Light-driven Electron Transfer Research Articles

Photooxidase system of Rhodospirillum rubrum. I. Photooxidations catalyzed by chromatophores isolated from a mutant deficient in photooxidase activity

The aerobic photooxidations of reduced 2,6-dichlorophenolindophenol and of reaction-center bacteriochlorophyll ( P-870) have been investigated in membrane vesicles (chromatophores) isolated from a non-phototrophic Rhodospirillum rubrum strain. In aerobic suspensions of wild-type chromatophores, continuous light elicits an increase of the levels of 2,6-dichlorophenolindophenol and of oxidized P-870, which reach steady-state values shortly after the onset of illumination. In contrast, light induces in mutant suspensions a transient increase of the levels of 2,6-dichlorophenolindophenol and of oxidized P-870, which fall to low steady-state values within a few seconds. These observations suggest that the mutation has altered a redox constituent located on the low-potential side of the photochemical reaction center, between a pool of acceptors and oxygen. Since endogenous cyclic photophosphorylation is catalyzed by mutant chromatophores at normal rates, it appears that the constituent altered by the mutation does not belong to the cyclic electron-transfer chain responsible for photophosphorylation. However, the system which mediates the aerobic photooxidations and the cyclic system are not completely independent: endogenous photophosphorylation is inhibited by oxygen in wild-type chromatophores but not in mutant chromatophores; in addition, the inhibitor of cyclic electron flow, 2-heptyl-4-hydroxyquinoline- N-oxide, enhances the aerobic photooxidation of reduced 2,6-dichlorophenolindophenol by chromatophores from both strains. These results support a tentative branched model for light-driven electron transfer. In that model, the constituent altered in the mutant strain is located in a side electron-transfer chain which connects the cyclic acceptors to oxygen.

Some newly observed correlations between structure and photochemical activity in chlorophyllin a and several derivatives

The abilities of chlorophyllin a and four related pigments to mediate light-driven electron transfer from two different electron donors to three electron acceptors were compared. Sulfhydryl reagents are poor electron donors with all of the pigments, although the reaction rate can be increased dramatically by adding p-benzoquinone or p-hydroquinone as an intermediate electron carrier. Replacement of the central magnesium atom of chlorophyllin a with an atom of zinc gives a higher rate of NADP photoreduction via the enzyme ferredoxin-NADP reductase, but results in lower rates of methyl viologen and methyl red photoreduction. Bacteriochlorophyllin is active in methyl viologen and methyl red photoreduction, but not in NADP photoreduction. Additional structure-activity correlations were observed and are described.

85] Biogenesis and modulation of membrane properties in greening Chlamydomonas reinhardi, y-1 cells

This chapter describes the experimental conditions for the modulation of membrane composition and function during the greening process and the analyses utilized for their characterization. The light-driven electron transfer coupled to the generation of an energy-rich state or intermediate utilizable for the synthesis of ATP or active ion transport is associated with a membrane system consisting of closed, flattened sacks (thylakoids). These are located within specific organelles (plastids) as superimposed disks fused to form grana in higher plants and several algae. In certain algae, such as Chlorella, Chlamydomonas mutants, or Euglena, under appropriate physiological conditions such as growth in the dark or in media containing high concentration of a utilizable carbon source but poor in nitrogen supply, the photosynthetic membrane system might be diluted through cell division while the plastid as an organelle and the potentiality to reform the membranes are preserved. The process of reformation of the photosynthetic membranes (greening) offers an ideally suitable experimental system for the investigation of the mechanism of biological membrane formation as related to its two major aspects, the structural and functional ones.

Fast changes of enthalpy and volume on flash excitation of Chromatium chromatophores

We have used a capacitor microphone transducer to measure volume changes in suspensions of Chromatium chromatophores 100 μs to 20 ms after flash excitation. Volume changes in photochemical reactions can arise both from volume differences between reactants and products, and from enthalpy changes which heat or cool the solution. By measuring the volume changes at two temperatures, one can resolve the total changes into their two components. In Chromatium, light-driven electron transfer from cytochrome C555 to the primary or secondary electron acceptors causes a contraction of approx. 33 Å 3 per electron transferred. The relaxation of the initial volume change is altered by the presence of gramicidin, valinomycin, o-phenanthroline, or phosphate. From the effects of these agents, we conclude that the volume change is localized at the photochemical reaction center, and that part of the relaxation involves transfer of a proton from the solvent to the chromatophores. Electron transfer from cytochrome C555 to the primary or secondary electron acceptors does not cause a significant enthalpy change. We conclude that the free energy increase that accompanies this reaction is due to a negative entropy change.

Synthetic model complexes for studying light-driven electron transfer in photosynthesis

Mixed multimolecular layers of ethyl chlorophyllide a and lumiflavin were prepared and found to exhibit a difference absorption spectrum against multimolecular layers of pure ethyl chlorophyllide a, with a maximum at 698 nm and a minimum at 670 nm in the 520-800 nm range. Similarly, molecular complexes of chlorophyllic a and lumiflavin dissolved in aqueous phosphate buffer containing poly- N-vinylpyrrolidone exhibited a difference absorption spectrum against the uncomplexed components, with a maximum near 695 nm and a minimum near 660 nm in the same wavelength range. The maxima in both difference spectra can be bleached reversibly by red light. The bleached samples recovered in the dark at room temperature with half-lives of 46 and 75 msec, respectively. The photobleaching was accompanied by the generation of electron spin resonance signals which are interpretable in terms of the oxidized chlorophyll radical and the lumiflavin radical. The photochemical properties of these model complexes are compared with those of P700, and their possible relevance to the primary energy conversion mechanism in photosynthesis is discussed.

A flavin-chlorophyll a complex resembling the P700 of photosynthesis

A model molecular complex of chlorophyll a and FMN is found to have properties resembling the P700 of photosynthesis. The model complex shows an absorption maximum near 700 nm which can be bleached reversibly by light. ESR studies show that the photobleaching is accompanied by light-driven electron transfer from the chlorophyll a to the flavin. These observations are consistent with the previous suggestion that P700 may be a molecular complex of chlorophyll a and the prosthetic group (FAD) of chloroplast NADP +-reductase.

The nature of the coupling between light energy and active ion transport in Nitella translucens

Abstract 1. 1. The aim of the work was to decide whether the link between light and the light-driven active ion fluxes in Nitella translucens reflects the consumption of ATP produced by photophosphorylation in the process of ion transport, or is the consequence of a direct coupling between ion transport and photosynthetic electron-transfer reactions. 2. 2. The active uptake of Cl − appears to require the participation of the second light reaction of photosynthesis—that involving the photooxidation of water, for which Cl − is an essential cofactor. Thus the Cl − uptake is not supported by far-red light in which only cyclic photophosphorylation is possible, and is sensitive to low concentrations of dichlorophenyldimethylurea. 3. 3. By contrast the active uptake of K + can be supported by cyclic photophosphorylation alone, in far-red light or in the presence of low concentrations of dichlorophenyldimethylurea. 4. 4. Low concentrations of imidazole, which uncouples photophosphorylation in isolated chloroplasts, inhibit K + uptake while leaving the Cl − flux unaffected. 5. 5. It is suggested that K + uptake is supported by light energy through the utilization of ATP produced in photophosphorylation, but that the Cl − uptake is directly linked to the light-driven electron-transfer reactions associated with the evolution of O 2 in photosynthesis, and does not depend on photophosphorylation. 6. 6. This work provides further evidence for the occurrence of cyclic photophosphorylation in intact cells.