Evolution of carboxylating enzymes involved in paramylon synthesis (phosphoenolpyruvate carboxylase and carboxykinase) in heterotrophically grown Euglena gracilis

Euglena Central Metabolic Pathways and Their Subcellular Locations

Euglenids are a group of algae of great interest for biotechnology, with a large and complex metabolic capability. To study the metabolic network, it is necessary to know where the component enzymes are in the cell, but despite a long history of research into Euglena, the subcellular locations of many major pathways are only poorly defined. Euglena is phylogenetically distant from other commonly studied algae, they have secondary plastids bounded by three membranes, and they can survive after destruction of their plastids. These unusual features make it difficult to assume that the subcellular organization of the metabolic network will be equivalent to that of other photosynthetic organisms. We analysed bioinformatic, biochemical, and proteomic information from a variety of sources to assess the subcellular location of the enzymes of the central metabolic pathways, and we use these assignments to propose a model of the metabolic network of Euglena. Other than photosynthesis, all major pathways present in the chloroplast are also present elsewhere in the cell. Our model demonstrates how Euglena can synthesise all the metabolites required for growth from simple carbon inputs, and can survive in the absence of chloroplasts.

Development of screening strategies for the identification of paramylon-degrading enzymes

Enzymatic degradation of the β-1,3-glucan paramylon could enable the production of bioactive compounds for healthcare and renewable substrates for biofuels. However, few enzymes have been found to degrade paramylon efficiently and their enzymatic mechanisms remain poorly understood. Thus, the aim of this work was to find paramylon-degrading enzymes and ways to facilitate their identification. Towards this end, a Euglena gracilis-derived cDNA expression library was generated and introduced into Escherichia coli. A flow cytometry-based screening assay was developed to identify E. gracilis enzymes that could hydrolyse the fluorogenic substrate fluorescein di-β-D-glucopyranoside in combination with time-saving auto-induction medium. In parallel, four amino acid sequences of potential E. gracilis β-1,3-glucanases were identified from proteomic data. The open reading frame encoding one of these candidate sequences (light_m.20624) was heterologously expressed in E. coli. Finally, a Congo Red dye plate assay was developed for the screening of enzyme preparations potentially able to degrade paramylon. This assay was validated with enzymes assumed to have paramylon-degrading activity and then used to identify four commercial preparations with previously unknown paramylon degradation ability.

Succinate and Lactate Production from Euglena gracilis during Dark, Anaerobic Conditions

Euglena gracilis is a eukaryotic, unicellular phytoflagellate that has been widely studied in basic science and applied science. Under dark, anaerobic conditions, the cells of E. gracilis produce a wax ester that can be converted into biofuel. Here, we demonstrate that under dark, anaerobic conditions, E. gracilis excretes organic acids, such as succinate and lactate, which are bulk chemicals used in the production of bioplastics. The levels of succinate were altered by changes in the medium and temperature during dark, anaerobic incubation. Succinate production was enhanced when cells were incubated in CM medium in the presence of NaHCO₃. Excretion of lactate was minimal in the absence of external carbon sources, but lactate was produced in the presence of glucose during dark, anaerobic incubation. E. gracilis predominantly produced L-lactate; however, the percentage of D-lactate increased to 28.4% in CM medium at 30°C. Finally, we used a commercial strain of E. gracilis for succinate production and found that nitrogen-starved cells, incubated under dark, anaerobic conditions, produced 869.6 mg/L succinate over a 3-day incubation period, which was 70-fold higher than the amount produced by nitrogen-replete cells. This is the first study to demonstrate organic acid excretion by E. gracilis cells and to reveal novel aspects of primary carbon metabolism in this organism.

Transcriptomic study reveals widespread spliced leader trans-splicing, short 5'-UTRs and potential complex carbon fixation mechanisms in the euglenoid Alga Eutreptiella sp

Eutreptiella are an evolutionarily unique and ecologically important genus of microalgae, but they are poorly understood with regard to their genomic make-up and expression profiles. Through the analysis of the full-length cDNAs from a Eutreptiella species, we found a conserved 28-nt spliced leader sequence (Eut-SL, ACACUUUCUGAGUGUCUAUUUUUUUUCG) was trans-spliced to the mRNAs of Eutreptiella sp. Using a primer derived from Eut-SL, we constructed four cDNA libraries under contrasting physiological conditions for 454 pyrosequencing. Clustering analysis of the ∼1.9×10(6) original reads (average length 382 bp) yielded 36,643 unique transcripts. Although only 28% of the transcripts matched documented genes, this fraction represents a functionally very diverse gene set, suggesting that SL trans-splicing is likely ubiquitous in this alga's transcriptome. The mRNAs of Eutreptiella sp. seemed to have short 5'- untranslated regions, estimated to be 21 nucleotides on average. Among the diverse biochemical pathways represented in the transcriptome we obtained, carbonic anhydrase and genes known to function in the C4 pathway and heterotrophic carbon fixation were found, posing a question whether Eutreptiella sp. employs multifaceted strategies to acquire and fix carbon efficiently. This first large-scale transcriptomic dataset for a euglenoid uncovers many potential novel genes and overall offers a valuable genetic resource for research on euglenoid algae.

Kinetic and thermodynamic characterization of adenylyl cyclase from Euglena gracilis

Some kinetic and thermodynamic properties of the plasma membrane adenylyl cyclase (AC) from the protist Euglena gracilis were examined. The AC kinetics for Mg-ATP was hyperbolic with a K(m) value of 0.33-0.43 mM, whereas the inhibition exerted by 2('),5(')-dideoxyadenosine was of the mixed type with a K(i) of 80-147 microM. The V(m) value (0.9 or 1.8 nmol(mg protein)(-1)min(-1)) changed, depending upon the carbon source in the growth medium (lactic acid or glutamate plus malate). Lactic acid membrane AC was slightly more thermolabile (from 28 to 40 degrees C) and showed higher activation energy (range 15-25 degrees C). With lactate, the total and saturated fatty acid percentage content in the plasma membrane was significantly greater than with glutamate plus malate, whereas the percentage content of polyunsaturated (n-3) fatty acids was lower. The data suggest that the fatty acid composition, as changed by the carbon source in the growth medium, may modulate the AC activity in Euglena.

Presence of proteins recognized by mammalian cytochrome P-450 antibodies in Euglena gracilis

We have attempted to probe three microsomal cytochrome P-450 isozymes in Euglena gracilis using immunochemical methods. They cross-react with anti-rat cytochrome P4502C11, cytochrome P4502E, and cytochrome P4502B. Activities of alkoxyphenoxazone dealkylation have been tested in living cells. In untreated cultures, the amount of proteins recognized by anti-cytochrome P4502C11 or anti-cytochrome P4502E is high. Phenobarbital treatment increased the levels of microsomal proteins recognized by antibody to cytochrome P4502B, as well as dealkylases of pentoxyresorufin, but decreased the level of proteins recognized by anti-cytochrome P450C11 or cytochrome P4502E. These results suggest that these unicellular algae may contain different isozymes of microsomal cytochromes P-450, comparable to those in mammalian liver. They are cytochrome P-450 equivalents of mammalian isoenzymes 2C, 2E and 2B. However, we could not demonstrate ethanol induction of cytochrome P-450 equivalent to isoenzyme 2E. Its role in xeno- or endobiotic metabolism remains to be elucidated.

Microsomal ethanol-oxidizing system in Euglena gracilis. Similarities between Euglena and mammalian cell systems

1. ADH activity of Euglena grown with 50 mM ethanol decreased, but MEOS activity increased with a corresponding increase in the total amount of cytochrome P-450. 2. Phenobarbital treatment increased the total amount of cytochrome P-450. 3. CO and KCN, cytochrome P-450 ligands, diminished acetaldehyde formed from ethanol oxidation by MEOS. 4. The amounts of NAD(P)H cytochrome c reductases and cytochrome b5 type, components of microsomal monooxygenase reaction, have been spectrophotometrically measured. 5. NAD(P)H cytochrome c reductases activities were induced by phenobarbital. 6. DMSO, an inhibitor of rabbit MEOS, inhibited O2 consumption (11-20%) by Euglena grown with an ethanol, but not a lactate medium. 7. These studies indicate the presence of cytochrome P-450-dependent MEOS in Euglena similar to that in the mammalian hepatic cell.

Use of a microbial model for the determination of drug effects on cell metabolism and energetics: study of citrulline-malate

Euglena gracilis can be used as a microbial model to study the effect of drugs on lactate metabolism and gluconeogenetic synthesis. The cell growth and metabolism have been characterized in a 33 mM lactate medium, non-supplemented or supplemented by dl-malate or by l-citrulline alone or by the compound formed by the stoichiometric combination of the two components: the citrulline-malate (Stimol). The malate of the complex accelerated the ammonium disappearance, while the citrulline facilitated the lactate consumption. A synergistic action of the complex, by comparison with the additive effects of the individual components, on most of the parameters studied was detected. A remarkable resistance to anoxia, and a quicker recovery under aeration of the cells supplemented with CM, were evident: after carbonation for 2 min the total nucleotides in the medium were increased by 44 per cent with an unchanged energy charge; and after a prolonged (20 min) anoxia followed by an aeration, the capacities of the cells to synthesize ATP in the presence of excesses of both ADP and phosphate were two-fold higher in Stimol treated cells than in control.

Citrulline-malate effect on microsome phospholipids and cytochrome P450 in Euglena grown with ethanol

This study indicates for the first time the presence of cytochrome P450 in the microsomes of Euglena grown in lactate medium and substantiates the use of Euglena as a hepatic cell model. Similar effects of ethanol on Euglena and on rat hepatic microsomes were demonstrated: (i) decrements in the quantities of FA per milligram of proteins; (ii) increases in the proportions of PE; (iii) decreases in the proportions of PC; and (iv) production of cytochrome P450, degraded in P420. The citrulline-malate reestablishes in the microsomes the phospholipid environment and the cytochrome P450 concentration. These findings illustrate that the complex acts on the lipid peroxidation via the changes in cytochrome P450 activity.

Effects of a first exposure to ethanol on the compositions of neutral and polar lipids in Euglena gracilis Z, taken as a hepatic cell model: equilibration by citrulline-malate

In comparison to the lipid composition of Euglena cells fed with lactate, a first exposure of the cells to ethanol favors the production of neutral lipids containing mainly unsaturated fatty acids. The ethanol diminishes drastically the proportion of PC and weakly that of PE. In contrast, it increases slightly the proportion of DPG. The ethanol induces important changes in the fatty acid distributions of each lipid class, suggesting modifications of the elongation-desaturation system. On the one hand the proportion of unsaturated fatty acids is increased and, on the other hand, the last double bond is predominantly situated in the delta 6 position in place of delta 3. The addition of the complex citrulline-malate corrects most of these changes.

Induction by antimycin A of cyanide-resistant respiration in heterotrophic Euglena gracilis: Effects of growth, respiration and protein biosynthesis

The addition of antimycin A during the logarithmic phase of growth of heterotrophic Euglena gracilis cultures (in lactate or glucose medium) was immediately followed by decreased respiration and a cessation of grwoth. Induced cyanideresistent respiration appeared 5 h after the addition of the inhibitor then the cells started to grow again and could be cultured in the presence of antimycin A. Thus the cells exhibited a cyanide-and antimycin-resistant respiration which was, in addition, sensitive to salicylhydroxamic acid and propylgallate. Antimycin-adapted Euglena and control cells were compared for their biomass production and protein synthesis. The difference in growth yield between control and antimycin-adapted cells was not as high as would be expected if only the first phosphorylation site of the normal respiratory chain was active in the presence of antimycin A. Furthermore, the ability to incorporate labelled valine into proteins, under resting-cell conditions, was not changed. Strong correlations were established between the effects of respiratory effectors on O2 consumption and valine incorporation. These results suggest that sufficient energy for protein synthesis and growth is provided by the operation of the cyanide-resistant respiratory pathway in antimycin-adapted Euglena.

Paramylon synthesis by Euglena gracilis photoheterotrophically grown under low O2 pressure : Description of a mitochloroplast complex

Special culture conditions for Euglena gracilis Z and ZR are described. They induce interactions between the chloroplast and mitochondrial metabolisms leading to paramylon synthesis. When grown in continuous light under pure nitrogen and in the presence of lactate as the sole carbon source, sugar synthesis occurs during the first 24 h of culture with the participation of both mitochondria (using lactate) and of chloroplasts (fixing CO2 from lactate decarboxylation). The activities of ribulose bisphosphate carboxylase, phosphoenolpyruvate carboxylase, and phosphoenolpyruvate carboxykinase are very high and mitochondria and chloroplasts develop then a common network of vesicles in which paramylon grains can be seen. Electron micrographs demonstrate membrane continuity between the two types of organelles. Occasionally the mitochondrial matrix and the chloroplast stroma are separated by only a unit membrane.

Evolution of enzymes involved in carbon metabolism (phosphoenolpyruvate and ribulose-bisphosphate carboxylases, phosphoenolpyruvate carboxykinase) during the light-induced greening of Euglena gracilis strains Z and ZR

Phosphoenolpyruvate carboxykinase activity decreases when Euglena gracilis Z and ZR undergo light-induced chloroplast development in batch "resting" medium lacking utilizable organic carbon and CO2. This enzyme is present in heterotrophically grown cells (Briand et al. 1981) and assures gluconeogenesis. It was consistently more active in strain ZR. Decreased carboxykinase activities were accompanied by parallel increases in the activities of ribulose bisphosphate carboxylase and phosphoenolpyruvate carboxylase. The rates of O2 evolution in light were much lower than those of CO2 fixed simultaneously. The incorporation of (14)CO2 into early C-4 dicarboxylic acids was higher in green cells than in etiolated cells, and it was even higher in green cells assayed in light in the presence of (DCMU). A hypothesis has been proposed, according to which there is a possible cooperation of phosphoenolpyruvate carboxylase in photosynthetic CO2 fixation, especially under conditions of limiting CO2.High temperatures (34° C) depress carboxylation enzyme activities to a greater extent than that of the carboxykinase without a great effect on cellular chlorophyll content. In the presence of 25 μm DCMU, however, chlorophyll accumulation is reduced without any detectable changes in enzyme activities in the Z strain. The ZR strain displayed its characteristic resistance to DCMU.