Anionic Inhibition of PS II: Insights Derived from Carbonic Anhydrase
At high concentrations, a number of monovalent anions inhibit photosystem II (PSII)1. Often this inhibition can be overcome by added bicarbonate1. Sinclair2 has suggested that several anions inhibit PSII by replacing chloride ions on the oxygen-evolving mechanism. Thus both the Cl− and HCO 3 − binding sites on PSII appear to be targets for inhibition by other anions. This inhibition bears striking resemblance to similar effects on the enzyme carbonic anhydrase. Indeed all of the classic inhibitors of this enzyme, monovalent anions, acetazolamide, and imidazole also inhibit PS1I3,4. So close is the resemblance between the response of PSII and carbonic anhydrase to anions and other inhibitors, that we investigated the possibility that the PSI! complex actually has carbonic anhydrase activity. We used a mass spectrometer to measure the rate of oxygen exchange between doubly labelled C18O2 and H 2 16 O in the presence and absence of osmotically shacked, washed maize thylakoids. Our results indicate that such thylakoids do demonstrate carbonic anhydrase activity. Moreover, the exchange activity was definitely greater in the dark than when the experiment was done in room light. PSII particles also showed carbonic anhydrase activity. These results and others suggest that some anion effects on the PSII complex (including that of bicarbonate) can be understood in terms of a specific interaction with a polypeptide that has carbonic anhydrase activity. Since a great deal is known about the binding of anions to carbonic anhydrase, this knowledge can be profitably applied to PSII.
KeywordsCarbonic Anhydrase Thylakoid Membrane Isotopic Exchange Carbonic Anhydrase Activity PSII Complex
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