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 ⇄ I 700 and pG ⇄ pR were determined by the fluence saturation curves of the structural volume change, Δ V r, analyzed with model functions taking photoselection into account. The quantum yield of the photoreversion, Φ I700 → Pr = 0.22 ± 0.12, proved to be ca . 1.4 times larger than the known quantum yield of the forward photoreaction (assuming Φ Pr → I700 = Φ 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 (Φ pR → pG = 0.07) compared with the known value for the thermal forward process to the signaling state pB (Φ pG → pR ≥ Φ 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.