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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,
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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.
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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
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