REDUCTION OF CARBON DIOXIDE COUPLED WITH THE OXYHYDROGEN REACTION IN ALGAE

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Algae after dark: mechanisms to cope with anoxic/hypoxic conditions

Chlamydomonas reinhardtii is a unicellular, soil-dwelling (and aquatic) green alga that has significant metabolic flexibility for balancing redox equivalents and generating ATP when it experiences hypoxic/anoxic conditions. The diversity of pathways available to ferment sugars is often revealed in mutants in which the activities of specific branches of fermentative metabolism have been eliminated; compensatory pathways that have little activity in parental strains under standard laboratory fermentative conditions are often activated. The ways in which these pathways are regulated and integrated have not been extensively explored. In this review, we primarily discuss the intricacies of dark anoxic metabolism in Chlamydomonas, but also discuss aspects of dark oxic metabolism, the utilization of acetate, and the relatively uncharacterized but critical interactions that link chloroplastic and mitochondrial metabolic networks.

Lichen symbiosis: nature's high yielding machines for induced hydrogen production

Hydrogen is a promising future energy source. Although the ability of green algae to produce hydrogen has long been recognized (since 1939) and several biotechnological applications have been attempted, the greatest obstacle, being the O2-sensitivity of the hydrogenase enzyme, has not yet been overcome. In the present contribution, 75 years after the first report on algal hydrogen production, taking advantage of a natural mechanism of oxygen balance, we demonstrate high hydrogen yields by lichens. Lichens have been selected as the ideal organisms in nature for hydrogen production, since they consist of a mycobiont and a photobiont in symbiosis. It has been hypothesized that the mycobiont's and photobiont's consumption of oxygen (increase of COX and AOX proteins of mitochondrial respiratory pathways and PTOX protein of chrolorespiration) establishes the required anoxic conditions for the activation of the phycobiont's hydrogenase in a closed system. Our results clearly supported the above hypothesis, showing that lichens have the ability to activate appropriate bioenergetic pathways depending on the specific incubation conditions. Under light conditions, they successfully use the PSII-dependent and the PSII-independent pathways (decrease of D1 protein and parallel increase of PSaA protein) to transfer electrons to hydrogenase, while under dark conditions, lichens use the PFOR enzyme and the dark fermentative pathway to supply electrons to hydrogenase. These advantages of lichen symbiosis in combination with their ability to survive in extreme environments (while in a dry state) constitute them as unique and valuable hydrogen producing natural factories and pave the way for future biotechnological applications.

A novel screening protocol for the isolation of hydrogen producing Chlamydomonas reinhardtii strains

BACKGROUND

Sealed Chlamydomonas reinhardtii cultures evolve significant amounts of hydrogen gas under conditions of sulfur depletion. However, the eukaryotic green alga goes through drastic metabolic changes during this nutritional stress resulting in cell growth inhibition and eventually cell death. This study aimed at isolating C. reinhardtii transformants which produce hydrogen under normal growth conditions to allow a continuous hydrogen metabolism without the stressful impact of nutrient deprivation.

RESULTS

To achieve a steady photobiological hydrogen production, a screening protocol was designed to identify C. reinhardtii DNA insertional mutagenesis transformants with an attenuated photosynthesis to respiration capacity ratio (P/R ratio). The screening protocol entails a new and fast method for mutant strain selection altered in their oxygen production/consumption balance. Out of 9000 transformants, four strains with P/R ratios varying from virtually zero to three were isolated. Strain apr1 was found to have a slightly higher respiration rate and a significantly lower photosynthesis rate than the wild type. Sealed cultures of apr1 became anaerobic in normal growth medium (TAP) under moderate light conditions and induced [FeFe]-hydrogenase activity, yet without significant hydrogen gas evolution. However, Calvin-Benson cycle inactivation of anaerobically adapted apr1 cells in the light led to a 2-3-fold higher in vivo hydrogen production than previously reported for the sulfur-deprived C. reinhardtii wild type.

CONCLUSION

Attenuated P/R capacity ratio in microalgal mutants constitutes a platform for achieving steady state photobiological hydrogen production. Using this platform, algal hydrogen metabolism can be analyzed without applying nutritional stress. Furthermore, these strains promise to be useful for biotechnological hydrogen generation, since high in vivo hydrogen production rates are achievable under normal growth conditions, when the photosynthesis to respiration capacity ratio is lowered in parallel to down regulated assimilative pathways.

Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae)

Unicellular green algae have the ability to operate in two distinctly different environments (aerobic and anaerobic), and to photosynthetically generate molecular hydrogen (H2). A recently developed metabolic protocol in the green alga Chlamydomonas reinhardtii permitted separation of photosynthetic O2-evolution and carbon accumulation from anaerobic consumption of cellular metabolites and concomitant photosynthetic H2-evolution. The H2 evolution process was induced upon sulfate nutrient deprivation of the cells, which reversibly inhibits photosystem-II and O2-evolution in their chloroplast. In the absence of O2, and in order to generate ATP, green algae resorted to anaerobic photosynthetic metabolism, evolved H2 in the light and consumed endogenous substrate. This study summarizes recent advances on green algal hydrogen metabolism and discusses avenues of research for the further development of this method. Included is the mechanism of a substantial tenfold starch accumulation in the cells, observed promptly upon S-deprivation, and the regulated starch and protein catabolism during the subsequent H2-evolution. Also discussed is the function of a chloroplast envelope-localized sulfate permease, and the photosynthesis-respiration relationship in green algae as potential tools by which to stabilize and enhance H2 metabolism. In addition to potential practical applications of H2, approaches discussed in this work are beginning to address the biochemistry of anaerobic H2 photoproduction, its genes, proteins, regulation, and communication with other metabolic pathways in microalgae. Photosynthetic H2 production by green algae may hold the promise of generating a renewable fuel from nature's most plentiful resources, sunlight and water. The process potentially concerns global warming and the question of energy supply and demand.

Trails of green alga hydrogen research - from hans gaffron to new frontiers

This paper summarizes aspects of the history of photosynthetic hydrogen research, from the pioneering discovery of Hans Gaffron over 60 years ago to the potential exploitation of green algae in commercial H(2)-production. The trail started as a mere scientific curiosity, but promises to be a most important discovery, one that leads photosynthesis research to important commercial applications. Progress achieved in the field of photosynthetic hydrogen production by green algae includes elucidation of the mechanism, the ability to modify photosynthesis by physiological means and to produce bulk amounts of H(2) gas, and cloning of the [Fe]-hydrogenase genes in several green algal species.

Aerobic hydrogenase activity in Anacystis nidulans. The oxyhydrogen reaction

1. The oxyhydrogen reaction of Anacystis nidulans was studied manometrically and polarographically in whole cells and in cell-free preparations; the activity was found to be associated with the particulate fraction. 2. Besides O2, the isolated membranes reduced artificial electron acceptors of positive redox potential; the reactions were unaffected by O2 levels less than 10--15%; aerobically the artificial acceptors were reduced simultaneously with O2. 3. H2-supported O2 uptake was inhibited by CO, KCN and 2-n-heptyl-8-hydroxyquinoline-N-oxide. Inhibition by CO was partly reversed by strong light. Uncouplers stimulated the oxyhydrogen reaction. 4. The kinetic properties of O2 uptake by isolated membranes were the same in presence of H2 and of other respiratory substrates. 5. Low rates of H2 evolution by the membrane preparations were found in presence of dithionite; methyl viologen stimulated the reaction. 6. The results indicate that under certain growth conditions Anacystis synthesizes a membrane-bound hydrogenase which appears to be involved in phosphorylative electron flow from H2 to O2 through the respiratory chain.

The differential action of metronidazole on nitrogen fixation, hydrogen metabolism, photosynthesis and respiration in Anabaena and Scenedesmus

Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) at 1--2 mM levels has been shown to be a selective inhibitor of nitrogenase activity in Anabanena. Two constitutive hydrogenases and photosynthesis are insensitive to metronidazole at these same concentrations. At higher concentrations metronidazole inhibits photosynthesis in Anabaena while photoreduction and to a lesser extent photohydrogen production are retarded in Scenedesmus. Respiration is slightly stimulated at high metronidazole levels in both algae. The reductant source for nitrogenase in Anabaena and photohydrogen production and photoreduction electron transport in Scenedesmus are discussed. Due to the activity to metronidazole as a selective inhibitor of ferredoxin-associated processes, it should prove to be useful in N2 fixation studies and in distinguishing between ferredoxin-linked reactions of different sensitivities and other activities not associated with low reduction potential components.

Observations on the photohydrogen producing activity during the synchronous cell cycle of Scenedesmus obliquus

In anaerobically adapted samples of synchronized cultures of the unicellular green alga Scenedesmus obliquus it was observed that both the rate and the maximum volume of hydrogen produced in the light changed in a parallel fashion over the life cycle. These two parameters of cells of the 16th h were 3 times greater than the comparable values for cells of the 8th h. Although both photosystems are involved in photohydrogen production the patterns seen over a complete life cycle (24 h) for hydrogen metabolism was inverse to that noted for changes in the photosynthetic capacity. The provision of either glucose, ethanol or acetate to 8th and 16th h cultures enhanced photohydrogen production of the 8th to the same level as the 16th h. From these findings, and also from the observation that the starch content is low at the 8th but 4 fold at the 16th h, it is apparent that in autotrophic cultures an endogenous organic compound, and not water, serves as the electron donor for photohydrogen production. Since free glucose was not detected the natural substrate is most likely starch. From experiments with monochromatic light and observations on the inhibitory action of DCMU and DBMIB on photohydrogen production we conclude that the major portion of the machinery for photohydrogen production in Scenedesmus requires both PS I and PS II participation and the input of electrons from the natural substrate proceeds through PS II.The alternate possibility that glucose, acetate and ethanol also act as inhibitors of reactions, most probably photophosphorylation, which compete with photohydrogen production was suggested by some experiments. The subsequent modulation of hydrogenase activity was discussed as a possible reason for the enhancement of photohydrogen production.

Light-dependent hydrogen evolution by Scenedesmus

The effect of glucose and the uncoupler Cl-CCP upon hydrogen production was studied in adapted cells of Scenedesmus obliquus D3. Cl-CCP at 10(-5)M concentration completely inhibited the evolution of H2 in the dark and increased the apparent rate of H2 evolution in the light. At 10(-5)M Cl-CCP, photosynthesis and photoreduction by anaerobically adapted algae were only temporarily inhibited; O2 evolution reappeared after approximately 1 hr of illumination if CO2 was present. Increasing the Cl-CCP concentration to 5 x 10(-5)M led to a maximum rate of photohydrogen production and fully inhibited H2 evolution, photoreduction and dark H2 evolution. H2 evolution was accompanied by a release of varying amounts of CO2 in the light, as well as in the dark. Dark CO2 production was stimulated by Cl-CCP. H2 evolution in the light was stimulated by adding glucose to autotrophically grown cells or by growing the cells heterotrophically with glucose; starvation had an opposite effect. Adapted cells released (14)CO2 from the 3 and/or 4 position of specifically labeled glucose, indicating that degradation occurred via the Embden-Meyerhof pathway. The amount of H2 released by autotrophically grown cells was the same either with continuous illumination or with short periods of light, followed by darkness. Scenedesmus mutant No. 11, which is unable to evolve O2 was not inhibited in its capacity to evolve H2 in the light. These data indicate that the evolution of H2 in the light by adapted Scenedesmus depends upon the degradation of organic material and does not require the production of free O2 by photosystem II.

Effect of hydrogen adaptation on fluorescence in normal and manganese-deficient algae

1. Steady-state fluorescence of normal cells of Ankistrodesmus braunii is about 40% higher under hydrogen-adapted as compared to aerobic conditions. 2. By contrast, fluorescence of manganese-deficient A. braunii, which aerobically is twice as strong as that of normal cells, decreases in the course of adaptation to a considerably lower level which is about as high as that of normal algae adapted to hydrogen. 3. The effects of DCMU on fluorescence of aerobic and anaerobic cells of A. braunii are similar to those brought about by manganese deficiency. 4. In Chlorella fusca (=pyrenoidosa), which develops a very high level of hydrogenase activity, fluorescence of normal cells is about equal under H2-adapted and under aerobic conditions. According to the current concept of the relation between fluorescence intensity and photosynthetic electron transport, these results seem to indicate that, in addition to system I, system II of photosynthesis is at least partly active in H2-adapted and photoreducing algae.