Bicarbonate-Reversible and Irreversible Inhibition of Photosystem II by Monovalent Anions

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Abstract
A number of inhibitory monovalent anions were tested for their primary site of action on photosystem II(PSII) in chloroplasts [of Zea mays]. The inhibitory effects of F-, HCO2-, NO2-, NO3- and CH3CO2- are all reversed by addition of a high concentration of HCO3-. This class of anions competitively inhibits H14CO3- binding to PSII. All of those anions tested reduced H14CO3- binding more in the light than in the dark. The primary inhibitory site of action of a number of monovalent anions is apparently at the HCO3- binding site(s) on the PSII complex. The carbonic anhydrase inhibitor gold cyanide, and also azide, inhibit PSII but at a site other than the HCO3- binding site. The unique ability of HCO3- to reverse the effects of inhibitory anions may reflect its singular ability to act as a proton donor/acceptor at the anion binding site. A similar role was proposed for non-substrate-bound HCO3- on carbonic anhydrase by Yeagle et al (1975).

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