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Significant strides toward producing biochemical fuels have been achieved by mimicking natural oxidative and photosynthetic phosphorylation. Here, different from these strategies, we explore boric acid as a fuel for tuneable synthesis of energy-storing molecules in a cell-like supramolecular architecture. Specifically, a proton locked in boric acid is released in a modulated fashion by the choice of polyols. As a consequence, controlled proton gradients across the lipid membrane are established to drive ATP synthase embedded in the biomimetic architecture, which facilitates tuneable ATP production. This strategy paves a unique route to achieve highly efficient bioenergy conversion, holding broad applications in synthesis and devices that require biochemical fuels.

The extrinsic and intrinsic membrane sectors of F1F0-ATPases are linked by a slender stalk 40-50 A in length. The stalk transmits the energy produced by oxidative or photosynthetic phosphorylation from the intrinsic sector, F0, to the catalytic sites in the extrinsic F1 sector. How this is achieved is unknown, but long-range conformational changes linked to transmembrane proton transport may be involved. In bacterial and chloroplast F1F0-ATPases, the stalk is probably a composite of subunits delta and epsilon, part of the gamma-subunit, and the extrinsic membrane domains of 2 subunits (identical or non-identical according to the species) that are bound to the membrane by their N-terminal regions. The stalk in the bovine mitochondrial enzyme appears to be more complex, and the gamma, delta, epsilon, OSCP, F6, b and d subunits all contribute to it. A bovine stalk complex has been assembled in vitro from bacterially expressed OSCP, F6, b and d, both in the presence and in the absence of F1-ATPase. One molecule of each of these subunits is present in the assembled complex, as there is also in each native F1F0-ATPase assembly. Providing that suitable crystals can be obtained, the stalk complex and the F1.stalk complex may permit the high resolution structure of bovine F1-ATPase to be extended into the stalk domain.

Uncouplers of photosynthetic phosphorylation, CCCP, TTFB and PCP, inhibited light-induced O2 evolution in the Hill reaction with SiMo (I50 approximately 20, 3 and 45 microM, respectively), but only insignificantly diminished SiMo photoreduction by pea chloroplasts. The same properties were exhibited by the ADRY agent ANT2p. CCCP, TTFB and PCP are oxidizable compounds with redox potentials of +1.17, +1.18 and +1.09 V (pH 6.0), as determined by cyclic voltammetry. Similarly to NH2OH, the tested uncouplers can apparently serve as electron donors for photosystem II.

Robinson, J. M. 1988. Does O2 photoreduction occur within chloroplasts in vivo? -Physiol. Plant. 72: 666–680. This discussion reviews evidence supporting the hypothesis that within intact chloroplasts in vivo, molecular O2 may serve as an alternative Hill oxidant (electron acceptor) on the reducing side of Photosystem I. Depending upon the availability of Hill oxidants such as NADP+ and NO−2, there is the potential within intact plastids in vivo, for photolytically derived reducing equivalents to reduce O2 to O−2 and H2O2 (the Mehler reaction). In chloroplasts of healthy tissues, the products of photosyn-thetic O2 reduction O−2 and H2O2) are rapidly removed by superoxide dismutase (EC 1.15.1.1) and L-ascorbate peroxidase (EC 1.11.1.11) to prevent toxicity. The presence of these two enzymes within chloroplasts in vivo reflects the potential for linear (non-cyclic) photosynthetic electron transport systems to draw upon molecular O2 as a terminal oxidant. In the intact plastid, O2 may act as an electron acceptor in the place of any other physiological Hill oxidant, e.g., NADP+, NO−2, and, presumably, oxidized thioredoxin. Under aerobic, physiological conditions, photo reduced ferre-doxin (Fdred), and/or reduced flavoprotein enzymes, e.g., ferredoxin:NADP+ oxidoreductase (EC 1.18.1.2), can donate electrons to O2; this reductive reaction appears to be non-enzymatic, but it is rapid. Stated from another viewpoint, O2 may serve as a Hill oxidant to support some linear electron flow when reductant supplies are in excess of reductant demands. For example, there are nitrogen assimilatory sites in the chloroplast, i.e., ferredoxin-nitrite reductase (NiR; EC 1.7.7.1) and glutamate synthase (ferredoxin) (GOGAT; EC 1.4.7.1), to which Fdred is allocated as reductant. Because NADH:nitrate reductase (NR; EC 1.6.6.1) is the rate limiting step of nitrogen assimilation, and, because NiR and GOGAT activities are in excess of NR activities by a factor of 2 or more, then an excess of unreacted Fdred could accumulate. Alternatively, the allocated Fdred would reduce the excess NiR and GOGAT sites, but the excess of reduced enzymes would not have substrates (e.g., NO−2, glutamine, and α-ketoglutarate) with which to react. Therefore, if ‘excess’ NiR and GOGAT binding sites were not employed, the available excess Fdred, and/or the reduced NiR and GOGAT proteins, would be susceptible to oxidation by O2. The resulting O2 photoreduction could account for nearly all of the observed in vivo Mehler type reactions. In vivo, apparent foliar O2 photoreduction occurs simultaneously with maximal CO2 photoassimilation, and, in high light, average rates have been determined by direct measurement to range from 10 to 40 μmol O2 consumed (mg Chl)−1 h−1. Therefore O2 reduction would support a low rate of linear (non-cyclic) electron flow which, in turn, could maintain a low, but significant rate of ATP production. However, there is not total agreement among researchers that the physiological role of O2 is that of serving as an alternative Hill oxidant in order to recycle unutilized Fdred or other photoreduced proteins. Also, there continues to be considerable controversy on whether or not O2 reduction supports significant photosynthetic phosphorylation. The total process of O2 photoreduction, and its physiological role(s), requires much more study before absolute functions can be assigned to O2 terminated, linear electron transport. Summary Molecular O2 possesses the physico-chemical properties that permit this molecule to serve as an alternative Hill oxidant within chloroplasts in vivo. Additionally, the physical and physiological properties within the chloroplast in vivo favor the potential for O2 to serve as an electron acceptor on the reducing side of Photosystem I. This may reflect an important ‘fail-safe mechanism’ which prevents over-reduction of linear photosynthetic electron transport chain proteins. This review has focused on the possibility that unutilized Fdred and/or other non-utilized, reduced plastid enzymes (e.g., NiR) may be electron donors to O2. It is hypothesized that this oxidation ultimately would be reflected as an in vivo Mehler reaction. However, it remains for future studies to establish without doubt, that in vivo, photoreduced chloroplast enzyme proteins can utilize O2 as a terminal electron acceptor. Further, that O2 photoreduction supports a significant level of photophosphorylation in vivo remains to be firmly established. Certainly, considerable evidence, gained with experiments utilizing isolates of intact chlo-roplasts and reconstituted chloroplast systems, supports the hypothesis that O2-terminated linear electron transport has the potential to support high rates of ATP production. However, in vivo studies e.g., with intact leaf tissues, which actually quantitate the relationship between O2 photoreduction and associated ATP production have not been conducted. These will be difficult experiments to perform, because, in vivo, it will be difficult to separate photosynthetic ATP production mediated by O2 from ATP production mediated by those other, more predominant Hill oxidants (e.g., NADP+, NO−2). Also, it continues to be a possibility that it is cyclic, and not pseudocyclic photophosphorylation that provides additional ATP to support photosynthetic cell metabolism. To establish beyond doubt that an in vivo role of the Mehler reaction is that of supplying ‘additional ATP’, remains for considerable future study.

Summary Photophosphorylation has been achieved in isolated chloroplasts of Phaseolus, without inhibition, by the addition of bovine serum albumin. Cyclic and non-cyclic photophosphorylation and four partial reactions of photosynthetic phosphorylation were studied in the system with chloroplasts of different ages. There seems to be evidence for a correlation between a minimum structural complexity of the chloroplast and the ability to carry out the reactions studied.

— Three different types of photomotion reactions, photokinesis, photo-topotaxis and photo-phobotaxis, have been investigated in the diatom Nitzschia communis. In photokinesis the active light appears to be absorbed by the photosynthetic pigments. Apparently, the acceleration of movement by light is due to an additional ATP supply from photosynthetic phosphorylation, as it is the case in the blue-green algae of the genus Phormidium. On the other hand, no correlations exist between photo-phobotaxis and photosynthesis, because red light does not induce photo-phobotactic responses in N. communis. The phobotactic action spectrum resembles that one of photo-topotaxis. In the visible region only violet, blue and blue-green light is active in both the reactions. However, it is not yet clear whether or not the stimuli are mediated by the same photoreceptor, mainly because of the different sensitivity of both the reactions to u.v.

The Mitchell hypothesis of chemiosmotic coupling in oxidative phosphorylation is examined in the light of experimental data on oxidative phosphorylation at present available. The following objections are brought against the theory: 1. Data on the magnitude of the phosphate potential against which ATP can be synthesized by the respiratory chain require, on the basis of chemiosmotic coupling, very effective proton and/or cation extrusion and/or anion uptake by the mitochondria. Experimental evidence for this is lacking. Indeed it has been shown by Chance and Mela that mitochondria in the controlled state do not extrude protons. 2. Although reversible ATP-driven proton (chloroplasts) or cation (mitochondria, erythrocytes) pumps have been demonstrated, this is insufficient evidence for the postulate that the establishment of a proton gradient is the primary energy-conserving event of the chloroplast or mitochondrial respiratory chain. Indeed, it is general experience that cations are required for the extrusion of protons by mitochondria. 3. The stoicheiometry of proton extrusion by mitochondria and of proton uptake by chloroplasts, and the kinetics of the latter process are also difficult to reconcile with the chemiosmotic hypothesis. 4. Further experimental evidence is presented confirming that, under the conditions of the oxygen-pulse experiments of Mitchell and Moyle, the extrusion of H+ is not associated with the oxidation of mitochondrial NADH. 5. The respiratory chain included in the chemiosmotic hypothesis is difficult to reconcile with our present knowledge of the chain.

The effect of light on the activity of herbicides within the cell is reviewed. The herbicides are grouped according to the way their phytotoxicity is influenced by light. The largest group, which includes the ureas, triazines and uracils, interfere with photosynthesis by blocking the Hill reaction. Lack of carbohydrates and other manifestations of toxicity appear in the light and these are discussed. The bipyridylium quaternary salts, diquat and paraquat, are not phytotoxic until a free radical is formed utilising light energy via photosynthesis. The free radical is re-oxidised by molecular oxygen, and a series of reactions are started leading to the formation of peroxide radicals which subsequently disrupts the cell. The phytotoxicity of 2,4-dinitrpphenol and related compounds is reduced by light. This type of compound combined with a herbicide which inhibits the Hill reaction and thus photosynthetic phosphorylation may act synergistically and mixtures of this type are discussed.

— Measurements were made of the 3.7 msec delayed light emission of chloroplasts treated with a variety of agents which affect the rate of electron transport (Hill reaction) or photosynthetic phosphorylation. The presence of the electron acceptors ferricyanide or pyocyanine increased delayed light emission. Inhibitors of electron transport (3-(3,4-dichlorophenyl)-1, -1-dimethylurea or 1,10(ortho)-penanthroline) inhibited delayed light emission. The addition of a phosphate acceptor system inhibited delayed light emission. This inhibition was reversed by inhibitors of the phosphorylation reaction, e.g. Dio-9 or phlorizin. From these results it was concluded that the 3.7 msec delayed light emission probably occurs as a result of back reactions of intermediates in the coupled electron transport and photosynthetic phosphorylation systems.