optoacoustic spectroscopy, employing photon densities sufficiently high to reach saturation and to establish the photoequilibria between the red-light-absorbing phyA form, P-r, and the first intermediate, I-700, and between the PYP ground state, pG, and the first intermediate, pR. The parameters for the photoequilibria P-r reversible arrow I-700 and pG reversible arrow pR were determined by the fluence saturation curves of the structural volume change, Delta V-r, analyzed with model functions taking photoselection into account. The quantum yield of the photoreversion, Phi(1700 --> Pr) = 0.22 +/- 0.12, proved to be ca 1.4 times larger than the known quantum yield of the forward photoreaction (assuming Phi(Pr --> 1700) = Phi(Pr --> Pfr)). This suggests that the chromophore-binding protein domain structures of P-r and I-700 are quite similar. in contrast, the photoreversion quantum yield in PYP for pR --> pG is small (Phi(pR --> pG) = 0.07) compared with the known value for the thermal forward process to the signaling state pB (Phi(pG --> pR) greater than or equal to Phi(pG --> pB) = 0.35). This is tentatively attributed to a main conformational change associated with the pG --> pR phototransformation. The results of this study emphasize the need of considering photoequilibria in photoreceptors when working with high-fluence laser pulses.