Purification and characterization of an L-arabinose isomerase from an isolated strain of Geobacillus thermodenitrificans producing D-tagatose

Production of L-ribose from L-ribulose by a triple-site variant of mannose-6-phosphate isomerase from Geobacillus thermodenitrificans

A triple-site variant (W17Q N90A L129F) of mannose-6-phosphate isomerase from Geobacillus thermodenitrificans was obtained by combining variants with residue substitutions at different positions after random and site-directed mutagenesis. The specific activity and catalytic efficiency (k(cat)/K(m)) for L-ribulose isomerization of this variant were 3.1- and 7.1-fold higher, respectively, than those of the wild-type enzyme at pH 7.0 and 70°C in the presence of 1 mM Co(2+). The triple-site variant produced 213 g/liter l-ribose from 300 g/liter L-ribulose for 60 min, with a volumetric productivity of 213 g liter(-1) h(-1), which was 4.5-fold higher than that of the wild-type enzyme. The k(cat)/K(m) and productivity of the triple-site variant were approximately 2-fold higher than those of the Thermus thermophilus R142N variant of mannose-6-phosphate isomerase, which exhibited the highest values previously reported.

Heterologous expression and characterization of Bacillus coagulans L-arabinose isomerase

Bacillus coagulans has been of great commercial interest over the past decade owing to its strong ability of producing optical pure L: -lactic acid from both hexose and pentose sugars including L: -arabinose with high yield, titer and productivity under thermophilic conditions. The L: -arabinose isomerase (L-AI) from Bacillus coagulans was heterologously over-expressed in Escherichia coli. The open reading frame of the L-AI has 1,422 nucleotides encoding a protein with 474 amino acid residues. The recombinant L-AI was purified to homogeneity by one-step His-tag affinity chromatography. The molecular mass of the enzyme was estimated to be 56 kDa by SDS-PAGE. The enzyme was most active at 70°C and pH 7.0. The metal ion Mn(2+) was shown to be the best activator for enzymatic activity and thermostability. The enzyme showed higher activity at acidic pH than at alkaline pH. The kinetic studies showed that the K (m), V (max) and k (cat)/K (m) for the conversion of L: -arabinose were 106 mM, 84 U/mg and 34.5 mM(-1)min(-1), respectively. The equilibrium ratio of L: -arabinose to L: -ribulose was 78:22 under optimal conditions. L: -ribulose (97 g/L) was obtained from 500 g/l of L: -arabinose catalyzed by the enzyme (8.3 U/mL) under the optimal conditions within 1.5 h, giving at a substrate conversion of 19.4% and a production rate of 65 g L(-1) h(-1).

The acid-tolerant L-arabinose isomerase from the mesophilic Shewanella sp. ANA-3 is highly active at low temperatures

Background

L-arabinose isomerases catalyse the isomerization of L-arabinose into L-ribulose at insight biological systems. At industrial scale of this enzyme is used for the bioconversion of D-galactose into D-tagatose which has many applications in pharmaceutical and agro-food industries. The isomerization reaction is thermodynamically equilibrated, and therefore the bioconversion rates is shifted towards tagatose when the temperature is increased. Moreover, to prevent secondary reactions it will be of interest to operate at low pH. The profitability of this D-tagatose production process is mainly related to the use of lactose as cheaper raw material. In many dairy products it will be interesting to produce D-tagatose during storage. This requires an efficient L-arabinose isomerase acting at low temperature and pH values.

Results

The gene encoding the L-arabinose isomerase from Shewanella sp. ANA-3 was cloned and overexpressed in Escherichia coli. The purified protein has a tetrameric arrangement composed by four identical 55 kDa subunits. The biochemical characterization of this enzyme showed that it was distinguishable by its maximal activity at low temperatures comprised between 15-35°C. Interestingly, this biocatalyst preserves more than 85% of its activity in a broad range of temperatures from 4.0 to 45°C. Shewanella sp. ANA-3 L-arabinose isomerase was also optimally active at pH 5.5-6.5 and maintained over 80% of its activity at large pH values from 4.0 to 8.5. Furthermore, this enzyme exhibited a weak requirement for metallic ions for its activity evaluated at 0.6 mM Mn²⁺. Stability studies showed that this protein is highly stable mainly at low temperature and pH values. Remarkably, T268K mutation clearly enhances the enzyme stability at low pH values. Use of this L-arabinose isomerase for D-tagatose production allows the achievement of attractive bioconversion rates of 16% at 4°C and 34% at 35°C.

Conclusions

Here we reported the purification and the biochemical characterization of the novel Shewanella sp. ANA-3 L-arabinose isomerase. Determination of the biochemical properties demonstrated that this enzyme was highly active at low temperatures. The generated T268K mutant displays an increase of the enzyme stability essentially at low pH. These features seem to be very attractive for the bioconversion of D-galactose into D-tagatose at low temperature which is very interesting from industrial point of view.

Identification and characterization of a novel L-arabinose isomerase from Anoxybacillus flavithermus useful in D-tagatose production

D-Tagatose is a highly functional rare ketohexose and many attempts have been made to convert D-galactose into the valuable D-tagatose using L-arabinose isomerase (L-AI). In this study, a thermophilic strain possessing L-AI gene was isolated from hot spring sludge and identified as Anoxybacillus flavithermus based on its physio-biochemical characterization and phylogenetic analysis of its 16s rRNA gene. Furthermore, the gene encoding L-AI from A. flavithermus (AFAI) was cloned and expressed at a high level in E. coli BL21(DE3). L-AI had a molecular weight of 55,876 Da, an optimum pH of 10.5 and temperature of 95°C. The results showed that the conversion equilibrium shifted to more D-tagatose from D-galactose by raising the reaction temperatures and adding borate. A 60% conversion of D-galactose to D-tagatose was observed at an isomerization temperature of 95°C with borate. The catalytic efficiency (k (cat) /K (m)) for D-galactose with borate was 9.47 mM(-1) min(-1), twice as much as that without borate. Our results indicate that AFAI is a novel hyperthermophilic and alkaliphilic isomerase with a higher catalytic efficiency for D-galactose, suggesting its great potential for producing D-tagatose.

The acid tolerant L-arabinose isomerase from the food grade Lactobacillus sakei 23K is an attractive D-tagatose producer

The araA gene encoding an L-arabinose isomerase (L-AI) from the psychrotrophic and food grade Lactobacillus sakei 23K was cloned, sequenced and over-expressed in Escherichia coli. The recombinant enzyme has an apparent molecular weight of nearly 220 kDa, suggesting it is a tetramer of four 54 kDa monomers. The enzyme is distinguishable from previously reported L-AIs by its high activity and stability at temperatures from 4 to 40 degrees C, and pH from 3 to 8, and by its low metal requirement of only 0.8 mM Mn(2+) and 0.8 mM Mg(2+) for its maximal activity and thermostability. Enzyme kinetic studies showed that this enzyme displays a high catalytic efficiency allowing D-galactose bioconversion rates of 20% and 36% at 10 and 45 degrees C, respectively, which are useful for commercial production of D-tagatose.

An L-arabinose isomerase from Acidothermus cellulolytics ATCC 43068: cloning, expression, purification, and characterization

The araA gene encoding an L-arabinose isomerase (L-AI) from the acido-thermophilic bacterium Acidothermus cellulolytics ATCC 43068 was cloned and overexpressed in Escherichia coli. The open reading frame of the L-AI consisted of 1,503 nucleotides encoding 501 amino acid residues. The recombinant L-AI was purified to homogeneity by heat treatment, ion-exchange chromatography, and gel filtration. The molecular mass of the enzyme was estimated to be approximately 55 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified enzyme was optimally active at 75 degrees C and pH 7.5. It required divalent metal ions, either Mn(2+) or Co(2+), for both enzymatic activity and thermostability improvement at higher temperatures. The enzyme showed relatively high activity and stability at acidic pH. It exhibited over 90% of its maximal activity at pH 6.0 and retained 80% of activity after 12 h incubation at pH 6.0. Catalytic property study showed that the enzyme had an interesting catalytic efficiency. Its apparent K (m), V (max), and catalytic efficiency (k (cat)/K (m)) for D-galactose was 28.9 mM, 4.9 U/mg, and 9.3 mM(-1) min(-1), respectively. The enzyme carried out the isomerization of D-galactose to D-tagatose with a conversion yield over 50% after 12 h under optimal conditions, suggesting its potential in D-tagatose production.

L-ribose production from L-arabinose by using purified L-arabinose isomerase and mannose-6-phosphate isomerase from Geobacillus thermodenitrificans

Two enzymes, L-arabinose isomerase and mannose-6-phosphate isomerase, from Geobacillus thermodenitrificans produced 118 g/liter L-ribose from 500 g/liter L-arabinose at pH 7.0, 70 degrees C, and 1 mM Co(2+) for 3 h, with a conversion yield of 23.6% and a volumetric productivity of 39.3 g liter(-1) h(-1).

Characterization of an L-arabinose isomerase from Bacillus subtilis

An isolated gene from Bacillus subtilis str. 168 encoding a putative isomerase was proposed as an L-arabinose isomerase (L-AI), cloned into Escherichia coli, and its nucleotide sequence was determined. DNA sequence analysis revealed an open reading frame of 1,491 bp, capable of encoding a polypeptide of 496 amino acid residues. The gene was overexpressed in E. coli and the protein was purified using nickel-nitrilotriacetic acid chromatography. The purified enzyme showed the highest catalytic efficiency ever reported, with a k(cat) of 14,504 min(-1) and a k(cat)/K(m) of 121 min(-1) mM(-1) for L-arabinose. A homology model of B. subtilis L-AI was constructed based on the X-ray crystal structure of E. coli L-AI. Molecular dynamics simulation studies of the enzyme with the natural substrate, L-arabinose, and an analogue, D-galactose, shed light on the unique substrate specificity displayed by B. subtilis L-AI only towards L-arabinose. Although L-AIs have been characterized from several other sources, B. subtilis L-AI is distinguished from other L-AIs by its high substrate specificity and catalytic efficiency for L-arabinose.

Differential selectivity of the Escherichia coli cell membrane shifts the equilibrium for the enzyme-catalyzed isomerization of galactose to tagatose

An Escherichia coli galactose kinase gene knockout (DeltagalK) strain, which contains the l-arabinose isomerase gene (araA) to isomerize d-galactose to d-tagatose, showed a high conversion yield of tagatose compared with the original galK strain because galactose was not metabolized by endogenous galactose kinase. In whole cells of the DeltagalK strain, the isomerase-catalyzed reaction exhibited an equilibrium shift toward tagatose, producing a tagatose fraction of 68% at 37 degrees C, whereas the purified l-arabinose isomerase gave a tagatose equilibrium fraction of 36%. These equilibrium fractions are close to those predicted from the measured equilibrium constants of the isomerization reaction catalyzed in whole cells and by the purified enzyme. The equilibrium shift in these cells resulted from the higher uptake and lower release rates for galactose, which is a common sugar substrate, than for tagatose, which is a rare sugar product. A DeltamglB mutant had decreased uptake rates for galactose and tagatose, indicating that a methylgalactoside transport system, MglABC, is the primary contributing transporter for the sugars. In the present study, whole-cell conversion using differential selectivity of the cell membrane was proposed as a method for shifting the equilibrium in sugar isomerization reactions.

Substrate specificity of a galactose 6-phosphate isomerase from Lactococcus lactis that produces d-allose from d-psicose

We purified recombinant galactose 6-phosphate isomerase (LacAB) from Lactococcus lactis using HiTrap Q HP and Phenyl-Sepharose columns. The purified LacAB had a final specific activity of 1.79units/mg to produce d-allose. The molecular mass of native galactose 6-phosphate isomerase was estimated at 135.5kDa using Sephacryl S-300 gel filtration, and the enzyme exists as a hetero-octamer of LacA and LacB subunits. The activity of galactose 6-phosphate isomerase was maximal at pH 7.0 and 30 degrees C, and enzyme activity was independent of metal ions. When 100g/L of d-psicose was used as the substrate, 25g/L of d-allose and 13g/L of d-altrose were simultaneously produced at pH 7.0 and 30 degrees C after 12h of incubation. The enzyme had broad specificity for various aldoses and ketoses. The interconversion of sugars with the same configuration except at the C2 position was driven by using a large amount of enzyme in extended reactions. The interconversion occurred via two isomerization reactions, i.e., the interconversion of d-allose<-->d-psicose<-->d-altrose, and d-allose to d-psicose reaction was faster than d-altrose to d-psicose reaction.

Cloning, sequencing, overexpression and characterization of L-rhamnose isomerase from Bacillus pallidus Y25 for rare sugar production

The L-rhamnose isomerase gene (L-rhi) encoding for L-rhamnose isomerase (L-RhI) from Bacillus pallidus Y25, a facultative thermophilic bacterium, was cloned and overexpressed in Escherichia coli with a cooperation of the 6xHis sequence at a C-terminal of the protein. The open reading frame of L-rhi consisted of 1,236 nucleotides encoding 412 amino acid residues with a calculated molecular mass of 47,636 Da, showing a good agreement with the native enzyme. Mass-produced L-RhI was achieved in a large quantity (470 mg/l broth) as a soluble protein. The recombinant enzyme was purified to homogeneity by a single step purification using a Ni-NTA affinity column chromatography. The purified recombinant L-RhI exhibited maximum activity at 65 degrees C (pH 7.0) under assay conditions, while 90% of the initial enzyme activity could be retained after incubation at 60 degrees C for 60 min. The apparent affinity (K(m)) and catalytic efficiency (k(cat)/K(m)) for L-rhamnose (at 65 degrees C) were 4.89 mM and 8.36 x 10(5) M(-1) min(-1), respectively. The enzyme demonstrated relatively low levels of amino acid sequence similarity (42 and 12%), higher thermostability, and different substrate specificity to those of E. coli and Pseudomonas stutzeri, respectively. The enzyme has a good catalyzing activity at 50 degrees C, for D: -allose, L-mannose, D-ribulose, and L-talose from D-psicose, L-fructose, D-ribose and L-tagatose with a conversion yield of 35, 25, 16 and 10%, respectively, without a contamination of by-products. These findings indicated that the recombinant L-RhI from B. pallidus is appropriate for use as a new source of rare sugar producing enzyme on a mass scale production.

Tagatose: properties, applications, and biotechnological processes

D-Tagatose has attracted a great deal of attention in recent years due to its health benefits and similar properties to sucrose. D-Tagatose can be used as a low-calorie sweetener, as an intermediate for synthesis of other optically active compounds, and as an additive in detergent, cosmetic, and pharmaceutical formulation. Biotransformation of D-tagatose has been produced using several biocatalyst sources. Among the biocatalysts, L-arabinose isomerase has been mostly applied for D-tagatose production because of the industrial feasibility for the use of D-galactose as a substrate. In this article, the characterization of many L-arabinose isomerases and their D-tagatose production is compared. Protein engineering and immobilization of the enzyme for increasing the conversion rate of D-galactose to D-tagatose are also reviewed.

Crystal Structure of Escherichia coli L-Arabinose Isomerase (ECAI), The Putative Target of Biological Tagatose Production

Escherichia coli L-arabinose isomerase (ECAI; EC 5.3.1.4) catalyzes the isomerization of L-arabinose to L-ribulose in vivo. This enzyme is also of commercial interest as it catalyzes the conversion of D-galactose to D-tagatose in vitro. The crystal structure of ECAI was solved and refined at 2.6 A resolution. The subunit structure of ECAI is organised into three domains: an N-terminal, a central and a C-terminal domain. It forms a crystallographic trimeric architecture in the asymmetric unit. Packing within the crystal suggests the idea that ECAI can form a hexameric assembly. Previous electron microscopic and biochemical studies supports that ECAI is hexameric in solution. A comparison with other known structures reveals that ECAI adopts a protein fold most similar to E. coli fucose isomerase (ECFI) despite very low sequence identity 9.7%. The structural similarity between ECAI and ECFI with regard to number of domains, overall fold, biological assembly, and active site architecture strongly suggests that the enzymes have functional similarities. Further, the crystal structure of ECAI forms a basis for identifying molecular determinants responsible for isomerization of arabinose to ribulose in vivo and galactose to tagatose in vitro.

Regiospecific flavonoid 7-O-methylation with Streptomyces avermitilis O-methyltransferase expressed in Escherichia coli

O-Methylation, commonly found in synthesis of secondary metabolites of plants and micro-organisms, appears to transfer a methyl group to the hydroxyl group of the recipient which increases the hydrophobicity of the recipient. O-Methyltransferase (OMT), , was isolated and characterized from Streptomyces avermitilis MA-4680. Its amino acid sequence showed 68% similarity with antibiotic C-1027 OMT and 53% similarity with the carminomycin 4-OMT. was expressed in E. coli as a His-tag fusion protein and showed that the methyl was transferred onto the 7-hydroxyl group of the isoflavones, daidzein and genistein, and the flavones, kaempferol and quercetin, as well as the flavanone naringenin. NMR and liquid chromatography-mass spectrometry were used to confirm the location of the methyl group on the recipient compound of naringenin, which was biotransformed into sakuranetin by E. coli transformant expressing (E. coli Sa-2). Therefore, E. coli Sa-2 would be used for the synthesis of the antifungal flavonoid, sakuranetin, through biotransformation.

Increase in d-tagatose Production Rate by Site-directed Mutagenesis of l-arabinose Isomerase from Geobacillus thermodenitrificans

Among single-site mutations of L-arabinose isomerase derived from Geobacillus thermodenitrificans, two mutants were produced having the lowest and highest activities of D-tagatose production. Site-directed mutagenesis at these sites showed that the aromatic ring at amino acid 164 and the size of amino acid 475 were important for D-tagatose production. Among double-site mutations, one mutant converted D-galactose into D-tagatose with a yield of 58% whereas the wild type gave 46% D-tagatose conversion after 300 min at 65 degrees C.