EGTA, a calcium chelator, inhibits electron transport in photosystem II of spinach chloroplasts at two different sites

Abstract

A fifteen minute incubation of spinach chloroplasts with the divalent Ca 2+ chelator, EGTA, in concentrations 50–250 μM, inhibits electron transport through both photosystems. All photosystem II partial reactions, including indophenol, ferricyanide and the DCMU-insensitive silicomolybdate reduction are inhibited from 70–100%. The photosystem II donor reaction, diphenyl carbazide → indophenol, is also inhibited, indicating that the inhibition site comes after the Mn 2+ site, and that the first Ca 2+ effect noted (site II) is not on the water oxidation enzyme, as is commonly assumed, but between the Mn 2+ site and plastoquinone A pool. The other photosystem II effect of EGTA (Ca 2+ site I), occurs in the region between plastoquinone A and P700 in the electron transport chain of chloroplasts. About 50% inhibition of the reaction ascorbate + TMPD → methyl viologen is given by incubation with 200 μM EGTA for 15 min. Ca 2+ site II activity can be restored with 20 mM CaCl 2. Ca 2+ site I responds to Ca 2+ and plastocyanin added jointly. More than 90% activity in the ascorbate + TMPD → methylviologen reaction can be restored. Various ways in which Ca 2+ ions could affect chloroplast structure and function are discussed. Since EGTA is more likely to penetrate chloroplast membranes than EDTA, which is known to remove CF 1, the coupling factor, from chloroplast membranes, and since Mg 2+ ions are ineffective in restoring activity, it is concluded that Ca 2+ may function in the electron transport chain of chloroplasts in a hitherto unsuspected manner.