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904 BIOCHEMICAL SOCIETY TRANSACTIONS Origin of the ‘Z scheme’ Severo Ochoa (1951) had suggested that the reduction of CO, could depend on the photochemical reduction of NADP+. This was proved by Vishniac and Ochoa with a chloroplast preparation supplied with a hydrogen donor at a less oxidizing potential than water. The potential span required for photosynthesis would be near to that of the oxyhydrogen reaction, from +0.32V to -0.41 V at pH7. The characteristic potential of cytochromefwas determined by H. E. Davenport as +0.35V at pH7. D. S. Bendall had found that a b-type cytochrome present in chloroplasts had a characteristic potential of +O at pH7. It seemed that the postulation of two forward reactions joined by a back reaction would rationalize the distribution of potentials. This hypothetical scheme, later referred to as the Z scheme, was developed with F. Bendall (Takamiya, 1975, page 55). The scheme survived because the potentials of other components of the chloroplast were found to be near to one or other of the two cytochrome components (Hill, 1965). Hill, R. (1965) Essays Biochern. 1, 121-151 Holt, A. S. & French, C. S. (1 949) in Photosynthesis inPlanrs (Franck,J. & Loomis, W. E., eds.), pp. 277-285, Iowa State College Press, Iowa Kamen, M. D. (1949) in PhotosynthesisinPlants(Franck, J. & Loomis, W. E.,eds.), pp. 365-380, Iowa State College Press, Iowa Keilin, D. (1966) in The History of Cell Respiration and Cytochrome, Cambridge University Press, Cambridge Krebs, H. A. (1972) Biogr. Mem. Fellows R . SOC.18,629-652 Ochoa, S . (1951) Exp. Biol. 5,29-51 Spoehr, H. A. (1926) Photosynthesis, Chemical Catalog Company, New York Takamiya, A. (ed.) (1975) Translation of Carbon and Nitrogen Assimilation by Kieta Shibata (1931), Japan Science Press, Tokyo van Niel, C. B. (1949) inphotosynthesis inPlunfs(Franck,J. & Loomis, W. E., eds.), pp. 437-495, Iowa State College Press, Iowa Variations in Photosynthetic Electron-Transport Pathways in Chlorella vulgaris JOHN SINCLAIR and AKINORI SARA1 Department of Biology, Carlton University, Ottawa, Ontario, Canada Photosynthesis in green plants and algae is known to involve O2 as both a product and a reactant. It is produced when water molecules are consumed in a reaction that feeds electrons into the photosynthetic electron-transport pathway. 0,is consumed during photorespiration (see review by Jackson & Volk, 1970), a light driven process that results in the release of CO,. This process appears to be intimately connected with the functioning of the enzyme ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39). O2 can also cause an inhibition of C 0 2fixation (Warburg effect), although the mechanism of this effect appears to be another aspect of photorespiration. O2can be reduced to H 2 0 2by Photosystem I in the Mehler reaction, which was first demonstrated by Mehler (1951) with isolated chloroplasts. The demonstration by Arnon et a/. (1967) that O2 could be reduced by ferredoxin in the Mehler reaction gave support to the idea that this reaction operated in uiuo. Evidence for the Mehler reaction in intact algal cells came from the work of Patterson & Myers (1973) with Anacysfisnidulans. They showed that H 2 0 2 was produced in a light-driven process, which was inhibited by 3-(3,4dichloropheny1)-1,I-dimethylurea. This process operated more rapidly at high O2 concentrations and low C 0 2 concentrations. Evidence that O2 could accept electrons from a site between the two photosystems was provided by Diner & Mauzerall (1973). The latter studied the evolution of 0,from Chlorella vulgaris and Phormidium luridum in the presence of different concentrations of 0,.They found that at low 0, 1978 576th MEETING, LONDON 905 concentrations (412p.p.m. or less) there was a non-linearity present at low light intensities when the rate of O2evolution was plotted against light intensity. To explain these findings, Diner & Mauzerall (1973) proposed that O2 could accept electrons from the large pool of plastoquinone, usually called A, situated between the two photosystems and also that this pool could accept electrons from a reductant R, which in turn received electrons from the reducing side of Photosystem I. In the absence of 0 2 at low light intensities, pool A became largely reduced owing to the action of component R and so decreased the rate of electron transport from Photosystem I1 and hence decreased O2evolution. A slightly altered version of this hypothesis was proposed by Schreiber & Vidaver (1974). who studied fluorescence transients with Scenedesmus. They suggested that O2 did not react with pool A directly, but was involved in a Mehler reaction and competed with component R for electrons from Photosystem I. In the absence of 02,electrons would more readily flow to component R and hence to pool A, which would thus become more reduced. The present study also involved investigating the effect of O2concentration on the rate of O2 evolution from the cells of Chlorella vulgaris, but used a modulated oxygen electrode. This device is basically an oxygen rate electrode in which the cells are layered on a shiny platinum electrode and illuminated from above by a light beam, whose intensity is periodically modulated. The cells respond to the light by producing waves of 02,which are detected by the platinum electrode and the amplitude and phase of the resultant electric current are measured with a lock-in amplifier. It has been shown by Joliot et al. (1966) that the amplitude is directly proportional to the rate of O2 evolution and that both the amplitude and phase are influenced by the rate-limiting reaction occurring between Photosystem I1 and the water-splitting act. It was found in this study that if the cells were immersed in a solution that was in equilibrium with a gas mixture containing at least 1 % (by volume) of 02,the phase of the oxygen signal was constant at all intensities of 650nm modulated light between 0.1 and 2 J.m-2*s-1. Over the same intensity range the amplitude was directly proportional to the light intensity. If the solution was in equilibrium with a gas mixture containing less than 1% 02,it was found that the phase lag between the oxygen signal and the light modulations decreased as the light intensity was lowered below about 0.5 J*m-2*s-1.At the same time the oxygen yield decreased so that the amplitude was no longer a linear function of the light intensity. Joliot et al. (1966) originally proposed that the rate of O2 production was directly proportional to the concentration of some oxidized precursor X+ and developed equations on this basis that satisfactorily described the results they and others (Sinclair & Arnason, 1974; Arnason & Sinclair, 1976) obtained with the modulated electrode. This simple proposal cannot account for the results obtained at low light intensities under anaerobic conditions. It is necessary to propose that under these conditions component X+ can be reduced by a reductant that is generated between the two photosystems. This proposal has beem investigated both by analysis and by computer simulation and does afford a good description of the experimental results. Evidence which lends support to this backflow of electrons from between the two systems to component X+ comes from measurements of the dark deactivation rates of the S-states associated with Photosystem 11. These S-states are oxidation states associated with the reaction centre and their disappearance requires an input of electrons. The rate of deactivation of the &state was speeded up approx. 2-fold by the removal of O2from the medium, while the rate of deactivation of the S2-state was speeded up almost 7-fold. Similar results were obtained by Diner (1977). If we identify component X+ with one of these S-states then the mechanism we have proposed might we11 give rise to the increased deactivation rates. In conclusion it is clear from the work presented here and that of Diner & Mauzerall (1973) that the removal of O2 from the bathing medium of algae causes changes in the photosynthetic electron-transport system. There is an increased backflow of electrons from the reducing side of Photosystem I, which reduces a redox pool or pools between the photosystems. This in turn gives rise to a backflow of electrons to the oxidizing side of Photosystem I1 and a decreased rate of O2evolution. Vol. 6 906 BIOCHEMICAL SOCIETY TRANSACTIONS Arnason, T. & Sinclair, J. (1976) Biochim. Biophys. Acta 430,517-523 Arnon, D. I., Tsujimoto, H. Y. & McSwain, B. D. (1967) Nature (London) 214, 562-566 Diner, B. (1977) Biochim. Biophys. Acta 460, 247-258 Diner, B. & Mauzerall, D. (1973) Biochim. Biophys. Acta 305, 329-352 Jackson, W. A. & Volk, R. J. (1970) Annu. Rev. Plant Physiol. 21, 385-432 Joliot, P., Hoffnung, M. & Chabaud, R. (1966) J. Chim. Phys. 63, 1423 Mehler, A. H. (1951) Archs. Biochem. Biophys. 33, 65-77 Patterson, C. 0. P. & Myers, J. (1973) Plant Physiol. 51, 104-109 Schreiber, U. & Vidaver, W. (1974) Biochim. Biophys. Acta 368,97-112 Sinclair, J. & Arnason, T. (1974) Biochim. Biophys. Acta 368, 393-400 Absorbance and Fluorescence Changes by Photosystem-I1 Electron Acceptors H. J. VAN GORKUM,* A.-L. ETIENNEt and A. C. F. GORREN* *Department of Biophysics, Huygens Laboratory, Wassenaarseweg 78, Leiden, The Netherlands, and tLaboratoire de Photosynth&e, CNRS 91 190 Gifsur- Yvette, France Electron-Paramagnetic-Resonance Studies of Photosystem 11 MICHAEL C. W. EVANS Department of Botany and Microbiology, University College, Cower Street, London WC1E 6BT, U.K. Electron-paramagnetic-resonance spectrometry has been used very successfully in the investigation of the reaction centres of Photosystem I and photosynthetic bacteria (Evans, 1977). It has had a number of advantages over optical spectrometry in that the measurements are not affected by the relatively high concentrations of light-harvesting chlorophyll and that the primary reactants include iron-sulphur proteins, iron-quinone complexes and reaction-centre chlorophyll components with poorly defined optical spectra, but good e.p.r. spectra. It has also been advantageous to carry out experiments at cryogenic temperatures when the primary reactions can be observed independently of secondary electron transport, when e.p.r. is again easier than optical spectroscopy. However, e.p.r. has a number of disadvantages; it is relatively insensitive, many components can only be observed under closely defined conditions, often only at low temperature, and there are limitations to the kinetic measurements that can be made. Photosystem I1 has proved to be extremely difficult to characterize by e.p.r. The original observation of Commoner et al. (1956), identified two organic radical signals in chloroplasts. One of these (Signal I) has been identified with the oxidized state of the Photosystem-I reaction-centre chlorophyll (P700) and the other (Signal 11) with 1978