The mathematical analysis described in the preceding paper (Biochim. Biophys. Acta (1977) 460, 65–75), in which the steady-state photooxidation of P-700 was compared with overall electron flux in Photosystem I chloroplast fragments, was applied to membrane fragments from the blue-green alga Nostoc muscorum (Strain 7119) noted for their high activity of both Photosystem I and Photosystem II. The same analysis, which gave good agreement between the photooxidation of P-700 and the overall light-induced electron flux (measured as NADP+ reduction) in Photosystem I chloroplast fragments, revealed in the algal membrane fragments two P-700 components: one responding to high light intensity (P-700 HI), the photooxidation of which was in good agreement with the overall electron flux (measured as NADP+ reduction by reduced 2,6-dichlorophenolindophenol), and the other component responding to low light intensity (P-700 LI), the photooxidation of which was not correlated with the reduction of NADP+ by reduced 2,6-dichlorophenolindophenol.

A mathematical analysis is described which measures the effects of actinic light intensity and concentration of an artificial electron donor on the steady-state light-induced redox level of a reaction-center pigment (e.g. P-700) and on the overall light-induced electron flux (e.g. reduction of NADP+). The analysis led to a formulation (somewhat similar to the Michaelis-Menten equation for enzyme kinetics) in which a parameter, I1/2, is defined as the actinic light intensity that, at a given concentration of electron donro, renders the reaction-center pigment half oxidized and half reduced. To determine the role of a presumed reaction-center pigment, I1/2 is compared with another parameter, equivalent to I1/2, that is obtained independently of the reaciton-center pigment by measuring the effect of actinic light intensity and concentration of electron donor on the overall electron flow. The theory was tested and validated in a model system with spinach Photosystem I chloroplast fragments by measurements of photooxidation of P-700 and light-induced reduction of NADP+ by reduced 2,6-dichlorophenolindophenol. A possible extension of this mathematical analysis to more general electron-transport systems is discussed.

The effects of magnesium and chloride ions on photosynthetic electron transport were investigated in membrane fragments of a blue-green alga, Nostoc muscorum (Strain 7119), noted for their stability and high rates of electron transport from water or reduced dichlorophenolindophenol to NADP+. Magnesium ions were required not only for light-induced electron transport from water to NADP+ but also for protection in the dark of the integrity of the water-photooxidizing system (Photosystem II). Membrane fragments suspended in the dark in a medium lacking Mg2+ lost the capacity to photoreduce NADP+ with water on subsequent illumination. Chloride ions could substitute, but less effectively, for each of these two effects of magnesium ions. By contrast, the photoreduction of NADP+ by DCIPH2 was independent of Mg2+ (or Cl−) for the protection of the electron transport system in the dark or during the light reaction proper. Furthermore, high concentrations of MgCl2 produced a strong inhibition of NADP+ photoreduction with DCIPH2 without significantly affecting the rate of NADP+ photoreduction with water. The implications of these findings for the differential involvement of Photosystem I and Photosystem II in the photoreduction of NADP+ with different electron donors are discussed.

Nostoc muscorum (Strain 7119) cells were disrupted and the accessory pigment phycocyanin was removed from membrane fragments by digitonin treatment. The phycocyanin-depleted membrane fragments retained both Photosystem I and Photosystem II activity, as evidenced by high rates of NADP+ photoreduction either by water or by reduced 2,6-dichlorophenolindophenol, indicating that phycocyanin is not an essential component for electron transport activity.

Photosynthetic enhancement of oxygen evolution (linked to CO 2 assimilation) in isolated chloroplasts was found to be governed by the supply of ATP. The addition of ATP (but not AMP) abolished enhancement that consistently occurred without added ATP. Enhancement in the H 2 O → NADP reaction by chloroplasts was investigated in the light of one recent report that the phenomenon occurs when pure ferredoxin is replaced by a crude preparation (PPNR) and another report that the phenomenon is governed by Mg ++ concentration. Fractionation of PPNR led to the isolation of a protein factor which when added to pure ferredoxin induced enhancement. However, the rate of NADP reduction with pure ferredoxin and without enhancement was greater than the maximum rate of NADP reduction with enhancement induced by either the protein factor of PPNR. The report that Mg ++ concentration governs enhancement was not confirmed.

This chapter discusses role of cytochromes and other metalloproteins in the photosynthetic electron transport. Current understanding of the mechanism of photosynthesis leans heavily on a concept of a photosynthetic, i.e., light-induced, electron transport. Photosynthetic phosphorylation (photophosphorylation) in chloroplasts is subdivided into two types, cyclic and noncyclic. The cyclic and noncyclic photophosphorylation jointly account for the basic feature of photosynthesis, i.e., conversion of radiant energy into chemical energy. The wavelength dependence of the phosphorylation associated with electron flow from an artificial electron donor (DPIPH2) to NADP resembles the cyclic system. It is well established that treating chloroplasts with ferricyanide in the dark chemically oxidizes several chloroplast constituents including cytochromes. The spectrum of the photoinduced decrease in absorbance had a maximum at 550 nm that was suggestive of an α-peak of a new cytochrome of a c-type.

Abstract—Recent work in our laboratory yielded new evidence that noncyclic electron transport in chloroplasts from water to ferredoxin (Fd) and N ADP is carried out solely by System II which, unexpectedly, was found to include not one but two photoreactions (IIa and IIb). The evidence suggests that these operate in series, being joined together by a ‘dark’ chain of electron carriers that includes (but is not limited to) cytochromeb559and plastocyanin (PC):H2O → IIbhv→ C550 → Cytb559rarr;PC→IIahv→ Fd → NADP.Photoreaction IIb involves an electron transfer from water to C550, a new chloroplast component distinct from cytochromes, whose photoreduction is observed as a decrease in absorb‐ance with a maximum at 550 nm. The photoreduction of CSSO proceeds effectively only in short‐wavelength System II light, is insensitive to low temperature (at least down to — 189°C). does not require plastocyanin, and is the first known System II photoreaction which is resistant to inhibition by DCMU oro‐phenanthroline. Photoreaction IIa involves an electron transfer from cytochromeb559to ferredoxin‐NADP and also proceeds effectively only in System II light. The photooxidation of cytochromeb559requires plastocyanin. Cytochromeb559is reduced by C550 in a reaction that is readily inhibited by DCMU oro‐phenanthroline. Thus, the site of DCMU (ando‐phenanthroline) inhibition of System II appears to lie between C550 and cytochromeb559. System I, comprising a single long‐wavelength light reaction and a cyclic electron transport chain that includes cytochromesb6andf, is viewed as operating in parallel to System II. The photoreduction of NADP by artificial electron donors via System I involves a portion of the cyclic electron transport chain and appears to be independent of plastocyanin. Chloroplast fragments have been prepared which either (a) exhibit System II activity (water → NADP) and lack functional cytochromefand P700 or (b) exhibit System I activity and lack plastocyanin. The present concept is consistent with the following: (i) No enhancement effect was found for NADP reduction by water where only System II is thought to be involved, but a large enhancement effect was observed in chloroplasts engaged in complete photosynthesis where both cyclic (System I) and noncyclic photophosphorylation (System II) are needed for CO2assimilation. (ii) The transfer of one electron from water to ferredoxin via System II requires optimally two quanta, but the transfer of one electron from reduced dye to ferredoxin via System I requires optimally only one quantum of light.

Previous reports from this laboratory described a new concept of three light reactions in plant photosynthesis comprising two short-wavelength (lambda < 700 nm) photoreactions belonging to Photosystem II and one long-wavelength (lambda > 700 nm) photoreaction belonging to Photosystem I. Among the electron carriers assigned to Photosystem II were cytochrome b(559) and plastocyanin and to Photosystem I, cytochrome f.According to a widely held view, the light-induced reduction of NADP by water requires the collaboration of Photosystems I and II and involves specifically cytochrome f and P700 (a portion of chlorophyll a peculiar to Photosystem I). By contrast, the new concept ascribes the light-induced reduction of NADP by water solely to the two photoreactions of Photosystem II, without the participation of Photosystem I and its components, cytochrome f and P700.Further evidence in support of the new concept has now been obtained from chloroplast fragments. Two kinds of chloroplast fragments have been prepared: (a) one with Photosystem II activity, capable-in the presence of plastocyanin-of photoreducing NADP with water but lacking P700 and functional cytochrome f and (b) another having only Photosystem I activity, lacking plastocyanin, and enriched in P700.

The conversion of radiant energy into chemical energy during photosynthesis is completed with the formation of ATP and reduced ferredoxin, an iron-sulfur protein with a reducing power equal to that of hydrogen gas.'-3 is agreement that this energy conversion process involves two photoacts, but there is disagreement as to their identity and their relation to the two photochemical reactions, cyclic and noncyclic photophosphorylation, on which the biosynthetic capacity of chloroplasts depends.

The iron-bearing protein ferredoxin is present in all photosynthetlc cells. It has now been shown that ferredoxin can catalyse, by two distinct photochemical reactions, the production of ATP in cell-free photosynthetic systems at rates comparable with the maximum rates of photosynthesis in vivo. This is strong evidence for the view that ferredoxin plays a key part in photosynthesis.

Sensitivity to low concentrations of desaspidin (5 x 10(-7)m) sharply distinguishes the photophosphorylations associated with the photooxidation of water from all other types of photophosphorylation by isolated chloroplasts. Contrary to recent reports in the literature, the effects of desapidin were not altered by changes in the redox conditions as influenced by the concentration of ascorbate and by the presence or absence of oxygen. Desaspidin consistently inhibited all types of cyclic photophosphorylation and the photophosphorylation coupled with the reduction of NADP by ascorbate-dichlorophenol indophenol. The same concentration of desaspidin gave little or no inhibition of photophosphorylation that are coupled with the photooxidation of water.

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