FORMATION BY NO OF NITROSYL ADDUCTS OF REDOX COMPONENTS OF THE PHOTOSYSTEM-II REACTION CENTER .1. NO BINDS TO THE ACCEPTOR-SIDE NON-HEME IRON

NO is a good electrophile and EPR spin probe, carrying an unpaired electron (S = 1 2). Exposure of Photosystem II reaction centers to NO results in the appearance of an EPR signal at g = 4. Dark titration with NO shows a Kd ≈ 30 μM in spinach chloroplasts and 250 μM in BBY preparations. Successive cycles of illumination at 200 K, followed by incubation at 245 K, results in binary oscillations of the amplitude of the g = 4 signal in spinach chloroplasts. This signal is small in states Q-AQB, Q-AQ-B, but large in states QAQB and QAQBH2 of the PS II acceptor side. NO slows electron transfer between QA and QB (Diner, B.A and Petrouleas, V. (1990) Biochim. Biophys. Acta 1015, 141-149) and modifies the Mössbauer spectrum of the non-heme Fe(II). These results strongly support the assignment of the g = 4 EPR signal to an acceptor side Fe(II)-NO adduct in an S = 3 2 state. Exchange coupling of this species with the S = 1 2 semiquinones results in an integral spin system, not detectable in X-band EPR. The g = 4 spectrum can be described by a spin hamiltonian. The rhombicity parameter E D is in all cases small (≤ 0.015), implying a near axial environment. NO can donate electrons to PS II. Charge recombination, following a light flash in the presence of DCMU, is blocked by NO (Km ≈ 30 μM, 25°C). NO also reacts reversibly in the dark with D+ (an oxidized secondary donor), resulting in the disappearance of Signal IIdark with a Kd of 3 μM. Pumping off of NO in the dark results in full recovery of the signal. © 1990.