Localization of glyoxylate cycle enzymes in glyoxysomes in Euglena

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.

The Craft of Peroxisome Purification-A Technical Survey Through the Decades

Purification technologies are one of the working horses in organelle proteomics studies as they guarantee the separation of organelle-specific proteins from the background contamination by other subcellular compartments. The development of methods for the separation of organelles was a major prerequisite for the initial detection and characterization of peroxisome as a discrete entity of the cell. Since then, isolated peroxisomes fractions have been used in numerous studies in order to characterize organelle-specific enzyme functions, to allocate the peroxisome-specific proteome or to unravel the organellar membrane composition. This review will give an overview of the fractionation methods used for the isolation of peroxisomes from animals, plants and fungi. In addition to "classic" centrifugation-based isolation methods, relying on the different densities of individual organelles, the review will also summarize work on alternative technologies like free-flow-electrophoresis or flow field fractionation which are based on distinct physicochemical parameters. A final chapter will further describe how different separation methods and quantitative mass spectrometry have been used in proteomics studies to assign the proteome of PO.

C2 metabolism in Euglena

Euglenoids are able to assimilate fatty acids and alcohols with various carbon-chain lengths, and ethanol is known to be one of the best carbon sources to support the growth of Euglena gracilis. Ethanol is first oxidized to acetate by the sequential reactions of alcohol dehydrogenase and acetaldehyde dehydrogenase in the mitochondria, and then converted to acetyl coenzyme A (acetyl-CoA). Acetyl-CoA is metabolized through the glyoxylate cycle which is a modified tricarboxylic acid (TCA) cycle in which isocitrate lyase (ICL) and malate synthase (MS) function to bypass the two decarboxylation steps of the TCA cycle, enabling the net synthesis of carbohydrates from C2 compounds. ICL and MS form a unique bifunctional enzyme localized in Euglena mitochondria, not in glyoxysome as in other eukaryotes. The unique glyoxylate and glycolate metabolism during photorespiration is also discussed in this chapter.

Increase in peroxisome number and the gene expression of putative glyoxysomal enzymes in Chlamydomonas cells supplemented with acetate

We cultured Chlamydomonas reinhardtii cells in a minimal culture medium supplemented with various concentrations of acetate, fatty acids, ethanol, fatty alcohols, or sucrose. The presence of acetate (0.5 or 1.0%, w/v) was advantageous for cell growth. To determine whether peroxisomes are involved in fatty acid and fatty alcohol metabolism, we investigated the dynamics of peroxisomes, including changes in their number and size, in the presence of acetate, ethanol, and sucrose. The total volume of peroxisomes increased when cells were grown with acetate, but did not change when cells were grown with ethanol or sucrose. We analyzed cell growth on minimal culture medium supplemented with various fatty acids (carbon chain length ranging from one to ten) to investigate which fatty acids are metabolized by C. reinhardtii. Among them, acetate caused the greatest increase in growth when added to minimal culture media. We analyzed the transcript levels of genes encoding putative glyoxysomal enzymes. The transcript levels of genes encoding malate synthase, malate dehydrogenase, isocitrate lyase, and citrate synthase increased when Chlamydomonas cells were grown on minimal culture medium supplemented with acetate. Our results suggest that Chlamydomonas peroxisomes are involved in acetate metabolism via the glyoxylate cycle.

Catabolite repression of chloroplast development in Euglena

Two-dimensional gel electrophoresis resolved total cellular protein from Euglena gracilis Klebs var. bacillaris Cori into 640 polypeptides detectable by silver staining. The addition of 84 mM ethanol to dark-grown resting carbon-starved cells increased the relative amounts of 6 polypeptides and decreased the relative amounts of 3 polypeptides. The addition of 84 mM malate to resting cells increased the relative amounts of 3 of the ethanol-induced polypeptides, suggesting that the induction of these polypeptides represents a generalized response to the provision of a utilizable carbon source, a nutritional shift up, rather than a specific response to ethanol addition. Exposure of dark-grown resting Euglena to light increases the relative amounts of 79 polypeptides encoded by the nuclear as well as the chloroplast genome and decreases the relative amounts of 72 polypeptides. Ethanol but not malate specifically inhibited all of the light-dependent changes in polypeptide levels, indicating that chloroplast development in Euglena is a catabolite-sensitive process.

Molecular characterization of a bifunctional glyoxylate cycle enzyme, malate synthase/isocitrate lyase, in Euglena gracilis

Euglena gracilis induced glyoxylate cycle enzymes when ethanol was fed as a sole carbon source. We purified, cloned and characterized a bifunctional glyoxylate cycle enzyme from E. gracilis (EgGCE). This enzyme consists of an N-terminal malate synthase (MS) domain fused to a C-terminal isocitrate lyase (ICL) domain in a single polypeptide chain. This domain order is inverted compared to the bifunctional glyoxylate cycle enzyme in Caenorhabditis elegans, an N-terminal ICL domain fused to a C-terminal MS domain. Purified EgGCE catalyzed the sequential ICL and MS reactions. ICL activity of purified EgGCE increased in the existence of acetyl-CoA at a concentration of micro-molar order. We discussed the physiological roles of the bifunctional glyoxylate cycle enzyme in these organisms as well as its molecular evolution.

Presence of glyoxylate cycle enzymes in the mitochondria of Euglena gracilis

Isocitrate lyase and malate synthase are specific enzymes of the glyoxylate cycle, used here as glyoxysomal markers. Both enzymes were found in the mitochondrial fraction after organelle fractionation by isopycnic centrifugation. Electron microscopy of this fraction indicated that mitochondria were the only recognizable organelles. Using an immunogold labeling method with anti-(malate synthase) antiserum, the only organelles stained in cells were the mitochondria. These results show that the glyoxylate cycle is present in mitochondria in Euglena.

Initial observations on the role of glutathione peroxidases in Euglena

The algae Euglena gracilis possesses two glutathione (GSH) peroxidase: a GSH peroxidase that reduces organic hydroperoxides as well as hydrogen peroxide (GSH peroxidase 1); and a GSH peroxidase associated with GSH transferase that is active only with organic hydroperoxide substrates (GSH peroxidase 2). Preliminary experiments with Euglena were conducted to explore the in vivo role of the GSH peroxidases. The enzymes were not induced in response to the stimulation of cellular processes that generate oxidant species, such as beta-oxidation or photosynthesis. The levels of GSH peroxidase 1 were approximately twofold higher in autotrophic cultures containing the herbicide DCMU. GSH peroxidase 1 was most active in stationary phase cells; while the levels of GSH peroxidase 2 were fairly constant throughout growth. Under conditions where lipid peroxidation was induced in Euglena, the addition of either GSH peroxidase plus GSH reduced the lipid peroxide levels more than tenfold.

Distribution of glyoxylate-cycle enzymes between microbodies and mitochondria in Aspergillus tamarii

The distribution of glyoxylate-cycle enzymes between microbodies and mitochondria was examined in ethanol-grown Aspergillus tamarii Kita. Particulate activities of catalase and the two glyoxylate by-pass enzymes, malate synthase and isocitrate lyase, were localized in the microbodies. The microbodies had a buoyant density of about 1.23 g cm(-3) after isopycnic centrifugation in linear sucrose gradients. Particulate activities of the other two glyoxycitrate synthase, together with that of succinate dehydrogenase were restricted to the mitochondria, which had a buoyant density of about 1.20 g cm(-3). Catalase also appeared to be localized in a second particle, perhaps the microbody inclusions or the Woronin bodies, having a buoyant density of about 1.26 g cm(-3).

[Cytochemical evidence of catalase in microbodies in Acetabularia mediterranea]

Direct evidence for the occurrence of microbodies in the marine green algae Acetabularia mediterranea is presented. The microbodies were characterized by means of their marker-enzyme catalase, (EC 1.11.1.6), the presence of which was demonstrated by electron microscopic cytochemical techniques. The function of microbodies in the metabolism of the Acetabularia cell is discussed.

The localization of glycollate-pathway enzymes in Euglena

Isolation of organelles from broken-cell suspensions of phototrophically grown Euglena gracilis Klebs was achieved by isopycnic centrifugation on sucrose gradients. 2. Equilibrium densities of 1.23g/cm3 for peroxisome-like particles, 1.22g/cm3 for mitochondria and 1.17g/cm3 for chloroplasts were recorded. 3. The enzymes glycollate dehydrogenase, glutamate-glyoxylate aminotransferase, serineglyoxylate aminotransferase, aspartate-alpha-oxoglutarate aminotransferase, hydroxy pyruvate reductase and malate dehydrogenase were present in peroxisome-like particles. 4. Unlike higher plants glycollate dehydrogenase and glutamate-glyoxylate aminotransferase were present in the mitochondria of Euglena. 5. Rates of glycollate and D-lactate oxidation were additive in the mitochondria, and, although glycollate dehydrogenase was inhibited by cyanide, D-lactate dehydrogenase activity was unaffected. 6. Glycollate oxidation was linked to O2 uptake in mitochondria but not in peroxisome-like particles. This glycollate-dependent O2 uptake was inhibited by antimycin A or cyanide. 7. The physiological significance of glycollate metabolism in Euglena mitochondria is discussed, with special reference to its role in photorespiration in algae.

[Distribution of microbody enzymes from Chlamydomonas on sucrose gradients]

The crude homogenate of cells from Chlamydomonas reinhardii was placed on a linear gradient from 30% to 60% sucrose and centrifuged for 4 hours at 60 000 g. After fractionation of the gradient the distribution of enzymes was determined. Hydroxypyruvate reductase and glycolate dehydrogenase, two markers for peroxisomes, appeared with one sharp peak at density 1.185 g/cm(3) within the gradient. Twenty-five percent of the hydroxypyruvate reductase was particulate and 75% was found as solubles in the top fractions. The peaks of both enzymes matched exactly the peak of the cytochrome oxidase which is a marker for mitochondria. The profile for malate dehydrogenase was the same as that for hydroxypyruvate reductase. Catalase, however, showed two peaks. One coincided with the peak of cytochrome oxidase and the other appeared at density 1.22 g/cm(3). More than 50% of the catalase moved into the gradient during centrifugation.

Changes in microbody-marker enzymes during growth of Tetrahymena pyriformis E

Marked differences in the rates of synthesis of several microbody (peroxisome) enzymes have been observed during the growth of Tetrahymena pyriformis (strain E) in shaking or static peptone medium. The specific activities of catalase and NADP + -isocitrate dehydrogenase, and of the mitochondrial enzyme fumarase, remained virtually constant during exponential growth under all conditions examined, whereas that of glycollate oxidase decreased dramatically in the course of growth. In contrast, isocitrate lyase and malate synthase were synthesized at increasing rates during growth on peptone medium with or without added acetate; highest specific activities were reached in late exponential to early stationary phase. The synthesis of these two enzymes, which appears to be coordinately regulated, was repressed in medium supplemented with glucose. Despite these variations in specific activity, glycollate oxidase as well as the key enzymes of the glyoxylate cycle remained associated with microbodies under all growth conditions tested.

The ultrastructure and cytochemistry of microbodies in dinoflagellates

Microbodies, similar in morphology to those of higher plants and animals, have been observed in thirteen genera of dinoflagellates. As in other unicellular algae, they did not show any staining after incubation in DAB/H2O2 medium although the mitochondrial cristae and chloroplast lamellae did give a positive reaction. The microbodies of Scrippsiella sweeneyae were found to be associated with a membranous reticulate structure, which may be involved in their formation.