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Chloroplast Fluorescence: Evidence for a Cyclic, Proton-Conducting Pathway in Oxygenic Photosynthesis (Photosystem II/Cyclic Proton Transport/Uncouplers) STEVEN W

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Chloroplast Fluorescence: Evidence for a Cyclic, Proton-Conducting Pathway in Oxygenic Photosynthesis (Photosystem II/Cyclic Proton Transport/Uncouplers) STEVEN W Proc. Natl. Acad. Sci. USA Vol. 84, pp. 8424-8428, December 1987 Biophysics Protonophores induce plastoquinol oxidation and quench chloroplast fluorescence: Evidence for a cyclic, proton-conducting pathway in oxygenic photosynthesis (photosystem II/cyclic proton transport/uncouplers) STEVEN W. MCCAULEY*, ANASTASIOS MELIS, GEORGE M.-S. TANG, AND DANIEL 1. ARNON Division of Molecular Plant Biology, University of California, Berkeley, CA 94720 Contributed by Daniel 1. Arnon, August 7, 1987 ABSTRACT The photosynthetic apparatus converts light terminal acceptor, PQH2 accumulates; electrons then "back into chemical energy by a series of reactions that give rise to a up" and accumulate on a specialized PQ (QA). The accumu- coupled flow of electrons and protons that generate reducing lation of QA is monitored by a rise in variable fluorescence power and ATP, respectively. A key intermediate in these (Fv) (6). reactions is plastoquinone (PQ), the most abundant electron We have found that in the absence of a terminal acceptor, and proton (hydrogen) carrier in photosynthetic membranes two chemically diverse proton-conducting ionophores (thylakoids). PQ ultimately transfers electrons to a terminal (uncouplers), 2,6-di-t-butyl-4-(2',2'-dicyanovinyl)phenol (SF electron acceptor by way of the Rieske Fe-S center of the 6847) and carbonylcyanide p-trifluoromethoxyphenylhydra- cytochrome bfcomplex. In the absence of a terminal acceptor, zone (FCCP), induced oxidation of PQH2 and dramatically electrons accumulate in the PQ pool, which is reduced to lowered chloroplast fluorescence (signifying oxidation of plastoquinol (PQH2), and also on a specialized PQ, QA, which Q-). The two protonophores produced the same effects when is reduced to an unprotonated semiquinone anion (Q-). The the only recognized pathway ofPQH2 oxidation by way ofthe accumulation of Q- is measured by a rise in fluorescence yield cytochrome bf complex was inhibited by 2,5-dibromo-3- and the accumulation of PQH2 is measured by absorption methyl-6-isopropyl-p-benzoquinone [DBMIB (dibromothy- difference spectrometry. We have found that in the absence of moquinone)] (4). Two other uncouplers, gramicidin and a terminal electron acceptor, two chemically diverse proton- nigericin, which are not protonophores but facilitate by other conducting ionophores (protonophores), 2,6-di-t-butyl-4- mechanisms proton movement across membranes (7), were (2',2'-dicyanovinyl)phenol (SF 6847) and carbonylcyanide p- ineffective. trifluoromethoxyphenylhydrazone (FCCP), induced oxidation These findings are consistent with the operation in PSII of of PQH2 and quenching of chloroplast fluorescence, signifying a cyclic, proton-conducting pathway that involves oxidation oxidation of Q-. The two protonophores produced the same of PQH2 by way of cytochrome b559 (1). We discuss here the effects even when the only recognized pathway of PQH2 PQ components of the cycle; the evidence pertaining to oxidation by way of the cytochrome bf complex was inhibited cytochrome b559 is being reported separately (8). by dibromothymoquinone. Two other uncouplers, gramicidin and nigericin, which are not protonophores but facilitate METHODS proton movement across membranes by other mechanisms, were ineffective. These findings are consistent with the oper- Chloroplasts were isolated from spinach leaves (Spinacia ation in the oxygen-generating photosystem (photosystem II) of oleracea var. Marathon) grown in a greenhouse in nutrient a cyclic, proton-conducting pathway. solution culture (9) and freshly harvested before each exper- iment. Previously described procedures were used for chlo- The recently described perspective on photosynthesis envi- rophyll estimation (9) and the preparation of thylakoids (10). sions the operation in the oxygen-generating photosystem These consisted of osmotically disrupted chloroplasts that (photosystem II; PSII) of a light-induced cyclic pathway for retained the capacity for complete noncyclic electron trans- conductance of protons (1). We now report effects of port from water to NADP+ and photosynthetic phosphoryl- uncouplers on chloroplast fluorescence and the redox state of ation (10). Where maintained, anaerobic conditions were plastoquinone (PQ) that are consistent with the operation of established by gassing the samples with N2 and including in such a pathway. the reaction mixture a glucose/glucose oxidase/catalase The photosynthetic apparatus converts light into chemical oxygen trap (11). energy by a series ofreactions that give rise to a coupled flow Chloroplast fluorescence and absorbance difference mea- of electrons and protons that generate reducing power and surements for quinones were performed as described (6, 12). ATP, respectively (2). A key intermediate in these reactions Differential extinction coefficients (mM-1 cm-1) of 13 at 263 is PQ, the most abundant redox component in photosynthetic nm for PQ and 19.6 at 257 nm for 2,5-dimethylbenzoquinone membranes (thylakoids) (3, 4). Because PQ-plastoquinol (DMQ) were used. DBMIB was kindly supplied by A. Trebst (PQH2) oxidoreductions (PQ + 2 e- + 2 H+ ;± PQH2) involve Bochum, F.R.G.), nigericin was provided transfers of hydrogen atoms (electrons plus protons), PQ is (Ruhr-Universitat, both the main electron and the main proton carrier in Abbreviations: PS, photosystem; PQ, plastoquinone; PQH2, thylakoids. plastoquinol; QA and QB, specialized membrane-bound forms of In functioning chloroplasts, PQH2 is ultimately oxidized by PQ; DMQ, 2,5-dimethylbenzoquinone; diuron (DCMU), 3-(3,4-di- a terminal electron acceptor, by way of the Rieske Fe-S chlorophenyl)-1,1-dimethylurea; DBMIB (dibromothymoquipone), center of the cytochrome bfcomplex (5). In the absence of a 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; SF 6847, 2,6-di- t-butyl-4-(2',2'-dicyanovinyl)phenol; FCCP, carbonylcyanide p- trifluoromethoxyphenylhydrazone; CCCP, carbonylcyanide m-chlo- The publication costs of this article were defrayed in part by page charge rophenylhydrazone; F,, variable fluorescence. payment. This article must therefore be hereby marked "advertisement" *Permanent address: Department of Physics, California Polytechnic in accordance with 18 U.S.C. §1734 solely to indicate this fact. University, 3801 West Temple Avenue, Pomona, CA 91768. 8424 Downloaded by guest on September 28, 2021 Biophysics: McCauley et al. Proc. Natl. Acad. Sci. USA 84 (1987) 8425 by Hoffman-LaRoche, and SF 6847 was provided by T f Sumitomo Chemical (Osaka, Japan). FCCP and gramicidin were purchased from Sigma. -DCMU RESULTS Control The carbonylcyanide phenylhydrazones [FCCP and carbon- _0 ylcyanide m-chlorophenylhydrazone (CCCP)] are widely 0) used proton-conducting ionophores that uncouple electron transport from ATP formation in oxidative and photosyn- thetic phosphorylation (7). In other studies these uncouplers 0.- were observed to influence the oxidizing (water-splitting) a,:3 side of PSII and were characterized as ADRY reagents 0 (accelerators of deactivation reactions of the water-splitting 1. - system Y) (13). a) )g, SF6847 0 A puzzling effect of FCCP and CCCP on PSII was that, at low concentrations, they quench chloroplast fluorescence I I emanating from PSII. This fluorescence reflects the redox FCCP F01 state of QA. QA is the first stable electron acceptor of PSII; F0 it is a specialized, tightly bound PQ that is reduced only to the unprotonated semiquinone form Q- (14). Fluorescence yield 0 1 2 3 4 5 is high when QA is predominantly in the reduced state (Q-) and decreases when it is in the oxidized state (15). Thus, the seconds quenching of fluorescence is tantamount to reoxidation of FIG. 1. Quenching of F, by protonophores SF 6847 and FCCP. QA. The reaction mixture contained osmotically disrupted chloroplasts Different explanations were put forward to explain fluo- (equivalent to 50 ,g of chlorophyll per ml), 5 mM MgCl2, 50 mM rescence quenching by FCCP or CCCP (16-18), including Tricine (pH 7.5), and, where added, 5 AM SF 6847 or 5 AM FCCP. "direct quenching actions ofoxygenated reaction products or The reaction mixtures were incubated for 2 min in the dark and then a between reduced electron carriers and illuminated by light filtered by Corning CS 4-96 and CS 3-69 filters. cyclic electron flow Intensity of illumination, 45 microeinsteinsm-2 s' (1 einstein = 1 such intermediates" (19) but excluding the protonophoric mol of photons). initial fluorescence. properties of these uncouplers. Since proton conductance in FO, PSII is of considerable conceptual interest (1), we reinvestig- ated the possibility that fluorescence quenching by FCCP conditions, including an ambient oxygen trap, whose effec- may be related to its proton-conducting properties. We were tiveness in excluding traces of oxygen was previously tested especially interested to determine whether another protono- by the oxygen-sensitive redox state of bound iron sulfur phore, chemically different from FCCP, would also quench centers of PSI (11). Fig. 3 shows that the protonophores SF chloroplast fluorescence. For this purpose we used SF 6847, 6847 and FCCP markedly quenched fluorescence in the a ditertiary phenol derivative, known as the most potent protonophoric uncoupler of electron transport and ATP formation (20). The effect of the two protonophores, FCCP II and SF 6847, was compared to that of two other uncouplers, which facilitate proton movement across membranes by ~+DCMU other mechanisms: gramicidin, a channel-forming ionophore, and nigericin, an ionophore that catalyzes the exchange ofK+ Control for
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