FORMATION BY NO OF NITROSYL ADDUCTS OF REDOX COMPONENTS OF THE PHOTOSYSTEM-II REACTION CENTER .2. EVIDENCE THAT HCO3- CO2 BINDS TO THE ACCEPTOR-SIDE NON-HEME IRON

We have demonstrated (Petrouleas and Diner, accompanying paper) that NO binds to the non-heme iron of the PS II reaction center. We show here that, in spinach chloroplasts, NO (Kd ≈ 30 μM), like formate, slows electron transfer between the primary and secondary quinone electron acceptors of PS II, QA and QB, respectively. In a series of saturating flashes given to dark-adapted chloroplasts treated with NO, this electron transfer is slowed by at least a factor of 10 from the second saturating flash excitation onward as compared to untreated chloroplasts. This slowing is completely reversed by the addition of NaHCO3 (10 mM), indicating that NO, like formate, displaces bicarbonate from the reaction center. The NO-enhanced dissociation of HCO-3 from the reaction center is pH-dependent, occurring much faster at pH 5.5 than at 7.4. In the reverse experiment, the S = 3 2 Fe(II)-NO EPR signal at g = 4 is diminished by the addition of NaHCO3, indicating that HCO-3 dissociates the NO ligation to the iron. These data argue in favor of HCO-3/CO2 as a ligand to the iron. Formate does not dissociate NO from the iron and it is possible that formate and NO displace HCO-3/CO2 by different mechanisms. © 1990.