Inactivation of Hydrogenase in Cell-free Extracts and Whole Cells of Chlamydomonas reinhardi by Oxygen

Light-Driven H2 Production in Chlamydomonas reinhardtii: Lessons from Engineering of Photosynthesis

In the green alga Chlamydomonas reinhardtii, hydrogen production is catalyzed via the [FeFe]-hydrogenases HydA1 and HydA2. The electrons required for the catalysis are transferred from ferredoxin (FDX) towards the hydrogenases. In the light, ferredoxin receives its electrons from photosystem I (PSI) so that H2 production becomes a fully light-driven process. HydA1 and HydA2 are highly O2 sensitive; consequently, the formation of H2 occurs mainly under anoxic conditions. Yet, photo-H2 production is tightly coupled to the efficiency of photosynthetic electron transport and linked to the photosynthetic control via the Cyt b6f complex, the control of electron transfer at the level of photosystem II (PSII) and the structural remodeling of photosystem I (PSI). These processes also determine the efficiency of linear (LEF) and cyclic electron flow (CEF). The latter is competitive with H2 photoproduction. Additionally, the CBB cycle competes with H2 photoproduction. Consequently, an in-depth understanding of light-driven H2 production via photosynthetic electron transfer and its competition with CO2 fixation is essential for improving photo-H2 production. At the same time, the smart design of photo-H2 production schemes and photo-H2 bioreactors are challenges for efficient up-scaling of light-driven photo-H2 production.

Protection of Oxygen-Sensitive Enzymes by Peptide Hydrogel

Molecular oxygen (O₂) is a highly reactive oxidizing agent and is harmful to many biological and industrial systems. Although O₂ often interacts via metals or reducing agents, a binding mechanism involving an organic supramolecular structure has not been described to date. In this work, the prominent dipeptide hydrogelator fluorenylmethyloxycarbonyl-diphenylalanine is shown to encage O₂ and significantly limit its diffusion and penetration through the hydrogel. Molecular dynamics simulations suggested that the O₂ binding mechanism is governed by pockets formed between the aromatic rings in the supramolecular structure of the gel, which bind O₂ through hydrophobic interactions. This phenomenon is harnessed to maintain the activity of the O₂-hypersensitive enzyme [FeFe]-hydrogenase, which holds promising potential for utilizing hydrogen gas for sustainable energy applications. Hydrogenase encapsulation within the gel allows hydrogen production following exposure to ambient O₂. This phenomenon may lead to utilization of this low molecular weight gelator in a wide range of O₂-sensitive applications.

Membrane Inlet Mass Spectrometry: A Powerful Tool for Algal Research

Since the first great oxygenation event, photosynthetic microorganisms have continuously shaped the Earth's atmosphere. Studying biological mechanisms involved in the interaction between microalgae and cyanobacteria with the Earth's atmosphere requires the monitoring of gas exchange. Membrane inlet mass spectrometry (MIMS) has been developed in the early 1960s to study gas exchange mechanisms of photosynthetic cells. It has since played an important role in investigating various cellular processes that involve gaseous compounds (O2, CO2, NO, or H2) and in characterizing enzymatic activities in vitro or in vivo. With the development of affordable mass spectrometers, MIMS is gaining wide popularity and is now used by an increasing number of laboratories. However, it still requires an important theory and practical considerations to be used. Here, we provide a practical guide describing the current technical basis of a MIMS setup and the general principles of data processing. We further review how MIMS can be used to study various aspects of algal research and discuss how MIMS will be useful in addressing future scientific challenges.

Flavodiiron-Mediated O2 Photoreduction Links H2 Production with CO2 Fixation during the Anaerobic Induction of Photosynthesis

Some microalgae, such as Chlamydomonas reinhardtii, harbor a highly flexible photosynthetic apparatus capable of using different electron acceptors, including carbon dioxide (CO₂), protons, or oxygen (O₂), allowing survival in diverse habitats. During anaerobic induction of photosynthesis, molecular O₂ is produced at photosystem II, while at the photosystem I acceptor side, the reduction of protons into hydrogen (H₂) by the plastidial [FeFe]-hydrogenases primes CO₂ fixation. Although the interaction between H₂ production and CO₂ fixation has been studied extensively, their interplay with O₂ produced by photosynthesis has not been considered. By simultaneously measuring gas exchange and chlorophyll fluorescence, we identified an O₂ photoreduction mechanism that functions during anaerobic dark-to-light transitions and demonstrate that flavodiiron proteins (Flvs) are the major players involved in light-dependent O₂ uptake. We further show that Flv-mediated O₂ uptake is critical for the rapid induction of CO₂ fixation but is not involved in the creation of the micro-oxic niches proposed previously to protect the [FeFe]-hydrogenase from O₂ By studying a mutant lacking both hydrogenases (HYDA1 and HYDA2) and both Flvs (FLVA and FLVB), we show that the induction of photosynthesis is strongly delayed in the absence of both sets of proteins. Based on these data, we propose that Flvs are involved in an important intracellular O₂ recycling process, which acts as a relay between H₂ production and CO₂ fixation.

Green Algal Hydrogenase Activity Is Outcompeted by Carbon Fixation before Inactivation by Oxygen Takes Place

Photoproduction of hydrogen by green algae is considered a transitory release valve of excess reducing power and a potential carbon-free source of sustainable energy. It is generally accepted that the transitory production of hydrogen is governed by fast inactivation of hydrogenase by oxygen. However, our data suggest that photosynthetic electron loss to competing processes, mainly carbon fixation, stops hydrogen production, supports hydrogen uptake, and precedes the inevitable inactivation by oxygen. Here, we show that when transitioning from dark anaerobiosis to light, hydrogen production ceases within 2 min, regardless of the presence of oxygen. Simultaneous monitoring of the active hydrogenase pool size shows that it remains entirely intact up to 4 min after illumination and is inactivated only later. Thus, our data reveal a window of 4 min in which the hydrogenase pool is not being degraded by oxygen. Furthermore, we show that electron loss, prominently to carbon fixation, outcompetes hydrogen production and leads to hydrogen uptake. Indeed, supplying additional reducing power to hydrogenase at the cessation point regenerates the accumulation of hydrogen. Our results imply the fast cessation of hydrogen production is governed by electron loss rather than oxygen inactivation, which takes place minutes later.

Structural Characterization of Poised States in the Oxygen Sensitive Hydrogenases and Nitrogenases

The crystallization of FeS cluster-containing proteins has been challenging due to their oxygen sensitivity, and yet these enzymes are involved in many critical catalytic reactions. The last few years have seen a wealth of innovative experiments designed to elucidate not just structural but mechanistic insights into FeS cluster enzymes. Here, we focus on the crystallization of hydrogenases, which catalyze the reversible reduction of protons to hydrogen, and nitrogenases, which reduce dinitrogen to ammonia. A specific focus is given to the different experimental parameters and strategies that are used to trap distinct enzyme states, specifically, oxidants, reductants, and gas treatments. Other themes presented here include the recent use of Cryo-EM, and how coupling various spectroscopies to crystallization is opening up new approaches for structural and mechanistic analysis.

Aerobic damage to [FeFe]-hydrogenases: activation barriers for the chemical attachment of O2

[FeFe]-hydrogenases are the best natural hydrogen-producing enzymes but their biotechnological exploitation is hampered by their extreme oxygen sensitivity. The free energy profile for the chemical attachment of O2 to the enzyme active site was investigated by using a range-separated density functional re-parametrized to reproduce high-level ab initio data. An activation free-energy barrier of 13 kcal mol(-1) was obtained for chemical bond formation between the di-iron active site and O2, a value in good agreement with experimental inactivation rates. The oxygen binding can be viewed as an inner-sphere electron-transfer process that is strongly influenced by Coulombic interactions with the proximal cubane cluster and the protein environment. The implications of these results for future mutation studies with the aim of increasing the oxygen tolerance of this enzyme are discussed.

Design of new strategy for green algal photo-hydrogen production: spectral-selective photosystem I activation and photosystem II deactivation

A new strategy in photosynthetic hydrogen (photo-H(2)) production from green algae was developed based on theory and successfully demonstrated. The new strategy applied a spectral-selective photosystem I (PSI) activating/photosystem II (PSII) deactivating radiation (or PSI light) that would drive a steady flow of electrons in the electron transport chain for delivery to hydrogenase for photo-H(2) production, but would reduce oxygen production through water photolysis below the respiratory oxygen consumption so that an anoxic condition would be maintained as required by hydrogenase. Implementing the strategy by using a PSI light (692 nm peak, 680-700 nm) on Chlamydomonas reinhardtii cells resulted in relatively sustained photo-H(2) production (total of 0.108 mL H(2)mg(-1)Chl, exceeding 0.066 mL H(2)mg(-1)Chl under white light). The strategy also proved successful and convenient in allowing cells to alternately switch between photo-H(2) production and a recovery period by simply turning on or off the PSI light.

Hydrogen photo-evolution upon S deprivation stepwise: an illustration of microalgal photosynthetic and metabolic flexibility and a step stone for future biotechnological methods of renewable H(2) production

The metabolic flexibility of some photosynthetic microalgae enables them to survive periods of anaerobiosis in the light by developing a particular photofermentative metabolism. The latter entails compounds of the photosynthetic electron transfer chain and an oxygen-sensitive hydrogenase in order to reoxidize reducing equivalents and to generate ATP for maintaining basal metabolic function. This pathway results in the photo-evolution of hydrogen gas by the algae. A decade ago, Melis and coworkers managed to reproduce such a condition in a laboratory context by depletion of sulfur in the algal culture media, making the photo-evolution by the algae sustainable for several days (Melis et al. in Plant Physiol 122:127-136, 2000). This observation boosted research in algal H(2) evolution. A feature, which due to its transient nature was long time considered as a curiosity of algal photosynthesis suddenly became a phenomenon with biotechnological potential. Although the Melis procedure has not been developed into a biotechnological process of renewable H(2) generation so far, it has been a useful tool for studying microalgal metabolic and photosynthetic flexibility and a possible step stone for future H(2) production procedures. Ten years later most of the critical steps and limitations of H(2) production by this protocol have been studied from different angles particularly with the model organism Chlamydomonas reinhardtii, by introducing various changes in culture conditions and making use of mutants issued from different screens or by reverse genomic approaches. A synthesis of these observations with the most important conclusions driven from recent studies will be presented in this review.

A cell-free microtiter plate screen for improved [FeFe] hydrogenases

BACKGROUND

[FeFe] hydrogenase enzymes catalyze the production and dissociation of H(2), a potential renewable fuel. Attempts to exploit these catalysts in engineered systems have been hindered by the biotechnologically inconvenient properties of the natural enzymes, including their extreme oxygen sensitivity. Directed evolution has been used to improve the characteristics of a range of natural catalysts, but has been largely unsuccessful for [FeFe] hydrogenases because of a lack of convenient screening platforms.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe an in vitro screening technology for oxygen-tolerant and highly active [FeFe] hydrogenases. Despite the complexity of the protocol, we demonstrate a level of reproducibility that allows moderately improved mutants to be isolated. We have used the platform to identify a mutant of the Chlamydomonas reinhardtii [FeFe] hydrogenase HydA1 with a specific activity approximately 4 times that of the wild-type enzyme.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate the feasibility of using the screen presented here for large-scale efforts to identify improved biocatalysts for energy applications. The system is based on our ability to activate these complex enzymes in E. coli cell extracts, which allows unhindered access to the protein maturation and assay environment.

Electrochemical kinetic investigations of the reactions of [FeFe]-hydrogenases with carbon monoxide and oxygen: comparing the importance of gas tunnels and active-site electronic/redox effects

A major obstacle for future biohydrogen production is the oxygen sensitivity of [FeFe]-hydrogenases, the highly active catalysts produced by bacteria and green algae. The reactions of three representative [FeFe]-hydrogenases with O(2) have been studied by protein film electrochemistry under conditions of both H(2) oxidation and H(2) production, using CO as a complementary probe. The hydrogenases are DdHydAB and CaHydA from the bacteria Desulfovibrio desulfuricans and Clostridium acetobutylicum , and CrHydA1 from the green alga Chlamydomonas reinhardtii . Rates of inactivation depend on the redox state of the active site 'H-cluster' and on transport through the protein to reach the pocket in which the H-cluster is housed. In all cases CO reacts much faster than O(2). In the model proposed, CaHydA shows the most sluggish gas transport and hence little dependence of inactivation rate on H-cluster state, whereas DdHydAB shows a large dependence on H-cluster state and the least effective barrier to gas transport. All three enzymes show a similar rate of reactivation from CO inhibition, which increases upon illumination: the rate-determining step is thus assigned to cleavage of the labile Fe-CO bond, a reaction likely to be intrinsic to the atomic and electronic state of the H-cluster and less sensitive to the surrounding protein.

How oxygen attacks [FeFe] hydrogenases from photosynthetic organisms

Green algae such as Chlamydomonas reinhardtii synthesize an [FeFe] hydrogenase that is highly active in hydrogen evolution. However, the extreme sensitivity of [FeFe] hydrogenases to oxygen presents a major challenge for exploiting these organisms to achieve sustainable photosynthetic hydrogen production. In this study, the mechanism of oxygen inactivation of the [FeFe] hydrogenase CrHydA1 from C. reinhardtii has been investigated. X-ray absorption spectroscopy shows that reaction with oxygen results in destruction of the [4Fe-4S] domain of the active site H-cluster while leaving the di-iron domain (2Fe(H)) essentially intact. By protein film electrochemistry we were able to determine the order of events leading up to this destruction. Carbon monoxide, a competitive inhibitor of CrHydA1 which binds to an Fe atom of the 2Fe(H) domain and is otherwise not known to attack FeS clusters in proteins, reacts nearly two orders of magnitude faster than oxygen and protects the enzyme against oxygen damage. These results therefore show that destruction of the [4Fe-4S] cluster is initiated by binding and reduction of oxygen at the di-iron domain-a key step that is blocked by carbon monoxide. The relatively slow attack by oxygen compared to carbon monoxide suggests that a very high level of discrimination can be achieved by subtle factors such as electronic effects (specific orbital overlap requirements) and steric constraints at the active site.

The role of PerR in O2-affected gene expression of Clostridium acetobutylicum

In the strict anaerobe Clostridium acetobutylicum, a PerR-homologous protein has recently been identified as being a key repressor of a reductive machinery for the scavenging of reactive oxygen species and molecular O(2). In the absence of PerR, the full derepression of its regulon resulted in increased resistance to oxidative stress and nearly full tolerance of an aerobic environment. In the present study, the complementation of a Bacillus subtilis PerR mutant confirmed that the homologous protein from C. acetobutylicum acts as a functional peroxide sensor in vivo. Furthermore, we used a transcriptomic approach to analyze gene expression in the aerotolerant PerR mutant strain and compared it to the O(2) stimulon of wild-type C. acetobutylicum. The genes encoding the components of the alternative detoxification system were PerR regulated. Only few other targets of direct PerR regulation were identified, including two highly expressed genes encoding enzymes that are putatively involved in the central energy metabolism. All of them were highly induced when wild-type cells were exposed to sublethal levels of O(2). Under these conditions, C. acetobutylicum also activated the repair and biogenesis of DNA and Fe-S clusters as well as the transcription of a gene encoding an unknown CO dehydrogenase-like enzyme. Surprisingly few genes were downregulated when exposed to O(2), including those involved in butyrate formation. In summary, these results show that the defense of this strict anaerobe against oxidative stress is robust and by far not limited to the removal of O(2) and its reactive derivatives.

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.

Atomic resolution modeling of the ferredoxin:[FeFe] hydrogenase complex from Chlamydomonas reinhardtii

The [FeFe] hydrogenases HydA1 and HydA2 in the green alga Chlamydomonas reinhardtii catalyze the final reaction in a remarkable metabolic pathway allowing this photosynthetic organism to produce H(2) from water in the chloroplast. A [2Fe-2S] ferredoxin is a critical branch point in electron flow from Photosystem I toward a variety of metabolic fates, including proton reduction by hydrogenases. To better understand the binding determinants involved in ferredoxin:hydrogenase interactions, we have modeled Chlamydomonas PetF1 and HydA2 based on amino-acid sequence homology, and produced two promising electron-transfer model complexes by computational docking. To characterize these models, quantitative free energy calculations at atomic resolution were carried out, and detailed analysis of the interprotein interactions undertaken. The protein complex model we propose for ferredoxin:HydA2 interaction is energetically favored over the alternative candidate by 20 kcal/mol. This proposed model of the electron-transfer complex between PetF1 and HydA2 permits a more detailed view of the molecular events leading up to H(2) evolution, and suggests potential mutagenic strategies to modulate electron flow to HydA2.

Maturation of hydrogenases

Enzymes possessing the capacity to oxidize molecular hydrogen have developed convergently three class of enzymes leading to: [FeFe]-, [NiFe]-, and [FeS]-cluster-free hydrogenases. They differ in the composition and the structure of the active site metal centre and the sequence of the constituent structural polypeptides but they show one unifying feature, namely the existence of CN and/or CO ligands at the active site Fe. Recent developments in the analysis of the maturation of [FeFe]- and [NiFe]- hydrogenases have revealed a remarkably complex pattern of mostly novel biochemical reactions. Maturation of [FeFe]-hydrogenases requires a minimum of three auxiliary proteins, two of which belong to the class of Radical-SAM enzymes and other to the family of GTPases. They are sufficient to generate active enzyme when their genes are co-expressed with the structural genes in a heterologous host, otherwise deficient in [FeFe]-hydrogenase expression. Maturation of the large subunit of [NiFe]-hydrogenases depends on the activity of at least seven core proteins that catalyse the synthesis of the CN ligand, have a function in the coordination of the active site iron, the insertion of nickel and the proteolytic maturation of the large subunit. Whereas this core maturation machinery is sufficient to generate active hydrogenase in the cytoplasm, like that of hydrogenase 3 from Escherichia coli, additional proteins are involved in the export of the ready-assembled heterodimeric enzyme to the periplasm via the twin-arginine translocation system in the case of membrane-bound hydrogenases. A series of other gene products with intriguing putative functions indicate that the minimal pathway established for E. coli [NiFe]-hydrogenase maturation may possess even higher complexity in other organisms.

Inhibition and aerobic inactivation kinetics of Desulfovibrio fructosovorans NiFe hydrogenase studied by protein film voltammetry

We have used protein film voltammetry to study the NiFe hydrogenase from Desulfovibrio fructosovorans. We show how measurements of transient activity following the addition in the electrochemical cell of H(2), CO, or O(2) allow simple and virtually instantaneous determinations of the Michaelis constant, inhibition constant, or rate of inactivation, respectively, thus opening new opportunities to study the active site of NiFe hydrogenases. The binding and release of CO occur within a fraction of a second, and we determine and discuss how its affinity for the active site changes as the driving force for the H(+)/H(2) reaction is continuously varied. Inactivation by O(2) is a slow, bimolecular process (with pH-independent rate constant approximately 3 x 10(4) s(-1) M(-1) at 40 degrees C, under one atm of H(2)) that leads to a mixture of fully oxidized states, and unlike the case of CO inhibition, the active site is not fully protected by H(2). This experimental approach could be used to study the reaction of other multicentered metalloenzymes with their gaseous substrates or inhibitors.

Molecular dynamics and experimental investigation of H(2) and O(2) diffusion in [Fe]-hydrogenase

The [Fe]-hydrogenase enzymes are highly efficient H(2) catalysts found in ecologically and phylogenetically diverse microorganisms, including the photosynthetic green alga, Chlamydomonas reinhardtii. Although these enzymes can occur in several forms, H(2) catalysis takes place at a unique [FeS] prosthetic group or H-cluster, located at the active site. Significant to the function of hydrogenases is how the surrounding protein structure facilitates substrate-product transfer, and protects the active site H-cluster from inactivation. To elucidate the role of protein structure in O(2) inactivation of [Fe]-hydrogenases, experimental and theoretical investigations have been performed. Molecular dynamics was used to comparatively investigate O(2) and H(2) diffusion in CpI ([Fe]-hydrogenase I from Clostridium pasteurianum). Our preliminary results suggest that H(2) diffuses more easily and freely than O(2), which is restricted to a small number of allowed pathways to and from the active site. These O(2) pathways are located in the conserved active site domain, shown experimentally to have an essential role in active site protection.

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.

Characterization of the oxygen tolerance of a hydrogenase linked to a carbon monoxide oxidation pathway in Rubrivivax gelatinosus

A hydrogenase linked to the carbon monoxide oxidation pathway in Rubrivivax gelatinosus displays tolerance to O2. When either whole-cell or membrane-free partially purified hydrogenase was stirred in full air (21% O2, 79% N2), its H2 evolution activity exhibited a half-life of 20 or 6 h, respectively, as determined by an anaerobic assay using reduced methyl viologen. When the partially purified hydrogenase was stirred in an atmosphere containing either 3.3 or 13% O2 for 15 min and evaluated by a hydrogen-deuterium (H-D) exchange assay, nearly 80 or 60% of its isotopic exchange rate was retained, respectively. When this enzyme suspension was subsequently returned to an anaerobic atmosphere, more than 90% of the H-D exchange activity was recovered, reflecting the reversibility of this hydrogenase toward O2 inactivation. Like most hydrogenases, the CO-linked hydrogenase was extremely sensitive to CO, with 50% inhibition occurring at 3.9 microM dissolved CO. Hydrogen production from the CO-linked hydrogenase was detected when ferredoxins of a prokaryotic source were the immediate electron mediator, provided they were photoreduced by spinach thylakoid membranes containing active water-splitting activity. Based on its appreciable tolerance to O2, potential applications of this hydrogenase are discussed.

Induction, localization and metal content of hydrogenase in the green alga Chlamydomonas reinhardtii

The hydrogenase enzyme occurring in Chlamydomonas reinhardtii is induced by anaerobic adaptation of the cells. In aerobically growing cells, antibodies against the hydrogenase failed to detect either active or inactive enzyme. However, already 10 min after the onset of anaerobic adaptation, the protein could be detected. The maximal amount of enzyme was reached after 2-3 hours anaerobiosis. Addition of nickel or iron to the growth medium did not influence activity. In atomic absorption experiments, a Ni/Fe ratio of about 1:250 was measured. We, therefore, propose the hydrogenase from C. reinhardtii to be of the Fe-only type. Adaptation in the presence of uncouplers of phosphorylation showed this process to be energy-dependent. From protein synthesis inhibition experiments, it is concluded that the protein is synthesized on cytoplasmic ribosomes and, therefore, must be nuclear encoded. After isolation of intact chloroplasts from adapted cells, the active enzyme was shown, by Western-blotting analysis, to be located in the chloroplasts.

Characterization and purification of a hydrogenase from the eukaryotic green alga Scenedesmus obliquus

Several catalytic properties of the hydrogenase from Scenedesmus obliquus have been examined to optimize the purification conditions. The Km-value for H2-evolution in the presence of the most effective electron mediator methylviologen is 0.66 mM. The pH-optimum is 6.3, the temperature-optimum is 50 degrees C and the energy of activation is 38.4 +/- 2 kJ.mol-1. The soluble hydrogenase from the green alga, Scenedesmus obliquus, was purified 1290-fold to homogeneity. The enzyme consists of two subunits with molecular masses of 55 kDa and 36 kDa. The molecular weight of the native enzyme, determined by gel filtration, is 150 +/- 5 kDa.

Isolation, characterization and N-terminal amino acid sequence of hydrogenase from the green alga Chlamydomonas reinhardtii

Hydrogenase from Chlamydomonas reinhardtii was purified to homogeneity by five column-chromatography steps under strict anaerobic conditions. The cells were disrupted by mild treatment with detergent. The enzyme was purified 6100-fold, resulting in a specific activity for H2 evolution of 935 mumol.min-1.mg protein-1 at 25 degrees C, using reduced methyl viologen as electron donor. The optimal temperature for hydrogen evolution is 60 degrees C, the optimal pH value is 6.9. The Km value for methyl viologen is 0.83 mM, for ferredoxin, 35 microM. From SDS/PAGE gels, the protein was judged to be pure. On non-denaturing gels, run under nitrogen, a single band was detected after activity staining. This band corresponded to the single band observed on denaturing SDS gels, which had an apparent molecular mass of 48 kDa. If the band was cut out of the native gel and incubated with reduced methyl viologen, hydrogen evolution could be measured. The purified enzyme contains 4 Fe atoms/mol. The amino acid composition and the N-terminal amino acid sequence (24 residues) of the protein were determined. No significant amino acid sequence homologies could be found to any sequences from prokaryotic hydrogenases.

Hydrogenase-Mediated Activities in Isolated Chloroplasts of Chlamydomonas reinhardii

Isolated intact chloroplasts of Chlamydomonas reinhardii were found to catalyze photoreduction of CO(2) in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea when adapted under an atmosphere of H(2) demonstrating the association of a hydrogenase and anaerobic adaptation system with these plastids. The specific activity of photoreduction was approximately one third that detected in cells and protoplasts. Photoreduction was found to have a lower osmoticum optimum relative to aerobically maintained chloroplasts (50 millimolar versus 120 millimolar mannitol). 3-Phosphoglycerate (3-PGA) stimulated photoreduction up to a peak at 0.25 millimolar beyond which inhibition was observed. In the absence of 3-PGA, inorganic phosphate had no effect on photoreduction but in the presence of 3-PGA, inorganic phosphate also stimulated the reaction. Carbonyl cyanide-p-trifluoromethoxyphenylhydrazone and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone inhibited photoreduction but inhibition by the former could be partially overcome by exogenously added ATP. The intact plastid can also catalyze photoevolution of H(2) while lysed chloroplast extracts catalyzed the reduction of methyl viologen by H(2). Both reactions occurred at rates approximately one-third of those found in cells. The oxyhydrogen reaction in the presence or absence of CO(2) was not detected.

Effects of Light Intensity and Oxidized Nitrogen Sources on Hydrogen Production by Chlamydomonas reinhardii

Chlamydomonas reinhardii cells, after a period of dark anaerobic adaptation, evolve H(2) not only in the dark but also in the light. Our results show that high irradiances impair prolonged H(2) evolution, while under low irradiances or darkness H(2) evolution proceeds for more than 50 hours. NO(3) (-) and NO(2) (-) suppress H(2) evolution both in the dark or under low irradiance. Apparently the cells prefer these oxidized nitrogen sources to protons as electron acceptors, since both NO(3) (-) and NO(2) (-) become reduced to NH(4) (+), which is excreted to the culture medium in high amounts. H(2) evolution started once these oxidized anions were largely depleted from the medium. Moreover, H(2) evolution was consistently associated with NH(4) (+) excretion even if NH(4) (+) was already present in high amounts in the medium. This observation indicates that the cells utilize not only their carbohydrate but also their protein reserves as sources of reducing power for H(2) evolution. This conclusion was supported by the observation that when nitrogen-starved cells were made anaerobic in a nitrogen-free medium, they not only evolved H(2) at very high rates but excreted concomitantly NH(4) (+) up to concentrations in the millimolar range.

Purification of Hydrogenase from Chlamydomonas reinhardtii

A method is described which results in a 2750-fold purification of hydrogenase from Chlamydomonas reinhardtii, yielding a preparation which is approximately 40% pure. With a saturating amount of ferredoxin as the electron mediator, the specific activity of pure enzyme was calculated to be 1800 micromoles H(2) produced per milligram protein per minute. The molecular weight was determined to be 4.5 x 10(4) by gel filtration and 4.75 x 10(4) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme has an abundance of acidic side groups, contains iron, and has an activation energy of 55.1 kilojoules per mole for H(2) production; these properties are similar to those of bacterial hydrogenases. The enzyme is less thermally stable than most bacterial hydrogenases, however, losing 50% of its activity in 1 hour at 55 degrees C. The K(m) of purified hydrogenase for ferredoxin is 10 micromolar, and the binding of these proteins to each other is enhanced under slightly acidic conditions. Purified hydrogenase also accepts electrons from a variety of artificial electron mediators, including sodium metatungstate, sodium silicotungstate, and several viologen dyes. A lag period is frequently observed before maximal activity is expressed with these artificial electron mediators, although the addition of sodium thiosulfate at least partially overcomes this lag.

Chloroplast Respiration : A MEANS OF SUPPLYING OXIDIZED PYRIDINE NUCLEOTIDE FOR DARK CHLOROPLASTIC METABOLISM

A spinach (Spinacia oleracia var. America) chloroplast particle fortified with ferredoxin, fructose-1,6-bisphosphate, or ribose-5-phosphate and NADP has been shown to generate NADPH by the oxidation of glyceraldehyde-3 phosphate to glycerate-3-phosphate (PGA) and to reduce ferredoxin with the NADPH. The resulting reduced ferredoxin can reduce O(2) to H(2)O(2), nitrite to ammonia, or protons to H(2). Hydrogen production was the result of adding hydrogenase from Chlamydomonas reinhardii to the chloroplast preparation. The predicted stoichiometry of 1 PGA:1 O(2) in the absence of and 2 PGA:1 O(2) in the presence of catalase was observed indicating H(2)O(2) as the end product of O(2) reduction. The predicted stoichiometry of 3 PGA:1 nitrite:1 ammonia was also observed. A scheme is presented to account for a sustained generation of NADP and ATP necessary for the dissimilation of starch in the darkened chloroplast. The unifying term chloroplast respiration is introduced to account for those reactions in which reduced ferredoxin interacts with physiological acceptors other than NADP or nitrite, hydrogen, or O(2) respiration when nitrite, protons, or O(2) is the ultimate electron acceptor.

The effect of electron carriers and other ligands on oxygen stability of clostridial hydrogenase

The effects of various electron carriers, a substrate (H2) and a reversible inhibitor (CO) on the rate of irreversible oxygen inactivation of clostridial hydrogenase (ferredoxin: H+ oxidoreductase, EC 1.18.3.1) have been studied kinetically. Some electron carriers (e.g., clostridial ferredoxin and methyl viologen) greatly stabilize the enzyme, some (FAD, FMN) drastically reduce its stability, while others (benzyl viologen and methylene blue) only slightly alter the stability. Competitive experiments indicate that stabilizers and destabilizers do not compete with each other for binding with the active center of hydrogenase. Hydrogen and CO do not affect the rate of the oxygen inactivation. On the basis of the results obtained herein and kinetic data on hydrogenase catalysis from the literature, it is concluded that the active center of this hydrogenase comprises at least three different independent subsites. The first one (presumably an iron atom of the iron-sulfur cluster) binds H2 and CO and does not contribute to the oxygen stability. The second one binds stabilizers like methyl viologen while the third one binds destabilizers like FMN and FAD.

Production of superoxide radicals by soluble hydrogenase from Alcaligenes eutrophus H16

The soluble hydrogenase (hydrogen-NAD+ oxidoreductase, EC 1.12.1.2) of Alcaligenes eutrophus H16 was shown to be stabilized by oxidation with oxygen and ferricyanide as long as electron donors and reducing compounds were absent. The simultaneous presence of H2, NADH and O2 in the enzyme solution, however, caused an irreversible inactivation of hydrogenase that was dependent on the O2 concentration. The half-life periods of 4 degrees C under partial pressures of 0.1, 5, 20 and 50% O2 were 11, 5, 2.5 and 1.5 h respectively. Evidence has been obtained that hydrogenase produces superoxide free radical anions (O2-.), which were detected by their ability to oxidize hydroxylamine to nitrite. The correlation between O2 concentration, nitrite formation and inactivation rates and the stabilization of hydrogenase by addition of superoxide dismutase indicated that superoxide radicals are responsible for enzyme inactivation. During short-term activity measurements (NAD+ reduction, H2 evolution from NADH), hydrogenase activity was inhibited by O2 only very slightly. In the presence of 0.7 mM-O2 an inhibition of about 20% was observed.