Formation by NO of nitrosyl adducts of redox components of the Photosystem II reaction center. I. NO binds to the acceptor-side non-heme iron
1990
Petrouleas, V. | Diner, B.A.
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 approximately 30 microM in spinach chloroplasts and 250 microM in BBY preparations. Successive cycles of illumination at 200 K, followed by incubation at 245 K, results in binary oscillations of die amplitude of the g = 4 signal in spinach chloroplasts. This signal is small in states (QA-)QB, (QA-)QB- and QA(QB-), but large in states (QA)QB and (QA)QB(H2) of the PS II acceptor side. NO slows electon transfer between QA and QB (Diner, B.A and Petrouleas, V. (1990) Biochim. Biophys. Acta 1015, 141-149) and modifies the Mossbauer 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 (less than or equal to 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 approximately 30 microM, 25 degrees C). NO also reacts reversibly in the dark with D+ (an oxidized secondary donor), resulting in the disappearance of Signal II(dark) with a Kd of 3 microM. Pumping off of NO in the dark results in full recovery of the signal.
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