General Biochemical Characterization of Thermostable Pullulanase and Glucoamylase from Clostridium thermohydrosulfuricum

Cell extracts of Clostridium thermohydrosulfuricum, an anaerobic bacterium which ferments starch into ethanol at 65°C, contained both pullulanase and glucoamylase activities. The general physiochemical and catalytic properties of these enzyme activities were compared. Pullulanase and glucoamylase activities were stable and optimally active at 85 and 75°C, respectively. The pH optima for activity and pH stability ranges were, respectively, 5.5 to 6 and 4.5 to 5.5 for pullulanase and 4 to 6 and 5 to 6 for glucoamylase. The apparent [S]_0.5v and V_max for pullulanase activity on pullulan were 0.33 mg/ml and 2.6 U/mg of protein. The apparent [S]_0.5v and V_max for glucoamylase activity on starch were of 0.41 mg/ml and 0.31 U/mg of protein. These enzymes were active and stable in the presence of air or 10% (vol/vol) ethanol. These enzyme activities allowed the organism to actively degrade raw starch into glucose in the absence of significant α-amylase activity.     

Continuous Production of Thermostable beta-Amylase with Clostridium thermosulfurogenes: Effect of Culture Conditions and Metabolite Levels on Enzyme Synthesis and Activity

A beta-amylase-overproducing mutant of Clostridium thermosulfurogenes was grown in continuous culture on soluble starch to produce thermostable beta-amylase. Enzyme productivity was reasonably stable over periods of weeks to months. The pH and temperature optima for beta-amylase production were pH 6.0 and 60 degrees C, respectively. Enzyme concentration was maximized by increasing biomass concentration by using high substrate concentrations and by maintaining a low growth rate. beta-Amylase concentration reached 90 U ml at a dilution rate of 0.07 h in a 3% starch medium. A further increase in enzyme activity levels was limited by acetic acid inhibition of growth and low beta-amylase productivity at low growth rates.     

Partial Purification and Characterization of a Thermostable Actinomycete beta-Amylase

A thermostable amylase, possibly a beta-amylase from Thermoactinomyces sp. no. 2 isolated from soil, is reported. The enzyme was purified 36-fold by acetone precipitation, ion-exchange chromatography, and Sephadex G-200 gel filtration, and the molecular weight was estimated at 31,600. The enzyme was characterized by demonstration of optimum activity at 60 degrees C and pH 7 and by retention of 70% activity at 70 degrees C (30 min). It was stimulated by Mn and Fe but strongly inhibited by Hg. Maltose was the only detectable product of hydrolysis of starches and was quantitatively highest in plantain starch hydrolysate.     

Simultaneous and Enhanced Production of Thermostable Amylases and Ethanol from Starch by Cocultures of Clostridium thermosulfurogenes and Clostridium thermohydrosulfuricum

Clostridium thermohydrosulfuricum and Clostridium thermosulfurogenes produced ethanol and amylases with different components as primary metabolites of starch fermentation. Starch fermentation parameters were compared in mono- and cocultures of these two thermoanaerobes to show that the fermentation was dramatically improved as a consequence of coordinate action of amylolytic enzymes and synergistic metabolic interactions between the two species. Under given monoculture fermentation conditions, neither species completely degraded starch during the time course of the study, whereas in coculture, starch was completely degraded. In monoculture starch fermentation, C. thermohydrosulfuricum produced lower levels of pullulanase and glucoamylase, whereas C. thermosulfurogenes produced lower levels of β-amylase and glucoamylase. In coculture fermentation, improvement of starch metabolism by each species was noted in terms of increased amounts and rates of increased starch consumption, amylase production, and ethanol formation. The single-step coculture fermentation completely degraded 2.5% starch in 30 h at 60°C and produced 9 U of β-amylase per ml, 1.3 U of pullulanase per ml, 0.3 U of glucoamylase per ml, and >120 mM ethanol with a yield of 1.7 mol/mol of glucose in starch. The potential industrial applications of the coculture fermentation and the physiological basis for the interspecies metabolic interactions are discussed.     

Differential amylosaccharide metabolism of Clostridium thermosulfurogenes and Clostridium thermohydrosulfuricum.

Clostridium thermosulfurogenes displayed faster growth on either glucose, maltose, or starch than Clostridium thermohydrosulfuricum. Both species grew faster on glucose than on starch or maltose. The fermentation end product ratios were altered based on higher ethanol and lactate yields on starch than on glucose. In C. thermohydrosulfuricum, glucoamylase, pullulanase, and maltase were mainly responsible for conversion of starch and maltose into glucose, which was accumulated by a putative glucose permease. In C. thermosulfurogenes, beta-amylase was primarily responsible for degradation of starch to maltose, which was accumulated by a putative maltose permease and then hydrolyzed by glucoamylase. Regardless of the growth substrate, the rates of glucose, maltose, and starch transformation were higher in C. thermosulfurogenes than in C. thermohydrosulfuricum. Both species had a functional Embden-Meyerhof glycolytic pathway and displayed the following catabolic activities: ferredoxin-linked pyruvate dehydrogenase, acetate kinase, NAD(P)-ethanol dehydrogenase, NAD(P)-ferredoxin oxidoreductase, hydrogenase, and fructose-1,6-diphosphate-activated lactate dehydrogenase. Ferredoxin-NAD reductase activity was higher in C. thermohydrosulfuricum than NADH-ferredoxin oxidase activity, but the former activity was not detectable in C. thermosulfurogenes. Both NAD- and NADP-linked ethanol dehydrogenases were unidirectional in C. thermosulfurogenes but reversible in C. thermohydrosulfuricum. The ratio of hydrogen-producing hydrogenase to hydrogen-consuming hydrogenase was higher in C. thermosulfurogenes. Two biochemical models are proposed to explain the differential saccharide metabolism on the basis of species enzyme differences in relation to specific growth substrates.     

Raw starch adsorption-desorption purification of a thermostable beta-amylase from Clostridium thermosulfurogenes

The beta-amylase from Clostridium thermosulfurogenes was readily adsorbed onto raw starch. The adsorbed beta-amylase was eluted from raw starch by using boiled soluble starch solution as an elutant. The soluble starch treated beta-amylase could not adsorb onto raw starch which indicates that the soluble and insoluble substrate binding sites of the beta-amylase may be the same. The beta-amylase was purified to homogeneity by raw starch adsorption-desorption techniques and octyl-Sepharose chromatography. It had a specific activity of 4188 units/mg protein. The insoluble substrate adsorption-desorption technique may be used for the purification of other enzymes.     

Thermostable Amylolytic Enzymes from a New Clostridium Isolate

A new Clostridium strain was isolated on starch at 60°C. Starch, pullulan, maltotriose, and maltose induced the synthesis of α-amylase and pullulanase, while glucose, ribose, fructose, and lactose did not. The formation of the amylolytic enzymes was dependent on growth and occurred predominantly in the exponential phase. The enzymes were largely cell bound during growth of the organism with 0.5% starch, but an increase of the starch concentration in the growth medium was accompanied by the excretion of α-amylase and pullulanase into the culture broth; but also by a decrease of total activity. α-Amylase, pullulanase, and α-glucosidase were active in a broad temperature range (40 to 85°C) and displayed temperature optima for activity at 60 to 70°C. During incubation with starch under aerobic conditions at 75°C for 2 h, the activity of both enzymes decreased to only 90 or 80%. The apparent K_m values of α-amylase, pullulanase, and α-glucosidase for their corresponding substrates, starch, pullulan, and maltose were 0.35 mg/ml, 0.63 mg/ml, and 25 mM, respectively.     

Comparative Characterization of Extracellular and Intracellular Hydrocarbons of Clostridium pasteurianum

Extracellular and intracellular hydrocarbons produced by Clostridium pasteurianum VKM 1774 during cultivation on glucose-containing media in an argon atmosphere or in the presence of carbon dioxide and molecular hydrogen were analyzed by gas-liquid chromatography. Intracellular hydrocarbons were 50-55% (C₂₅-C₃₅) n-alkanes. Carbon dioxide and molecular hydrogen stimulated synthesis of extracellular hydrocarbons, which comprised 90-95% (C₁₁-C₂₄) n-alkanes.     

Biochemical and Proteomic Characterization of a Novel Extracellular β-Glucosidase from Trichoderma citrinoviride

β-Glucosidases are of pivotal importance in bioconversion of carbonic biomass into fermentable and other useful metabolites, food industry, biotransformation, glyco-trimming of metabolome, etc. Trichoderma citrinoviride when grown on delignified Lantana camara produced a β-glucosidase and secreted it out in the medium. The extracellularly secreted β-glucosidase of T. citrinoviride was homogeneity purified and then characterized for its kinetic properties and proteomic characteristics. The 90 kDa enzyme was monomeric in nature, optimally active at pH 5.5 and the catalytic reaction rate was highest at 55°C. Uniquely, the enzyme was insensitive to inhibition by glucose (up to 5 mM). It also possessed catalytic ability of transglycosylation, as it could catalyze conversion of geraniol into its glucoside. MALDI-TOF assisted proteomic analysis revealed its high degree of sequence similarity with family 3 glycoside hydrolases.     

Biochemical and Genetic Characterization of an Extracellular Protease from Pseudomonas fluorescens CY091

Pseudomonas fluorescens CY091 cultures produce an extracellular protease with an estimated molecular mass of 50 kDa. Production of this enzyme (designated AprX) was observed in media containing CaCl₂ or SrCl₂ but not in media containing ZnCl₂, MgCl₂, or MnCl₂. The requirement of Ca²⁺ (or Sr²⁺) for enzyme production was concentration dependent, and the optimal concentration for production was determined to be 0.35 mM. Following ammonium sulfate precipitation and ion-exchange chromatography, the AprX in the culture supernatant was purified to near electrophoretic homogeneity. Over 20% of the enzyme activity was retained in the AprX sample which had been heated in boiling water for 10 min, indicating that the enzyme is highly resistant to heat inactivation. The enzyme activity was almost completely inhibited in the presence of 1 mM 1,10-phenanthroline, but only 30% of the activity was inhibited in the presence of 1 mM EGTA. The gene encoding AprX was cloned from the genome of P. fluorescens CY091 by isolating cosmid clones capable of restoring the protease production in a nonproteolytic mutant of strain CY091. The genomic region of strain CY091 containing the aprX gene was located within a 7.3-kb DNA fragment. Analysis of the complete nucleotide sequence of this 7.3-kb fragment revealed the presence of a cluster of genes required for the production of extracellular AprX in P. fluorescens and Escherichia coli. The AprX protein showed 50 to 60% identity in amino acid sequence to the related proteases produced by Pseudomonas aeruginosa and Erwinia chrysanthemi. Two conserved sequence domains possibly associated with Ca²⁺ and Zn²⁺ binding were identified. Immediately adjacent to the aprX structural gene, a gene (inh) encoding a putative protease inhibitor and three genes (aprD, aprE, and aprF), possibly required for the transport of AprX, were also identified. The organization of the gene cluster involved in the synthesis and secretion of AprX in P. fluorescens CY091 appears to be somewhat different from that previously demonstrated in P. aeruginosa and E. chrysanthemi.     

Phylogenetic Analysis and Biochemical Characterization of a Thermostable Dihydropyrimidinase from Alkaliphilic Bacillus sp. TS-23

Two degenerate primers established from the alignment of highly conserved amino acid sequences of bacterial dihydropyrimidinases (DHPs) were used to amplify a 330-bp gene fragment from the genomic DNA of Bacillus sp. TS-23 and the amplified DNA was successfully used as a probe to clone a dhp gene from the strain. The open reading frame of the gene consisted of 1422 bp and was deduced to contain 472 amino acids with a molecular mass of 52 kDa. The deduced amino acid sequence exhibited greater than 45% identity with that of prokaryotic d-hydantoinases and eukaryotic DHPs. Phylogenetic analysis showed that Bacillus sp. TS-23 DHP is grouped together with Bacillus stearothermophilus d-hydantoinase and related to dihydroorotases and allantoinases from various organisms. His₆-tagged DHP was over-expressed in Escherichia coli and purified by immobilized metal affinity chromatography to a specific activity of 3.46 U mg⁻¹ protein. The optimal pH and temperature for the purified enzyme were 8.0 and 60 °C, respectively. The half-life of His₆-tagged DHP was 25 days at 50 °C. The enzyme activity was stimulated by Co²⁺ and Mn²⁺ ions. His₆-tagged DHP was most active toward dihydrouracil followed by hydantoin derivatives. The catalytic efficiencies (k_cat/K_m) of the enzyme for dihydrouracil and hydantoin were 2.58 and 0.61 s⁻¹ mM⁻¹, respectively.     

Overexpression and Biochemical Characterization of a Thermostable Phytase from Bacillus subtilis US417 in Pichia pastoris

The overexpression of the native gene encoding the thermostable Bacillus subtilis US417 phytase using Pichia pastoris system is described. The phytase gene, in which the sequence encoding the signal peptide was replaced by that of the α-factor of Saccharomyces cerevisiae, was placed under the control of the methanol-inducible promoter of the alcohol oxidase 1 gene and expressed in Pichia pastoris. Small-scale expression experiments and activity assays were used to screen positive colonies. A recombinant strain was selected and produces 43 and 227 U/mL of phytase activity in shake flasks and in high-cell-density fermentation, respectively. The purified phytase was glycosylated protein and varied in size (50–65 kDa). It has a molecular mass of 43 kDa when it was deglycosylated. The purified r-PHY maintains 100 % of its activity after 10 min incubation at 75 °C and pH 7.5. This thermostable phytase, which is also active over broad pH ranges, may be useful as feed additives, since it can resist the temperature used in the feed-pelleting process.     

On-line monitoring of pullulanase production during continuous culture of Clostridium thermosulfurogenes

Summary An automated turbidimetric immunoassay for pullulanase from Clostridium thermosulfurogenes was developed for on-line bioprocess monitoring. Measurements are based on the turbidity caused by specific aggregation between pullulanase molecules present in the cultivation medium and antibodies against these enzymes. The time for one assay cycle is 3.0 min. Pullulanase concentrations between 10 units (U)/l and 1000 U/l could be measured. Standard deviations of less than 2% were found. The analysis system could be successfully employed for on-line product monitoring during a 240-h cultivation of C. thermosulfurogenes. An average correlation factor of 0.975 was obtained for the pullulanase data from on-line turbidimetric and the off-line reference assay, demonstrating the accuracy of this on-line method. Offprint requests to: Th. Scheper     

Purification and Characterization of Pea Epicotyl beta-Amylase

The most abundant β-amylase (EC 3.2.1.2) in pea (Pisum sativum L.) was purified greater than 880-fold from epicotyls of etiolated germinating seedlings by anion exchange and gel filtration chromatography, glycogen precipitation, and preparative electrophoresis. The electrophoretic mobility and relative abundance of this β-amylase are the same as that of an exoamylase previously reported to be primarily vacuolar. The enzyme was determined to be a β-amylase by end product analysis and by its inability to hydrolyze β-limit dextrin and to release dye from starch azure. Pea β-amylase is an approximate 55 to 57 kilodalton monomer with a pl of 4.35, a pH optimum of 6.0 (soluble starch substrate), an Arrhenius energy of activation of 6.28 kilocalories per mole, and a K_m of 1.67 milligrams per milliliter (soluble starch). The enzyme is strongly inhibited by heavy metals, p-chloromer-curiphenylsulfonic acid and N-ethylmaleimide, but much less strongly by iodoacetamide and iodoacetic acid, indicating cysteinyl sulfhydryls are not directly involved in catalysis. Pea β-amylase is competitively inhibited by its end product, maltose, with a K_i of 11.5 millimolar. The enzyme is partially inhibited by Schardinger maltodextrins, with α-cyclohexaamylose being a stronger inhibitor than β-cycloheptaamylose. Moderately branched glucans (e.g. amylopectin) were better substrates for pea β-amylase than less branched or non-branched (amyloses) or highly branched (glycogens) glucans. The enzyme failed to hydrolyze native starch grains from pea and glucans smaller than maltotetraose. The mechanism of pea β-amylase is the multichain type. Possible roles of pea β-amylase in cellular glucan metabolism are discussed.     

Continuous Production of Thermostable β-Amylase with Clostridium thermosulfurogenes: Effect of Culture Conditions and Metabolite Levels on Enzyme Synthesis and Activity

A β-amylase-overproducing mutant of Clostridium thermosulfurogenes was grown in continuous culture on soluble starch to produce thermostable β-amylase. Enzyme productivity was reasonably stable over periods of weeks to months. The pH and temperature optima for β-amylase production were pH 6.0 and 60°C, respectively. Enzyme concentration was maximized by increasing biomass concentration by using high substrate concentrations and by maintaining a low growth rate. β-Amylase concentration reached 90 U ml⁻¹ at a dilution rate of 0.07 h⁻¹ in a 3% starch medium. A further increase in enzyme activity levels was limited by acetic acid inhibition of growth and low β-amylase productivity at low growth rates.     

Biochemical Characterization and Structural Analysis of a Bifunctional Cellulase/Xylanase from Clostridium thermocellum

We expressed an active form of CtCel5E (a bifunctional cellulase/xylanase from Clostridium thermocellum), performed biochemical characterization, and determined its apo- and ligand-bound crystal structures. From the structures, Asn-93, His-168, His-169, Asn-208, Trp-347, and Asn-349 were shown to provide hydrogen-bonding/hydrophobic interactions with both ligands. Compared with the structures of TmCel5A, a bifunctional cellulase/mannanase homolog from Thermotoga maritima, a flexible loop region in CtCel5E is the key for discriminating substrates. Moreover, site-directed mutagenesis data confirmed that His-168 is essential for xylanase activity, and His-169 is more important for xylanase activity, whereas Asn-93, Asn-208, Tyr-270, Trp-347, and Asn-349 are critical for both activities. In contrast, F267A improves enzyme activities.     

Regulation and genetic enhancement of beta-amylase production in Clostridium thermosulfurogenes.

We studied the general mechanism for regulation of beta-amylase synthesis in Clostridium thermosulfurogenes. beta-Amylase was expressed at high levels only when the organism was grown on maltose or other carbohydrates containing maltose units. Three kinds of mutants altered in beta-amylase production were isolated by using nitrosoguanidine treatment, enrichment on 2-deoxyglucose, and selection of colonies with large clear zones on iodine-stained starch-glucose agar plates. beta-Amylase was produced only when maltose was added to cells growing on sucrose in wild-type and catabolite repression-resistant mutant strains, but the differential rate of enzyme synthesis in constitutive mutants was constant regardless of the presence of maltose. In carbon-limited chemostats of wild-type and catabolite repression-resistant mutant stains, beta-amylase was expressed on maltose but not on glucose or sucrose. beta-Amylase synthesis was immediately repressed by the addition of glucose. Therefore, we concluded that beta-amylase synthesis in C. thermosulfurogenes was inducible and subject to catabolite repression. The addition of cAMP did not eliminate the repressive effect of glucose. The mutants were generally characterized in terms of beta-amylase production, growth properties, fermentation product formation, and alterations in glucose isomerase and glucoamylase activities. A hyperproductive mutant produced eightfold more beta-amylase on starch medium than the wild type and more rapidly fermented starch to ethanol.     

Catalytic mechanism of xylose (glucose) isomerase from Clostridium thermosulfurogenes. Characterization of the structural gene and function of active site histidine

The gene coding for thermophilic xylose (glucose) isomerase of Clostridium thermosulfurogenes was isolated and its complete nucleotide sequence was determined. The structural gene (xylA) for xylose isomerase encodes a polypeptide of 439 amino acids with an estimated molecular weight of 50,474. The deduced amino acid sequence of thermophilic C. thermosulfurogenes xylose isomerase displayed higher homology with those of thermolabile xylose isomerases from Bacillus subtilis (70%) and Escherichia coli (50%) than with those of thermostable xylose isomerases from Ampullariella (22%), Arthrobacter (23%), and Streptomyces violaceoniger (24%). Several discrete regions were highly conserved throughout the amino acid sequences of all these enzymes. To identify the histidine residue of the active site and to elucidate its function during enzymatic xylose or glucose isomerization, histidine residues at four different positions in the C. thermosulfurogenes enzyme were individually modified by site-directed mutagenesis. Substitution of His101 by phenylalanine completely abolished enzyme activity whereas substitution of other histidine residues by phenylalanine had no effect on enzyme activity. When His101 was changed to glutamine, glutamic acid, asparagine, or aspartic acid, approximately 10-16% of wild-type enzyme activity was retained by the mutant enzymes. The Gln101 mutant enzyme was resistant to diethylpyrocarbonate inhibition which completely inactivated the wild-type enzyme, indicating that His101 is the only essential histidine residue involved directly in enzyme catalysis. The constant Vmax values of the Gln101, Glu101, Asn101, and Asp101 mutant enzymes over the pH range of 5.0-8.5 indicate that protonation of His101 is responsible for the reduced Vmax values of the wild-type enzyme at pH below 6.5. Deuterium isotope effects by D-[2-2H]glucose on the rate of glucose isomerization indicated that hydrogen transfer and not substrate ring opening is the rate-determining step for both the wild-type and Gln101 mutant enzymes. These results suggest that the enzymatic sugar isomerization does not involve a histidine-catalyzed proton transfer mechanism. Rather, essential histidine functions to stabilize the transition state by hydrogen bonding to the C5 hydroxyl group of the substrate and this enables a metal-catalyzed hydride shift from C2 to C1.     

Identification and Biochemical Characterization of a Thermostable Malate Dehydrogenase from the Mesophile Streptomyces coelicolor A3(2)

We identified and characterized a malate dehydrogenase from Streptomyces coelicolor A3(2) (ScMDH). The molecular mass of ScMDH was 73,353.5 Da with two 36,675.0 Da subunits as analyzed by matrix-assisted laser-desorption ionization–time-of-flight mass spectrometry (MALDI-TOF-MS). The detailed kinetic parameters of recombinant ScMDH are reported here. Heat inactivation studies showed that ScMDH was more thermostable than most MDHs from other organisms, except for a few extremely thermophile bacteria. Recombinant ScMDH was highly NAD⁺-specific and displayed about 400-fold (k _cat) and 1,050-fold (k _cat?K _m) preferences for oxaloacetate reduction over malate oxidation. Substrate inhibition studies showed that ScMDH activity was inhibited by excess oxaloacetate (K _i=5.8 mM) and excess L-malate (K _i=12.8 mM). Moreover, ScMDH activity was not affected by most metal ions, but was strongly inhibited by Fe²⁺ and Zn²⁺. Taken together, our findings indicate that ScMDH is significantly thermostable and presents a remarkably high catalytic efficiency for malate synthesis.