Induction and characterization of -galactosidase in an extreme thermophile

Application of the thermostable β-galactosidase, BgaB, from Geobacillus stearothermophilus as a versatile reporter under anaerobic and aerobic conditions

Use of thermophilic organisms has a range of advantages, but the significant lack of engineering tools limits their applications. Here we show that β-galactosidase from Geobacillus stearothermophilus (BgaB) can be applicable in a range of conditions, including different temperatures and oxygen concentrations. This protein functions both as a marker, promoting colony color development in the presence of a lactose analogue S-gal, and as a reporter enabling quantitative measurement by a simple colorimetric assay. Optimal performance was observed at 70 °C and pH 6.4. The gene was introduced into G. thermoglucosidans. The combination of BgaB expressed from promoters of varying strength with S-gal produced distinct black colonies in aerobic and anaerobic conditions at temperatures ranging from 37 to 60 °C. It showed an important advantage over the conventional β-galactosidase (LacZ) and substrate X-gal, which were inactive at high temperature and under anaerobic conditions. To demonstrate the versatility of the reporter, a promoter library was constructed by randomizing sequences around −35 and −10 regions in a wild type groES promoter from Geobacillus sp. GHH01. The library contained 28 promoter variants and encompassed fivefold variation. The experimental pipeline allowed construction and measurement of expression levels of the library in just 4 days. This β-galactosidase provides a promising tool for engineering of aerobic, anaerobic, and thermophilic production organisms such as Geobacillus species.

A multifunctional GH39 glycoside hydrolase from the anaerobic gut fungus Orpinomyces sp. strain C1A

Background. The anaerobic gut fungi (phylum Neocallimastigomycota) represent a promising source of novel lignocellulolytic enzymes. Here, we report on the cloning, expression, and characterization of a glycoside hydrolase family 39 (GH39) enzyme (Bgxg1) that is highly transcribed by the anaerobic fungus Orpinomycessp. strain C1A under different growth conditions. This represents the first study of a GH39-family enzyme from the anaerobic fungi. Methods. Using enzyme activity assays, we performed a biochemical characterization of Bgxg1 on a variety of substrates over a wide range of pH and temperature values to identify the optimal enzyme conditions and the specificity of the enzyme. In addition, substrate competition studies and comparative modeling efforts were completed. Results. Contrary to the narrow range of activities (β-xylosidase or α-L-iduronidase) observed in previously characterized GH39 enzymes, Bgxg1 is unique in that it is multifunctional, exhibiting strong β-xylosidase, β-glucosidase, β-galactosidase activities (11.5 ± 1.2, 73.4 ± 7.15, and 54.6 ± 2.26 U/mg, respectively) and a weak xylanase activity (10.8 ± 1.25 U/mg), as compared to previously characterized enzymes. Further, Bgxg1 possesses extremely high affinity (as evident by the lowest K m values), compared to all previously characterized β-glucosidases, β-galactosidases, and xylanases. Physiological characterization revealed that Bgxg1 is active over a wide range of pH (3-8, optimum 6) and temperatures (25-60 °C, optimum 39 °C), and possesses excellent temperature and thermal stability. Substrate competition assays suggest that all observed activities occur at a single active site. Using comparative modeling and bioinformatics approaches, we putatively identified ten amino acid differences between Bgxg1 and previously biochemically characterized GH39 β-xylosidases that we speculate could impact active site architecture, size, charge, and/or polarity. Discussion. Collectively, the unique capabilities and multi-functionality of Bgxg1 render it an excellent candidate for inclusion in enzyme cocktails mediating cellulose and hemicellulose saccharification from lignocellulosic biomass.

A reporter gene system for the precise measurement of promoter activity in Thermus thermophilus HB27

We developed a reporter gene system that enables precise analysis of promoter activity in Thermus thermophilus HB27. The reporter vector employs a promoterless β-galactosidase gene of Thermus spp. strain T2. However, T. thermophilus HB27 strain has three genes (TTP0042, TTP0220 and TTP0222) whose products have β-galactosidase activity, which would interfere with correct measurements of promoter activities. Thus, to eliminate this background activity, we disrupted all three of these genes to generate a host strain for measuring promoter expression as β-galactosidase activity. In addition, T. thermophilus strains also produce carotenoids called thermoxanthins that are yellow pigments. To avoid the influence of these carotenoids on the β-galactosidase assay, we also disrupted the phytoene synthase gene (crtB). The reporter gene system developed here is a powerful tool for studying transcriptional activity and the mechanisms that regulate gene expression in T. thermophilus HB27. We also showed that the crtB gene cassette could be used in repeated gene-disruption experiments to screen transformants by colony colour, thus eliminating the need for antibiotic resistance markers.

Construction of new cloning vectors that employ the phytoene synthase encoding gene for color screening of cloned DNA inserts in Thermus thermophilus

Strains of Thermus thermophilus produce unique carotenoids called thermozeaxanthins and their colonies are light-yellow pigmented. Here, we developed a new cloning system allowing for the rapid and convenient detection of recombinants by color screening based on carotenoid production in T. thermophilus. We constructed two cloning vectors that overexpress the crtB gene encoding a phytoene synthase under the strong promoter of the slpA gene. Phytoene synthase is one of essential enzymes for the production of carotenoids. We also isolated a carotenoid-overproducing mutant that formed orange colonies. Because disruption of crtB in the carotenoid-overproducing mutant resulted in white colonies, we used the disruptant as a host strain. Whereas transformants carrying a new cloning vector, pTRK1-PRslpA-crtBcas, grew into unusual red-pigmented colonies probably because of the extreme accumulation of thermozeaxanthins, those carrying the vector with a foreign DNA inserts formed white colonies. Thus, recombinants can be detected easily by color screening (red/white screening) in T. thermophilus. This cloning system requires no additional chromogenic substrate in the medium. We also constructed a promoter-probe vector, pTRK1-crtBmcs-PP, employing the open reading frame of crtB with multiple cloning sites. Using this vector, a series of colony-color phenotypes is observed probably depending on promoter activities of foreign DNA inserts, which enables the rapid probing of promoters. These vectors are useful to simplify cloning procedures and to identify the promoters of different strengths in T. thermophilus.

Characterization of a glycoside hydrolase family 1 β-galactosidase from hot spring metagenome with transglycosylation activity

A novel, thermostable, alkalophilic β-D-galactosidase (Mbgl) was isolated from a metagenome of geothermal springs in northern Himalayan region of India. Mbgl was 447 amino acids in size and had conserved catalytic residues E170 and E358, indicating that it belonged to family 1 of glycosyl hydrolases showing maximum homology (89 %) with uncharacterized β-galactosidase of Eubacterium, Meiothermus ruber DSM1279. Temperature and pH optima of Mbgl were 65 °C and 8.0 respectively, and it retained 80 % activity even at pH 10.0. Mbgl was active as a homotetramer, recognized β-(1,4)-D-galactoside as the preferred glycosidic bond, and preferentially hydrolyzed pNPgal with K(m) 3.33 mM and k(cat) 2,000 s(-1). It displayed high transglycosylation activity with wide acceptor specificity including hexoses and pentoses leading to the formation of prebiotic galacto-oligosaccharides whereas its lactose hydrolysis potential was low.

Applications of β-gal-III isozyme from Bacillus coagulans RCS3, in lactose hydrolysis

Bacillus coagulans RCS3 isolated from hot water springs secreted five isozymes i.e. β-gal I-V of β-galactosidase. β-gal III isozyme was purified using DEAE cellulose and Sephadex G 100 column chromatography. Its molecular weight characterization showed a single band at 315kD in Native PAGE, while two subunits of 50.1 and 53.7 kD in SDS PAGE. β-Gal III had pH optima in the range of 6-7 and temperature optima at 65°C. It preferred nitro-aryl-β-d-galactoside as substrate having K(m) of 4.16 mM with ONPG. More than 85% and 80% hydrolysis of lactose (1-5%, w/v) was recorded within 48 h of incubation at 55°C and 50°C respectively and pH range of 6-7. About 78-86% hydrolysis of lactose in various brands of standardized milk was recorded at incubation temperature of 50°C. These results marked the applications of β-gal III in processing of milk/whey industry.

Protein engineering of a cold-active beta-galactosidase from Arthrobacter sp. SB to increase lactose hydrolysis reveals new sites affecting low temperature activity

We examined variants of an especially cold-active beta-galactosidase (BgaS) to better understand features affecting enzyme activity at temperature extremes. We targeted locations corresponding to a region in the LacZ enzyme previously shown to increase activity and decrease thermostability. Changes in this region of BgaS consistently caused the elimination or reduction of activity. A gene (bgaS3) encoding a loss of function variant was subjected to random mutagenesis to restore activity and discover potential interactions important in cold activity. Gene sequences from the resulting library indicated that only two amino acid alterations, E229D and V405A, were required to restore activity. Genes with combinations of these mutations were constructed and their enzymes purified. Enzymes with the E229D/V405A/G803D alterations (BgaS6), or E229D/V405A (BgaS7) had similar thermal optima and thermostabilities as BgaS. BgaS7, however, showed a 2.5-fold increase in catalytic activity at 15 degrees C and hydrolyzed 80% of lactose in skim milk in less than half the time of BgaS at 2.5 degrees C. Computer-generated models predicted that the substitutions at positions 229 and 405 yielded fewer contacts at the enzyme's activating interface. Results from regional saturation mutagenesis supported this hypothesis and suggested that not easily predicted, subtle, cooperative intramolecular interactions contributed to thermal adaptation.

The enzymes from extreme thermophiles: bacterial sources, thermostabilities and industrial relevance

This review on enzymes from extreme thermophiles (optimum growth temperature greater than 65 degrees C) concentrates on their characteristics, especially thermostabilities, and their commercial applicability. The enzymes are considered in general terms first, with comments on denaturation, stabilization and industrial processes. Discussion of the enzymes subsequently proceeds in order of their E.C. classification: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. The ramifications of cloned enzymes from extreme thermophiles are also discussed.

Three forms of thermostable lactose-hydrolase from Thermus sp. IB-21: cloning, expression, and enzyme characterization

Three thermostable lactose-hydrolases, namely, two beta-glycosidases (bglA and bglB) and one beta-galactosidase (bgaA) genes were cloned from the genomic library of Thermus sp. IB-21. The bglA, bglB, and bgaA consisted of 1311 bp (436 amino acid residues), 1296 bp (431 aa), and 1938 bp (645 aa) of nucleotides with predicted molecular masses of 49,066, 48,679, and 72,714 Da, respectively. These enzymes were overexpressed in Escherichia coli BL21(DE3) using pET21b(+) vector system. The recombinant enzymes were purified to homogeneity by a heat precipitation (70 degrees C, 40 min) and a Ni2+-affinity chromatography. The molecular masses of the purified enzymes estimated by SDS-PAGE agreed with their predicted values. All the purified enzymes showed their optimal pH at around 5.0-6.0. In contrast, the temperature profiles for activity and thermostability patterns were different for each enzyme. BglB beta-glycosidase displayed the best lactose hydrolysis activity of the three enzymes without substrate inhibition up to 200 mM lactose at 70 degrees C and pH 7.0. The specific activities (U/mg) of BglA, BglB, and BgaA on 138 mM lactose at 70 degrees C and pH 7.0 were 36.8, 160.3, and 8.5, respectively.

Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima

AIMS

Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima for the hydrolysis of lactose and the production of galacto-oligosaccharides.

METHODS AND RESULTS

A putative beta-galactosidase gene of Thermotoga maritima was expressed in Escherichia coli as a carboxyl terminal His-tagged recombinant enzyme. The gene encoded a 1100-amino acid protein with a calculated molecular weight of 129,501. The expressed enzyme was purified by heat treatment, His-tag affinity chromatography, and gel filtration. The optimum temperatures for beta-galactosidase activity were 85 and 80 degrees C with oNPG and lactose, respectively. The optimum pH value was 6.5 for both oNPG and lactose. In thermostability experiments, the enzyme followed first-order kinetics of thermal inactivation and its half-life times at 80 and 90 degrees C were 16 h and 16 min, respectively. Mn2+ was the most effective divalent cation for beta-galactosidase activity on both oNPG and lactose. The Km and Vmax values of the thermostable enzyme for oNPG at 80 degrees C were 0.33 mm and 79.6 micromol oNP min(-1) mg(-1). For lactose, the Km and Vmax values were dependent on substrate concentrations; 1.6 and 63.3 at lower concentrations up to 10 mm of lactose and 27.8 mm and 139 micromol glucose min(-1) mg(-1) at higher concentrations, respectively. The enzyme displayed non-Michaelis-Menten reaction kinetics with substrate activation, which was explained by simultaneous reactions of hydrolysis and transgalactosylation.

CONCLUSIONS

The results suggest that the thermostable enzyme may be suitable for both the hydrolysis of lactose and the production of galacto-oligosaccharides.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings of this work contribute to the knowledge of hydrolysis and transgalactosylation performed by beta-galactosidase of hyperthermophilic bacteria.

Overproduction of Thermus sp. Strain T2 beta-galactosidase in Escherichia coli and preparation by using tailor-made metal chelate supports

A novel thermostable chimeric beta-galactosidase was constructed by fusing a poly-His tag to the N-terminal region of the beta-galactosidase from Thermus sp. strain T2 to facilitate its overexpression in Escherichia coli and its purification by immobilized metal-ion affinity chromatography (IMAC). The poly-His tag fusion did not affect the activation, kinetic parameters, and stability of the beta-galactosidase. Copper-iminodiacetic acid (Cu-IDA) supports enabled the most rapid adsorption of the His-tagged enzyme, favoring multisubunit interactions, but caused deleterious effects on the enzyme stability. To improve the enzyme purification a selective one-point adsorption was achieved by designing tailor-made low-activated Co-IDA or Ni-IDA supports. The new enzyme was not only useful for industrial purposes but also has become an excellent model to study the purification of large multimeric proteins via selective adsorption on tailor-made IMAC supports.

Affinity chromatography of polyhistidine tagged enzymes. New dextran-coated immobilized metal ion affinity chromatography matrices for prevention of undesired multipoint adsorptions

New immobilized metal ion affinity chromatography (IMAC) matrices containing a high concentration of metal-chelate moieties and completely coated with inert flexible and hydrophilic dextrans are here proposed to improve the purification of polyhistidine (poly-His) tagged proteins. The purification of an interesting recombinant multimeric enzyme (a thermoresistant beta-galactosidase from Thermus sp. strain T2) has been used to check the performance of these new chromatographic media. IMAC supports with a high concentration (and surface density) of metal chelate groups promote a rapid adsorption of poly-His tagged proteins during IMAC. However, these supports also favor the promotion of undesirable multi-punctual adsorptions and problems may arise for the simple and effective purification of poly-His tagged proteins: (a) more than 30% of the natural proteins contained in crude extracts from E. coli become adsorbed, in addition to our target recombinant protein, on these IMAC supports via multipoint weak adsorptions; (b) the multimeric poly-His tagged enzyme may become adsorbed via several poly-His tags belonging to different subunits. In this way, desorption of the pure enzyme from the support may become quite difficult (e.g., it is not fully desorbed from the support even using 200 mM of imidazole). The coating of these IMAC supports with dextrans greatly reduces these undesired multi-point adsorptions: (i) less than 2% of natural proteins contained in crude extracts are now adsorbed on these novel supports; and (ii) the target multimeric enzyme may be fully desorbed from the support using 60 mM imidazole. In spite of this dramatic reduction of multi-point interactions, this dextran coating hardly affects the rate of the one-point adsorption of poly-His tagged proteins (80% of the rate of adsorption compared to uncoated supports). Therefore, this dextran coating of chromatographic matrices seems to allow the formation of strong one-point adsorptions that involve small areas of the protein and support surface. However, the dextran coating seems to have dramatic effects for the prevention of weak or strong multipoint interactions that should involve a high geometrical congruence between the enzyme and the support surface.

Structure of the beta-galactosidase gene from Thermus sp. strain T2: expression in Escherichia coli and purification in a single step of an active fusion protein

The nucleotide sequence of both the bgaA gene, coding for a thermostable beta-galactosidase of Thermus sp. strain T2, and its flanking regions was determined. The deduced amino acid sequence of the enzyme predicts a polypeptide of 645 amino acids (Mr, 73,595). Comparative analysis of the open reading frames located in the flanking regions of the bgaA gene revealed that they might encode proteins involved in the transport and hydrolysis of sugars. The observed homology between the deduced amino acid sequences of BgaA and the beta-galactosidase of Bacillus stearothermophilus allows us to classify the new enzyme within family 42 of glycosyl hydrolases. BgaA was overexpressed in its active form in Escherichia coli, but more interestingly, an active chimeric beta-galactosidase was constructed by fusing the BgaA protein to the choline-binding domain of the major pneumococcal autolysin. This chimera illustrates a novel approach for producing an active and thermostable hybrid enzyme that can be purified in a single step by affinity chromatography on DEAE-cellulose, retaining the catalytic properties of the native enzyme. The chimeric enzyme showed a specific activity of 191,000 U/mg at 70 degrees C and a Km value of 1.6 mM with o-nitrophenyl-beta-D-galactopyranoside as a substrate, and it retained 50% of its initial activity after 1 h of incubation at 70 degrees C.

Identification of new enzyme activities of several strains of Thermus species

New thermostable enzyme activities of seven Thermus strains were compared using the API ZYM system. All the strains exhibited high levels of alpha- and beta-glycosidases, esterase (C4) and esterase-lipase (C8) activities intracellularly. Only T. thermophilus HB8 (ATCC 27634) showed alpha-glucosidase and esterase activities in the supernatant. According to the intensity of beta-galactosidase activity, Thermus strains were divided in three groups. Group 0, which showed a weak beta-galactosidase activity, included Thermus spp. ATCC 31674 (T351) and 27978 (X-1) as well as T. thermophilus ATCC 27634 (HB8). Group I which consisted of T. aquaticus ATCC 25104 (YT-1), ATCC 25105 (Y-VII-51B) and Thermus sp. ATCC 27737 (T2), had a specific activity of approximately 40.0 U mg-1 and galactose as inducer. T. aquaticus ATCC 31558 (group 2) was particularly effective for beta-galactosidase production (2840 U) with a specific activity of 98 U mg-1. For each strain, galactose (0.5%) was a better inducer of beta-galactosidase production than lactose (1%). The detection of beta-galactosidase activity was dependent on the derivative chromogenic substrates used (naphthyl or nitrophenol coupled to sugar). Oligosaccharides were synthesized from cellobiose, lactulose, maltose or lactose as substrates at high temperature in some strains of Thermus.

Purification and properties of a novel thermostable galacto-oligosaccharide-producing beta-galactosidase from Sterigmatomyces elviae CBS8119

A thermostable beta-galactosidase which catalyzed the production of galacto-oligosaccharide from lactose was solubilized from a cell wall preparation of Sterigmatomyces elviae CBS8119. The enzyme was purified to homogeneity by means of chromatography on DEAE-Toyopearl, Butyl-Toyopearl, Chromatofocusing, and p-aminobenzyl 1-thio-beta-D-galactopyranoside agarose columns. The molecular weight of the purified enzyme was estimated to be about 170,000 by gel filtration with a Highload-Superdex 200pg column and 86,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its isoelectric point, determined by polyacrylamide gel electrofocusing, was 4.1. The optimal temperature for enzyme activity was 85 degrees C. It was stable at temperatures up to 80 degrees C for 1 h. The optimal pH range for the enzyme was 4.5 to 5.0, it was stable at pH 2.5 to 7.0, and its activity was inhibited by Hg2+. The Km values for o-nitrophenyl-beta-D-galactopyranoside and lactose were 9.5 and 2.4 mM, respectively, and the maximum velocities for these substrates were 96 and 240 mumol/min per mg of protein, respectively. In addition, this enzyme possessed a high level of transgalactosylation activity. Galacto-oligosaccharides, including tri- and tetrasaccharides, were produced with a yield, by weight, of 39% from 200-mg/ml lactose.

Molecular cloning and sequence analysis of the crtB gene of Thermus thermophilus HB27, an extreme thermophile producing carotenoid pigments

We have cloned and sequenced a 1.5-kb chromosomal fragment of Thermus thermophilus which promoted the overproduction of carotenoids in T. thermophilus. An open reading frame (ORF-A) coding for a polypeptide with 289 amino acids was responsible for carotenoid overproduction. The putative ORF-A protein showed significant homology with the amino acid sequences of crtB gene products (phytoene syntheses) of other microorganisms. The clone containing the ORF-A on a multicopy plasmid produced about three times as much carotenoid as that produced by the host strain, suggesting that the crtB gene product is a rate-limiting enzyme for carotenoid biosynthesis in T. thermophilus.

Cloning and analysis of the beta-galactosidase-encoding gene from Clostridium thermosulfurogenes EM1

Clostridium thermosulfurogenes EM1 produced a thermostable (up to 70 degrees C) beta-galactosidase (beta Gal) with a pH optimum of 7 during growth on lactose. The gene (lacZ) encoding this enzyme was cloned and expressed in Escherichia coli using pUC18 as a vector. The nucleotide sequence of a 2.7-kb PstI fragment carrying the lacZ gene was determined. The open reading frame for lacZ, which encoded a protein of 716 amino acids with a calculated Mr of 83,728, was confirmed by the identity of its deduced aa sequence with the chemically determined N-terminal aa sequence of the purified beta Gal of C. thermosulfurogenes EM1. The structural gene was preceded by a possible promoter sequence, 5'-TTGTAG (-35), 5'-TAATAT (-10); and a ribosome-binding site, 5'-AGGAGG. The cloned beta Gal was found to be indistinguishable from the native enzyme. The Mr of the active beta Gal was 170,000, as determined by Superose 12HR gel filtration and gradient gel electrophoresis. This indicated that this enzyme is composed of two identical subunits. Comparison of the aa sequences of different beta Gal revealed that five large regions of similarity with the enzymes from E. coli (lacZ, ebgA), Klebsiella pneumoniae (lacZ), and Lactobacillus bulgaricus are present in the beta Gal of C. thermosulfurogenes EM1 and that the putative active site residues (Glu461 and Tyr503 in the E. coli lacZ-encoded beta Gal) are conserved (Glu389 and Tyr429). Therefore, the thermostable beta Gal of C. thermosulfurogenes EM1 is more closely related to the enzyme of E. coli than to the likewise thermostable one of Bacillus stearothermophilus.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that β-Galactosidase of Sulfolobus solfataricus Is Only One of Several Activities of a Thermostable β- d -Glycosidase

A survey of Sulfolobus isolates showed all to contain thermostable enzyme activities hydrolyzing various glycosidic compounds. Of those not previously reported, the β-glucosidase activity of Sulfolobus solfataricus isolate P2 was chosen for further study and found to have the same kinetics of inactivation, apparent molecular weight, and many (though not all) other biochemical properties of the β-galactosidase also present in this strain. The two activities copurified approximately 850-fold to apparent homogeneity. The enzyme, whose subunit M r was estimated to be 60,000 to 65,000 by gel permeation chromatography of the active enzyme and 70,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the denatured form, hydrolyzed a variety of low-molecular-weight, β-linked glycosides and could account for most of the corresponding activities found in crude extract. Kinetic analyses indicated that chromogenic β- d -galactosides and β- d -glucosides are hydrolyzed at a common active site and that β-glucosides and β-fucosides represent the preferred substrates. The liberation of aglycone from aryl β- d -glucosides was stimulated by alcohols in a manner suggesting specific interaction between alcohol and enzyme.

Substrate inhibition or activation kinetics of the beta-galactosidase from the extreme thermoacidophile archaebacterium Caldariella acidophila

The kinetics of the hydrolysis of p-nitrophenyl-beta-D-galactopyranoside (pNPG) by a thermophile, beta-galactosidase, was studied at different temperatures. This enzyme was isolated from the thermophilic microorganism archaebacterium Caldariella acidophila. The hydrolysis of pNPG by beta-galactosidase does not follow Michaelis-Menten law. This enzyme is inhibited by excess substrate at low temperatures and it is activated by excess substrate at high temperatures. A minimum mechanistic model is proposed to explain the behaviour. This model assumes the binding of an additional substrate molecule on the glycosidyl enzyme intermediate. This model is in good agreement with the postulated mechanism for beta-galactosidase from Escherichia coli. The kinetic parameters are calculated at six different temperatures.

Characterization of thermoanaerobacter glucose isomerase in relation to saccharidase synthesis and development of single-step processes for sweetener production

Regulation of glucose isomerase synthesis was studied in Thermoanaerobacter strain B6A, which fermented a wide variety of carbohydrates including glucose, xylose, lactose, starch, and xylan. Glucogenic amylase activities and beta-galactosidase were produced constitutively, whereas the synthesis of glucose isomerase was induced by either xylose or xylan. Production of these saccharidase activities was not significantly repressed by the presence of glucose or 2-deoxyglucose in the growth media. Glucose isomerase production was optimized by controlling the culture pH at 5.5 during xylose fermentation. The apparent temperature and pH optima for these cell-bound saccharidase activities were as follows: glucose isomerase, 80 degrees C, pH 7.0 to 7.5; glucogenic amylase, 70 degrees C, pH 5.0 to 5.5; and beta-galactosidase, 60 degrees C, pH 6.0 to 6.5 Glucose isomerase, glucogenic amylase, and beta-galactosidase were produced in xylose-grown cells that were active and stable at 60 to 70 degrees C and pH 6.0 to 6.5. Under single-step process conditions, these saccharidase activities in whole cells or cell extracts converted starch or lactose directly into fructose mixtures. A total of 96% of initial liquefied starch was converted into a 49:51 mixture of glucose and fructose, whereas 85% of initial lactose was converted into a 40:31:29 mixture of galactose, glucose, and fructose.

Cloning of alpha- and beta-galactosidase genes from an extreme thermophile, Thermus strain T2, and their expression in Thermus thermophilus HB27

The genes encoding thermostable alpha- and beta-galactosidases from an extremely thermophilic bacterium, Thermus strain T2, were cloned in Escherichia coli. The alpha-galactosidase gene was located just downstream from the beta-galactosidase gene. The genes were introduced into Thermus thermophilus HB27 with the aid of Thermus cryptic plasmid pTT8, and beta-galactosidases were expressed constitutively.

Thermostable beta-galactosidase from the archaebacterium Sulfolobus solfataricus. Purification and properties

A thermophilic and thermostable beta-galactosidase activity was purified to homogeneity from crude extracts of the archaebacterium Sulfolobus solfataricus, by a procedure including ion-exchange and affinity chromatography. The homogeneous enzyme had a specific activity of 116.4 units/mg at 75 degrees C with o-nitrophenyl beta-galactopyranoside as substrate. Molecular mass studies demonstrated that the S. solfataricus beta-galactosidase was a tetramer of 240 +/- 8 kDa composed of similar or identical subunits. Comparison of the amino acid composition of beta-galactosidase from S. solfataricus with that from Escherichia coli revealed a lower cysteine content and a lower Arg/Lys ratio in the thermophilic enzyme. A rabbit serum, raised against the homogeneous enzyme did not cross-react with beta-galactosidase from E. coli. The enzyme, characterized for its reaction requirements and kinetic properties, showed a thermostability and thermophilicity notably greater than those reported for beta-galactosidases from other mesophilic and thermophilic sources.

Purification and characterisation of an inducible beta-galactosidase from Corynebacterium murisepticum

The beta-galactosidase (EC 3.2.1.32) of Corynebacterium murisepticum (inducible by lactose and galactose) was purified by successive column chromatography on Sephadex G-200, DEAE-Sephadex A-50 and DEAE-cellulose (DE52). The enzyme was found to be a dimer of identical subunits of molecular mass 100,000 daltons. The Km values of the enzyme for the substrates lactose and o-nitrophenyl-beta-D-galactopyranoside (ONPG) are 16.7 mM and 4.4 mM, respectively, indicating, its low affinity for the substrates. The Ouchterlony immunodiffusion method exhibited immunological homogeneity of the enzyme preparation. The catalytic site of the enzyme does not take part in antigen-antibody reaction.

Thermostable enzymes

In general, enzyme thermostability is an intrinsic property, determined by the primary structure of the protein. However, external environmental factors including cations, substrates, co-enzymes, modulators, polyols and proteins often increase enzyme thermostability. With some exceptions, enzymes present in thermophiles are more stable than their mesophilic counterparts. Some organisms produce enzymes with different thermal stability properties when grown at lower and higher temperatures. There are commercial advantages in carrying out enzymic reactions at higher temperatures. Some industrial enzymes exhibit high thermostability. More stable forms of other industrial enzymes are eagerly being sought.

Molecular basis of isozyme formation of beta-galactosidases in Bacillus stearothermophilus: isolation of two beta-galactosidase genes, bgaA and bgaB

Bacillus stearothermophilus IAM11001 produced three beta-galactosidases, beta-galactosidase I, II, and III (beta-gal I, II, and III), which are detectable by polyacrylamide (nondenatured) gel electrophoresis. By connecting restriction fragments of the chromosomal DNA to plasmid vectors, followed by transformation of Escherichia coli, two beta-galactosidase genes (bgaA and bgaB) located close to each other on the chromosome were isolated. Identification of the gene products and Southern hybridization analyses with a 2.7-kilobase-pair EcoRI fragment containing the bgaA gene as probe revealed that a single bgaA gene exists on the genome and that beta-gal II and beta-gal III consist of a common subunit (the bgaA gene product; molecular weight, 120,000), but differ in their assembly (beta-gal II is a dimer, and beta-gal III is a tetramer). The bgaB gene product (molecular weight, 70,000) in Bacillus subtilis harboring pHG5 (a hybrid plasmid consisting of pUB110 and a 2.9-kilobase-pair EcoRI fragment) was estimated to be the beta-gal I protein from its heat stability. Southern hybridization and immunological testing indicated that the two genes have no homology.

A correlation between protein thermostability and resistance to proteolysis

The loss of activity due to proteolysis of purified L-asparaginase and beta-galactosidase from different sources correlates with the thermal instability of the enzymes. A similar correlation is found when populations of soluble proteins from micro-organisms grown at different temperatures are compared for proteolytic susceptibility and thermal stability. It is proposed that there is a general correlation between the thermostability of proteins and their resistance to proteolysis.

Purification and characterization of beta-galactosidase from a strain of Bacillus coagulans

beta-Galactosidase from B. coagulans strain L4 is produced constitutively, has a mol. wt. of 4.3 x 10(5) and an optimal temperature of 55 degrees C. The optimal pH at 30 degrees C is 6.0 whereas at 55 degrees C it is 6.5. The energy of activation of enzyme activity is 41.9 kJ/mol (10 kcal/mol). No cations are required. The Km with ONPG as substrate is 4.2-5.6 mM and with lactose is 50 mM. The Ki for inhibition by galactose is 11.7-13.4 mM and for dextrose is 50 mM. Galactose inhibited competitively while dextrose inhibited noncompetitively. The purified and unprotected enzyme is 70% destroyed in 30 min at 55 degrees C whereas in the presence of 2 mg/ml of BSA 42% of the activity is destroyed in 30 min at 55 degrees C. An overall purification of 75.3-fold was achieved.