Characterization of acetohydroxyacid synthase from the hyperthermophilic bacterium Thermotoga maritima

Acetohydroxyacid synthase (AHAS) is the key enzyme in branched chain amino acid biosynthesis pathway. The enzyme activity and properties of a highly thermostable AHAS from the hyperthermophilic bacterium Thermotoga maritima is being reported. The catalytic and regulatory subunits of AHAS from T. maritima were over-expressed in Escherichia coli. The recombinant subunits were purified using a simplified procedure including a heat-treatment step followed by chromatography. A discontinuous colorimetric assay method was optimized and used to determine the kinetic parameters. AHAS activity was determined to be present in several Thermotogales including T. maritima. The catalytic subunit of T. maritima AHAS was purified approximately 30-fold, with an AHAS activity of approximately 160±27 U/mg and native molecular mass of 156±6 kDa. The regulatory subunit was purified to homogeneity and showed no catalytic activity as expected. The optimum pH and temperature for AHAS activity were 7.0 and 85 °C, respectively. The apparent K_m and V_max for pyruvate were 16.4±2 mM and 246±7 U/mg, respectively. Reconstitution of the catalytic and regulatory subunits led to increased AHAS activity. This is the first report on characterization of an isoleucine, leucine, and valine operon (ilv operon) enzyme from a hyperthermophilic microorganism and may contribute to our understanding of the physiological pathways in Thermotogales. The enzyme represents the most active and thermostable AHAS reported so far.     

Fructose-1,6-bisphosphatase from a hyper-thermophilic bacterium Thermotoga maritima: Characterization,metabolite stability,and its implications

Fructose-1,6-bisphosphatase gene from a hyperthermophilic bacterium Thermotoga maritima was cloned, and the recombinant protein was produced in E. coli, purified, and characterized. The fructose-1,6-bisphosphatase (FBPase) with a molecular mass of ca. 28 kDa was purified from the fusion protein cellulose-binding module (CBM)-intein-FBPase by affinity adsorption on regenerated amorphous cellulose followed by intein self-cleavage. The substrate fructose 1,6-bisphosphate was not stable at high temperatures, especially at high pHs. The degradation constants of fructose 1,6-bisphosphate, glucose-6-phosphate, and fructose-6-phosphate were determined at different temperatures (37, 60, and 80 °C) and pH 7.5 or 9.0. The k_cat and K_m values of FBPase were 8.57 s⁻¹ and 0.04 mM at 60 °C, as well as 58.7 s⁻¹ and 0.12 mM at 80 °C. This enzyme was very stable at its suboptimal temperatures, with half-life times of ca. 1330 and 55.6 h at 60 and 80 °C, respectively. At 60 °C, this enzyme had an estimated total turn-over number of 20,500,000 (mol product/mol enzyme) and weight-based total turn-over umber of 192,000 (kg product/kg enzyme), respectively. These results indicated that this enzyme would be a stable building block for cell-free synthetic pathway biotransformation (SyPaB) that can implement complicated biochemical reactions. In order to obtain high-yield desired products, we suggest that over-addition or over-expression of the enzymes responsible for converting easily degraded metabolites should be important to prevent unnecessary metabolite loss for in vitro or in vivo synthetic pathway design.     

Biochemical indicators of salt stress in Plantago maritima: Implications for environmental stress assessment

Our study is focused on native spontaneous species of saline ecosystems Plantago maritima. Plants were cultivated at several salt concentrations (0, 50, 100, 200, 300, 400 and 500 mM NaCl) in a glass greenhouse under semi-controlled conditions. Growth parameters, water parameters and ionic status were determined and they were used as criteria to assess the response of P. maritima under a salinity gradient. Catalase, guaiacaol and ascobate peroxidase activities, total protein and proline were also determined. Our results show that P. maritima is a facultative halophyte capable of expressing its maximum growth potential at relatively low concentrations of salt (less than 3 g l⁻¹ NaCl). At high doses of salt (concentrations > 200 mM), the decrease in the growth of P. maritima is associated to a decrease in the uptake of K⁺. There is a disruption of the water intake of their organs and therefore results an invasion of the cytoplasm by Na⁺ toxic ion. However, stressed plants use K⁺ more sparingly. They invest especially in the production of biomass expressed by the dry weight of the shoots, and they use Na⁺ and proline for osmotic adjustment. The halophyte studied is able to accumulate high levels of proline in response to increasing salt concentration. The accumulation of the amino compound, mainly in roots, is interpreted as an indicator of salt tolerance. Additionally, a significant correlation between the tolerance of the plants to salinity and the activity of several antioxidant enzymes has been observed. Hence, we suggest the possibility of using these activities as a biochemical indicator for salt tolerance in P. maritima. Our study points out two types of biomarkers of salt exposure: enzymatic biomarkers in the leaves and proline content in the roots. Both did show very good correlation with salt exposure, and thus may be considered good biomarkers of exposure with a very good dose–response relationship.     

Pyruvate decarboxylase activity of the acetohydroxyacid synthase of Thermotoga maritima

Acetohydroxyacid synthase (AHAS) catalyzes the production of acetolactate from pyruvate. The enzyme from the hyperthermophilic bacterium Thermotoga maritima has been purified and characterized (k_cat ~100 s^?1). It was found that the same enzyme also had the ability to catalyze the production of acetaldehyde and CO₂ from pyruvate, an activity of pyruvate decarboxylase (PDC) at a rate approximately 10% of its AHAS activity. Compared to the catalytic subunit, reconstitution of the individually expressed and purified catalytic and regulatory subunits of the AHAS stimulated both activities of PDC and AHAS. Both activities had similar pH and temperature profiles with an optimal pH of 7.0 and temperature of 85 °C. The enzyme kinetic parameters were determined, however, it showed a non-Michaelis-Menten kinetics for pyruvate only. This is the first report on the PDC activity of an AHAS and the second bifunctional enzyme that might be involved in the production of ethanol from pyruvate in hyperthermophilic microorganisms.     

The structure of putative N-acetyl glutamate kinase from Thermus thermophilus reveals an intermediate active site conformation of the enzyme

The de novo biosynthesis of arginine in microorganisms and plants is accomplished via several enzymatic steps. The enzyme N-acetyl glutamate kinase (NAGK) catalyzes the phosphorylation of the γ-COO^? group of N-acetyl-l-glutamate (NAG) by adenosine triphosphate (ATP) which is the second rate limiting step in arginine biosynthesis pathway. Here we report the crystal structure of putative N-acetyl glutamate kinase (NAGK) from Thermus thermophilus HB8 (TtNAGK) determined at 1.92 Å resolution. The structural analysis of TtNAGK suggests that the dimeric quaternary state of the enzyme and arginine insensitive nature are similar to mesophilic Escherichia coli NAGK. These features are significantly different from its thermophilic homolog Thermatoga maritima NAGK which is hexameric and arginine-sensitive. TtNAGK is devoid of its substrates but contains two sulfates at the active site. Very interestingly the active site of the enzyme adopts a conformation which is not completely open or closed and likely represents an intermediate stage in the catalytic cycle unlike its structural homologs, which all exist either in the open or closed conformation. Engineering arginine biosynthesis pathway enzymes for the production of l-arginine is an important industrial application. The structural comparison of TtNAGK with EcNAGK revealed the structural basis of thermostability of TtNAGK and this information could be very useful to generate mutants of NAGK with increased overall stability.     

The Gram-negative bacterium Azotobacter chroococcum NCIMB 8003 employs a new glycoside hydrolase family 70 4,6-α-glucanotransferase enzyme (GtfD) to synthesize a reuteran like polymer from maltodextrins and starch

BackgroundOriginally the glycoside hydrolase (GH) family 70 only comprised glucansucrases of lactic acid bacteria which synthesize α-glucan polymers from sucrose. Recently we have identified 2 novel subfamilies of GH70 enzymes represented by the Lactobacillus reuteri 121 GtfB and the Exiguobacterium sibiricum 255-15 GtfC enzymes. Both enzymes catalyze the cleavage of (α1 → 4) linkages in maltodextrin/starch and the synthesis of consecutive (α1 → 6) linkages. Here we describe a novel GH70 enzyme from the nitrogen-fixing Gram-negative bacterium Azotobacter chroococcum, designated as GtfD.MethodsThe purified recombinant GtfD enzyme was biochemically characterized using the amylose-staining assay and its products were identified using profiling chromatographic techniques (TLC and HPAEC-PAD). Glucans produced by the GtfD enzyme were analyzed by HPSEC-MALLS-RI, methylation analysis, 1D/2D [1]H/[13]C NMR spectroscopy and enzymatic degradation studies.ResultsThe A. chroococcum GtfD is closely related to GtfC enzymes, sharing the same non-permuted domain organization also found in GH13 enzymes and displaying 4,6-α-glucanotransferase activity. However, the GtfD enzyme is unable to synthesize consecutive (α1 → 6) glucosidic bonds. Instead, it forms a high molecular mass and branched α-glucan with alternating (α1 → 4) and (α1 → 6) linkages from amylose/starch, highly similar to the reuteran polymer synthesized by the L. reuteri GtfA glucansucrase from sucrose.ConclusionsIn view of its origin and specificity, the GtfD enzyme represents a unique evolutionary intermediate between family GH13 (α-amylase) and GH70 (glucansucrase) enzymes.General significanceThis study expands the natural repertoire of starch-converting enzymes providing the first characterization of an enzyme that converts starch into a reuteran-like α-glucan polymer, regarded as a health promoting food ingredient.     

Dihydroxyacetone production in an engineered Escherichia coli through expression of Corynebacterium glutamicum dihydroxyacetone phosphate dephosphorylase

Dihydroxyacetone (DHA) has several industrial applications such as a tanning agent in tanning lotions in the cosmetic industry; its production via microbial fermentation would present a more sustainable option for the future. Here we genetically engineered Escherichia coli (E. coli) for DHA production from glucose. Deletion of E. coli triose phosphate isomerase (tpiA) gene was carried out to accumulate dihydroxyacetone phosphate (DHAP), for use as the main intermediate or precursor for DHA production. The accumulated DHAP was then converted to DHA through the heterologous expression of Corynebacterium glutamicum DHAP dephosphorylase (cghdpA) gene. To conserve DHAP exclusively for DHA production we removed methylglyoxal synthase (mgsA) gene in the ΔtpiA strain. This drastically improved DHA production from 0.83 g/l (0.06 g DHA/g glucose) in the ΔtpiA strain bearing cghdpA to 5.84 g/l (0.41 g DHA/g glucose) in the ΔtpiAΔmgsA double mutant containing the same gene. To limit the conversion of intracellular DHA to glycerol, glycerol dehydrogenase (gldA) gene was further knocked out resulting in a ΔtpiAΔmgsAΔgldA triple mutant. This triple mutant expressing the cghdpA gene produced 6.60 g/l of DHA at 87% of the maximum theoretical yield. In summary, we demonstrated an efficient system for DHA production in genetically engineered E. coli strain.     

Characterization of α-phosphoglucomutase isozymes from Toxoplasma gondii

The Toxoplasma gondii genome project has revealed two putative isoforms (TgPGM-I and TgPGM-II) of α-phosphoglucomutase (EC 5.4.2.2). We obtained recombinant proteins of these isoforms from the Beverley strain of T. gondii and characterized their properties, particularly the kinetic properties of these isoforms. The specific activities of TgPGM-I and TgPGM-II for α-d-glucose 1-phosphate were 338 ± 9 and 84 ± 6 μmol/min/mg protein, respectively, at 37 °C under optimal conditions. The Kcat and Km values of TgPGM-I were 398 ± 11/s and 0.19 ± 0.03 mM and those for TgPGM-II were 93 ± 7/s and 3.53 ± 0.91 mM, respectively, for α-d-glucose 1-phosphate. Magnesium ions were the most effective divalent cations for both the enzyme activities. The maximum activities of both the enzymes were obtained in the presence of more than 0.2 mM α-d-glucose 1,6-bisphosphate. Although both enzymes were attached to the α-phosphohexomutase superfamily, amino acid sequence homology between TgPGM-I and TgPGM-II showed very low overall identity (25%). No α-phosphomannomutase (EC 5.4.2.8) activity was detected for either enzyme. The data indicated that TgPGM-I, but not TgPGM-II, may play an important role in α-d-glucose 6-phosphate production.     

Studies on the expression of recombinant fuculose-1-phosphate aldolase in E. coli

Fuculose-1-phosphate aldolase (Fuc-1-PA) is a dihydroxyacetone phosphate (DHAP) dependent aldolase with potential application in chiral synthesis. The influence of the growth medium on the expression of the enzyme in Escherichia coli has been studied. Complex LB medium, a defined medium (MD) and a semi-complex medium (MSC) have been compared in order to maximize aldolase production. The defined medium produced highest expression levels (700 activity units (AU)/g of dry cell weight (DCW)). The optimal induced isopropyl-β-thiogalactopyranoside (IPTG) concentration of 100 μM produces in the MD medium of 41 μmol/g dry cell weight of enzyme.     

Properties and mechanism of d-glucosaminate-6-phosphate ammonia-lyase: An aminotransferase family enzyme with d-amino acid specificity

Salmonella enterica serovar Typhimurium utilizes a wide range of growth substrates, some of which are relatively novel. One of these unusual substrates is d-glucosaminate, which is metabolized by the enzymes encoded in the dga operon. d-Glucosaminate is transported and converted to d-glucosaminate-6-phosphate (G6P) by a phosphotransferase system, composed of DgaABCD. The protein product of dgaE, d-glucosaminate-6-phosphate ammonia lyase (DGL), converts G6P to 2-keto-3-deoxygluconate-6-phosphate, which undergoes a retroaldol reaction catalyzed by the DgaF protein to give d-glyceraldehyde-3-phosphate and pyruvate. We have now developed an improved synthesis of G6P which gives a higher yield. The DGL reaction is of mechanistic interest because it is one of only a few enzymes in the pyridoxal-5′-phosphate (PLP) dependent aminotransferase superfamily known to catalyze reaction of a d-amino acid substrate. The pH dependence of DGL shows an optimum at 7.5–8.5, suggesting a requirement for a catalytic base. α-Glycerophosphate and inorganic phosphate are weak competitive inhibitors, with K_i values near 30 mM, and d-serine is neither a substrate nor an inhibitor. We have found in rapid-scanning stopped-flow experiments that DGL reacts rapidly with its substrate to form a quinonoid intermediate with λ_max = 480 nm, within the dead time (ca. 2 msec), which then rapidly decays (k = 279 s^− 1) to an intermediate with absorption between 330 and 350 nm, probably an aminoacrylate complex. We suggest a mechanism for DGL and propose that the unusual stereochemistry of the DGL reaction requires a catalytic base poised on the opposite face of the PLP-substrate complex from the other members of the aminotransferase superfamily.     

Gallic acid interference on polyphenolic amperometric biosensing using Trametes versicolor laccase

The present work reports the gallic acid (GA) interference on polyphenolic amperometric biosensing using Trametes versicolor laccase (TvLac). GA′ inhibitory effect on TvLac activity was investigated on the oxidation of caffeic acid (CA) by free TvLac and its immobilised form on modified polyethersulfone membrane (PES/TvLac), using spectrophotometric and amperometric biosensor detection methods. The results have indicated that GA presents inhibitory behaviour on TvLac activity in a concentration-dependent manner. The GA concentration leading to 50% activity lost, IC₅₀, was determined to be 19.15 ± 0.11 μM and 5.11 ± 0.19 μM for free and immobilised enzyme, respectively. The results have also shown that GA exhibited a competitive and a mixed inhibition types on the TvLac activity for spectrophotometric and amperometric biosensor methods, respectively. Further GA′ and CA′ cyclic voltammetry studies have demonstrated that GA′ oxidation products interfered with CA′ redox reaction products. In fact, a decrease of the reduction current was observed at cyclic voltammograms of CA, when mixed with GA. Therefore, the GA′ interference on polyphenolic amperometric biosensing is the result of the combination of two factors: on one hand, we have the inhibitory enzymatic effect, and on the other, the reaction of GA′ oxidation products with the o-quinones obtained by the enzymatic oxidation of CA. Both gave rise to the amperometric signal decreasing effect.     

Enhancing GDP-fucose production in recombinant Escherichia coli by metabolic pathway engineering

Guanosine 5′-diphosphate (GDP)-fucose is the indispensible donor substrate for fucosyltransferase-catalyzed synthesis of fucose-containing biomolecules, which have been found involving in various biological functions. In this work, the salvage pathway for GDP-fucose biosynthesis from Bacterioides fragilis was introduced into Escherichia coli. Besides, the biosynthesis of guanosine 5′-triphosphate (GTP), an essential substrate for GDP-fucose biosynthesis, was enhanced via overexpression of enzymes involved in the salvage pathway of GTP biosynthesis. The production capacities of metabolically engineered strains bearing different combinations of recombinant enzymes were compared. The shake flask fermentation of the strain expressing Fkp, Gpt, Gmk and Ndk obtained the maximum GDP-fucose content of 4.6 ± 0.22 μmol/g (dry cell mass), which is 4.2 fold that of the strain only expressing Fkp. Through fed-batch fermentation, the GDP-fucose content further rose to 6.6 ± 0.14 μmol/g (dry cell mass). In addition to a better productivity than previous fermentation processes based on the de novo pathway for GDP-fucose biosynthesis, the established schemes in this work also have the advantage to be a potential avenue to GDP-fucose analogs encompassing chemical modification on the fucose residue.     

Synthesis and biological evaluation of novel inhibitors against 1,3,8-trihydroxynaphthalene reductase from Magnaporthe grisea

1,3,8-Trihydroxynaphthalene reductase (3HNR) is an essential enzymes that is involved in fungal melanin biosynthesis. Based on the structural informations of active site of 3HNR, a series of β-nitrostyrene compounds were rationally designed and synthesized. The enzymatic activities of these compounds showed that most of them exhibited high inhibitory activities (<5.0 μM) against 3HNR; compound 3-2 exhibit the highest inhibitory activity (IC₅₀ = 0.29 μM). In particular, some of these compounds had moderate fungicidal activity against Magnaporthe grisea. Compound 3-4 showed high in vivo activities against M. grisea (EC₅₀ = 9.5 ppm). Furthermore, compound 3-2 was selected as a representative molecule, and the probable binding mode of this compound and the surrounding residues in the active site of 3HNR was elucidated by using molecular dock. The positive results suggest that β-nitrostyrene derivatives are most likely to be promising leads toward the discovery of novel agent of rice blast.     

Engineering of an N-acetylneuraminic acid synthetic pathway in Escherichia coli

N-acetylneuraminic acid (NeuAc) has recently drawn much attention owing to its wide applications in many aspects. Besides extraction from natural materials, production of NeuAc was recently focused on enzymatic synthesis and whole-cell biocatalysis. In this study, we designed an artificial NeuAc biosynthetic pathway through intermediate N-acetylglucosamine 6-phosphate in Escherichia coli. In this pathway, N-acetylglucosamine 2-epimerase (slr1975) and glucosamine-6-phosphate acetyltransferase (GNA1) were heterologously introduced into E. coli from Synechocystis sp. PCC6803 and Saccharomyces cerevisiae EBY100, respectively. By derepressing the feedback inhibition of glucosamine-6-phosphate synthase, increasing the accumulation of N-acetylglucosamine and pyruvate, and blocking the catabolism of NeuAc, we were able to produce 1.62 g l^?1 NeuAc in recombinant E. coli directly from glucose. The NeuAc yield reached 7.85 g l^?1 in fed-batch fermentation. This process offered an efficient fermentative method to produce NeuAc in microorganisms using glucose as carbon source and can be optimized for further improvement.     

Antitrypanosomal alkaloids from Polyalthia suaveolens (Annonaceae): Their effects on three selected glycolytic enzymes of Trypanosoma brucei

In continuation of our study on medicinal plants of Cameroon, stem barks of Polyalthia suaveolens were phytochemically studied. This investigation yielded a new indolosesquiterpene alkaloid, named polysin (1) and four hitherto known alkaloids (2–5). Polysin (1) appeared as a competitive reversible inhibitor (K_i = 10 μM) of phosphofructo kinase (PFK) of Trypanosoma brucei with respect to fructose-6-phosphate (K_i/K_M = 0.05) and could be used in the design of new trypanocidal drugs. The other isolated compounds (2–5) also exhibited interesting inhibitory effects on selected glycolytic enzymes (PFK, glyceraldehyde-3-phosphate dehydrogenase and aldolase).     

Engineering of photosynthetic mannitol biosynthesis from CO2 in a cyanobacterium

d-Mannitol (hereafter denoted mannitol) is used in the medical and food industry and is currently produced commercially by chemical hydrogenation of fructose or by extraction from seaweed. Here, the marine cyanobacterium Synechococcus sp. PCC 7002 was genetically modified to photosynthetically produce mannitol from CO₂ as the sole carbon source. Two codon-optimized genes, mannitol-1-phosphate dehydrogenase (mtlD) from Escherichia coli and mannitol-1-phosphatase (mlp) from the protozoan chicken parasite Eimeria tenella, in combination encoding a biosynthetic pathway from fructose-6-phosphate to mannitol, were expressed in the cyanobacterium resulting in accumulation of mannitol in the cells and in the culture medium. The mannitol biosynthetic genes were expressed from a single synthetic operon inserted into the cyanobacterial chromosome by homologous recombination. The mannitol biosynthesis operon was constructed using a novel uracil-specific excision reagent (USER)-based polycistronic expression system characterized by ligase-independent, directional cloning of the protein-encoding genes such that the insertion site was regenerated after each cloning step. Genetic inactivation of glycogen biosynthesis increased the yield of mannitol presumably by redirecting the metabolic flux to mannitol under conditions where glycogen normally accumulates. A total mannitol yield equivalent to 10% of cell dry weight was obtained in cell cultures synthesizing glycogen while the yield increased to 32% of cell dry weight in cell cultures deficient in glycogen synthesis; in both cases about 75% of the mannitol was released from the cells into the culture medium by an unknown mechanism. The highest productivity was obtained in a glycogen synthase deficient culture that after 12 days showed a mannitol concentration of 1.1 g mannitol L⁻¹ and a production rate of 0.15 g mannitol L⁻¹ day⁻¹. This system may be useful for biosynthesis of valuable sugars and sugar derivatives from CO₂ in cyanobacteria.