Biochemical characterization of a thermostable l-arabinose isomerase from a thermoacidophilic bacterium, Alicyclobacillus hesperidum URH17-3-68

Improving Catalytic Efficiency of L-Arabinose Isomerase from Lactobacillus plantarum CY6 towards D-Galactose by Molecular Modification

L-Arabinose isomerase (L-AI) has been commonly used as an efficient biocatalyst to produce D-tagatose via the isomerization of D-galactose. However, it remains a significant challenge to efficiently synthesize D-tagatose using the native (wild type) L-AI at an industrial scale. Hence, it is extremely urgent to redesign L-AI to improve its catalytic efficiency towards D-galactose, and herein a structure-based molecular modification of Lactobacillus plantarum CY6 L-AI (LpAI) was performed. Among the engineered LpAI, both F118M and F279I mutants showed an increased D-galactose isomerization activity. Particularly, the specific activity of double mutant F118M/F279I towards D-galactose was increased by 210.1% compared to that of the wild type LpAI (WT). Besides the catalytic activity, the substrate preference of F118M/F279I was also largely changed from L-arabinose to D-galactose. In the enzymatic production of D-tagatose, the yield and conversion ratio of F118M/F279I were increased by 81.2% and 79.6%, respectively, compared to that of WT. Furthermore, the D-tagatose production of whole cells expressing F118M/F279I displayed about 2-fold higher than that of WT cell. These results revealed that the designed site-directed mutagenesis is useful for improving the catalytic efficiency of LpAI towards D-galactose.

Low-calorie bulk sweeteners: Recent advances in physical benefits, applications, and bioproduction

At present, with the continuous improvement of living standards, people are paying increasing attention to dietary nutrition and health. Low sugar and low energy consumption have become important dietary trends. In terms of sugar control, more and more countries have implemented sugar taxes in recent years. Hence, as the substitute for sugar, low-calorie sweeteners have been widely used in beverage, bakery, and confectionary industries. In general, low-calorie sweeteners consist of high-intensity and low-calorie bulk sweeteners (some rare sugars and sugar alcohols). In this review, recent advances and challenges in low-calorie bulk sweeteners are explored. Bioproduction of low-calorie bulk sweeteners has become the focus of many researches, because it has the potential to replace the current industrial scale production through chemical synthesis. A comprehensive summary of the physicochemical properties, physiological functions, applications, bioproduction, and regulation of typical low-calorie bulk sweeteners, such as D-allulose, D-tagatose, D-mannitol, sorbitol, and erythritol, is provided.

Production of d-Tagatose by Whole-Cell Conversion of Recombinant Bacillus subtilis in the Absence of Antibiotics

Simple Summary

d-tagatose is a valuable monosaccharide in the food industry produced from lactose by β-galactosidase and arabinose isomerase. To improve its production safety, d-alanine-deficient heterologous gene expression systems were constructed without antibiotics. The integrated expression and co-expression plasmids were used in different systems, also exploiting the need for d-alanine during cellular metabolism. The integration of the β-galactosidase gene in recombinant is uniquely innovative and promising, applying common knockout techniques to the expression of target genes and the production of high-value products.

Abstract

d-tagatose is a popular functional monosaccharide produced from lactose by β-galactosidase and arabinose isomerase. In this study, two d-alanine-deficient heterologous gene expression systems were constructed, B. subtilis 168 D1 and B. subtilis 168 D2, using overlapping extension PCR and the CRE/loxP system. The lacZ gene for β-galactosidase was integrated into a specific locus of the chassis B. subtilis 168 D2. A mutually complementary plasmid pMA5 with the alanine racemase gene alrA attached to it was constructed and used to assemble recombinant plasmids overexpressing β-galactosidase and arabinose isomerase. Afterward, an integrated recombinant was constructed by the plasmid expressing the arabinose isomerase gene araA of E. coli transform-competent B. subtilis 168 D2 cells. The co-expressing plasmids were introduced into alanine racemase knockout B. subtilis 168 D1. Whole-cell bioconversion was performed using the integrated recombinant with a maximum yield of 96.8 g/L d-tagatose from 500 g/L lactose, and the highest molar conversions were 57.2%. B. subtilis 168 D1/pMA5-alrA-araA-lacZ is capable of single-cell one-step production of d-tagatose. This study provides a new approach to the production of functional sugars.

Development of recyclable magnetic cross-linked enzyme aggregates for the synthesis of high value rare sugar d-tagatose in aqueous phase catalysis

In this study, a high value rare sugar d-tagatose was synthesized using recyclable magnetic catalysts.

Characterization of an L-Arabinose Isomerase from Bacillus velezensis and Its Application for L-Ribulose and L-Ribose Biosynthesis

L-Ribulose and L-ribose are two high-value unnatural sugars that can be biosynthesized by sugar isomerases. In this paper, an L-arabinose isomerase (BvAI) from Bacillus velezensis CICC 24777 was cloned and overexpressed in Escherichia coli BL21 (DE3) strain. The maximum activity of recombinant BvAI was observed at 45 °C and pH 8.0, in the presence of 1.0 mM Mn²⁺. Approximately 207.2 g/L L-ribulose was obtained from 300 g/L L-arabinose in 1.5 h by E. coli harboring BvAI. In addition, approximately 74.25 g/L L-ribose was produced from 300 g/L L-arabinose in 7 h by E. coli co-expressing BvAI and L-RI from Actinotalea fermentans ATCC 43279 (AfRI). This study provides a feasible approach for producing L-ribose from L-arabinose using a co-expression system harboring L-Al and L-RI.

Biochemical Characterization of Heat-Tolerant Recombinant L-Arabinose Isomerase from Enterococcus faecium DBFIQ E36 Strain with Feasible Applications in D-Tagatose Production

D-Tagatose is a ketohexose, which presents unique properties as a low-calorie functional sweetener possessing a sweet flavor profile similar to D-sucrose and having no aftertaste. Considered a generally recognized as safe (GRAS) substance by FAO/WHO, D-tagatose can be used as an intermediate for the synthesis of other optically active compounds as well as an additive in detergent, cosmetic, and pharmaceutical formulations. This study reports important features for L-arabinose isomerase (EC 5.3.1.4) (L-AI) use in industry. We describe arabinose (araA) gene virulence analysis, gene isolation, sequencing, cloning, and heterologous overexpression of L-AI from the food-grade GRAS bacterium Enterococcus faecium DBFIQ E36 in Escherichia coli and assess biochemical properties of this recombinant enzyme. Recombinant L-AI (rL-AI) was one-step purified to homogeneity by Ni²⁺-agarose resin affinity chromatography and biochemical characterization revealed low identity with both thermophilic and mesophilic L-AIs but high degree of conservation in residues involved in substrate recognition. Optimal conditions for rL-AI activity were 50 °C, pH 5.5, and 0.3 mM Mn²⁺, exhibiting a low cofactor concentration requirement and an acidic optimum pH. Half-life at 45 °C and 50 °C were 1427 h and 11 h, respectively, and 21.5 h and 39.5 h at pH 4.5 and 5.6, respectively, showing the high stability of the enzyme in the presence of a metallic cofactor. Bioconversion yield for D-tagatose biosynthesis was 45% at 50 °C after 48 h. These properties highlight the technological potential of E. faecium rL-AI as biocatalyst for D-tagatose production.

Tagatose as a Potential Nutraceutical: Production, Properties, Biological Roles, and Applications

Nutraceuticals are gaining importance owing to their potential applications in numerous sectors including food and feed industries. Among the emerging nutraceuticals, d-tagatose occupies a significant niche because of its low calorific value, antidiabetic property and growth promoting effects on beneficial gut bacteria. As d-tagatose is present in minute quantities in naturally occurring food substances, it is produced mainly by chemical or biological means. Recently, attempts were made for bio-production of d-tagatose using l-arabinose isomerase enzyme to overcome the challenges of chemical process of production. Applications of d-tagatose for maintaining health and wellbeing are increasing due to growing consumer awareness and apprehension against modern therapeutic agents. This review outlines the current status on d-tagatose, particularly its production, properties, biological role, applications, and the future perspectives.

Biochemical properties of L-arabinose isomerase from Clostridium hylemonae to produce D-tagatose as a functional sweetener

d-Tagatose has gained substantial interest due to its potential functionalities as a sucrose substitute. In this study, the gene araA, encoding l-arabinose isomerase (l-AI) from Clostridium hylemonae (DSM 15053), was cloned and expressed in Escherichia coli BL21 (DE3). This gene consists of 1,506 nucleotides and encodes a protein of 501 amino acid residues with a calculated molecular mass of 56,554 Da. Since l-AI was expressed as an intracellular inclusion body, this enzyme was solubilized with guanidine hydrochloride, refolded, and activated with a descending concentration gradient of urea. The purified enzyme exhibited the greatest activity at 50°C, pH 7-7.5, and required 1 mM of Mg2+ as a cofactor. Notably, the catalytic efficiency (3.69 mM-1sec-1) of l-AI from C. hylemonae on galactose was significantly greater than that of other previously reported enzymes. The bioconversion yield of d-tagatose using the C. hylemonae l-arabinose isomerase at 60°C reached approximately 46% from 10 mM of d-galactose after 2 h. From these results, it is suggested that the l-arabinose isomerase from C. hylemonae could be utilized as a potential enzyme for d-tagatose production due to its high conversion yield at an industrially competitive temperature.

Rational Design of Bacillus coagulans NL01 l-Arabinose Isomerase and Use of Its F279I Variant in d-Tagatose Production

d-Tagatose is a prospective functional sweetener that can be produced by l-arabinose isomerase (AI) from d-galactose. To improve the activity of AI toward d-galactose, the AI of Bacillus coagulans was rationally designed on the basis of molecular modeling and docking. After alanine scanning and site-saturation mutagenesis, variant F279I that exhibited improved activity toward d-galactose was obtained. The optimal temperature and pH of F279I were determined to be 50 °C and 8.0, respectively. This variant possessed 1.4-fold catalytic efficiency compared with the wild-type (WT) enzyme. The recombinant Escherichia coli overexpressing F279I also showed obvious advantages over the WT in biotransformation. Under optimal conditions, 67.5 and 88.4 g L^-1 d-tagatose could be produced from 150 and 250 g L^-1 d-galactose, respectively, in 15 h. The biocatalyst constructed in this study presents a promising alternative for large-scale d-tagatose production.

Cloning, Expression, and Characterization of a Novel L-Arabinose Isomerase from the Psychrotolerant Bacterium Pseudoalteromonas haloplanktis

L-Arabinose isomerase (L-AI, EC 5.3.1.4) catalyzes the isomerization between L-arabinose and L-ribulose, and most of the reported ones can also catalyze D-galactose to D-tagatose, except Bacillus subtilis L-AI. In this article, the L-AI from the psychrotolerant bacterium Pseudoalteromonas haloplanktis ATCC 14393 was characterized. The enzyme showed no substrate specificity toward D-galactose, which was similar to B. subtilis L-AI but distinguished from other reported L-AIs. The araA gene encoding the P. haloplanktis L-AI was cloned and overexpressed in E. coli BL21 (DE3). The recombinant enzyme was purified by one-step nickel affinity chromatography . The enzyme displayed the maximal activity at 40 °C and pH 8.0, and showed more than 75 % of maximal activity from pH 7.5-9.0. Metal ion Mn²⁺ was required as optimum metal cofactor for activity simulation, but it did not play a significant role in thermostability improvement as reported previously. The Michaelis-Menten constant (K _m), turnover number (k _cat), and catalytic efficiency (k _cat/K _m) for substrate L-arabinose were measured to be 111.68 mM, 773.30/min, and 6.92/mM/min, respectively. The molecular docking results showed that the active site residues of P. haloplanktis L-AI could only immobilize L-arabinose and recognized it as substrate for isomerization.

Alicyclobacillus spp.: New Insights on Ecology and Preserving Food Quality through New Approaches

Alicyclobacillus spp. includes spore-forming and thermo-acidophilic microorganisms, usually recovered from soil, acidic drinks, orchards and equipment from juice producers. The description of the genus is generally based on the presence of ω-fatty acids in the membrane, although some newly described species do not possess them. The genus includes different species and sub-species, but A. acidoterrestris is generally regarded as the most important spoiler for acidic drinks and juices. The main goal of this review is a focus on the ecology of the genus, mainly on the species A. acidoterrestris, with a special emphasis on the different phenotypic properties and genetic traits, along with the correlation among them and with the primary source of isolation. Finally, the last section of the review reports on some alternative approaches to heat treatments (natural compounds and other chemical treatments) to control and/or reduce the contamination of food by Alicyclobacillus.

Engineering of Alicyclobacillus hesperidum L-arabinose isomerase for improved catalytic activity and reduced pH optimum using random and site-directed mutagenesis

A mutation, D478N, was obtained by an error-prone polymerase chain reaction using the L-arabinose isomerase (L-AI) gene from Alicyclobacillus hesperidum URH17-3-68 as the template. The mutated isomerase showed higher activity for D-galactose isomerization. The mutation site obtained from random mutagenesis was then introduced as a single-site mutation using site-directed mutagenesis. Single-site variants, D478N, D478Q, D478A, D478K, and D478R, were constructed. The optimum temperatures were all higher than 60 °C. D478A, D478N, and D478Q retained more than 80 % of the maximum relative activity of the wild-type L-AI at 75 °C. With the exception of the D478A variant, all variants showed decreased optimum pH values in the acidic range (6.0-6.5). All of the variant L-AIs could be significantly activated by the addition of Co(2+) and Mn(2+). D478N and D478Q showed higher catalytic efficiencies (k cat/K m) toward D-galactose than that of wild-type L-AI. In addition, the D478N and D478Q variants exhibited a much higher conversion ratio of D-galactose to D-tagatose at 6.0 than the wild-type L-AI. According to the molecular model, residue D478 was located on the surface of the enzyme and distant from the active site. It was supposed that the charged state of residue 478 may influence the optimum pH for substrate binding or isomerization.

The genus Geobacillus and their biotechnological potential

The genus Geobacillus comprises a group of Gram-positive thermophilic bacteria, including obligate aerobes, denitrifiers, and facultative anaerobes that can grow over a range of 45-75°C. Originally classified as group five Bacillus spp., strains of Bacillus stearothermophilus came to prominence as contaminants of canned food and soon became the organism of choice for comparative studies of metabolism and enzymology between mesophiles and thermophiles. More recently, their catabolic versatility, particularly in the degradation of hemicellulose and starch, and rapid growth rates have raised their profile as organisms with potential for second-generation (lignocellulosic) biorefineries for biofuel or chemical production. The continued development of genetic tools to facilitate both fundamental investigation and metabolic engineering is now helping to realize this potential, for both metabolite production and optimized catabolism. In addition, this catabolic versatility provides a range of useful thermostable enzymes for industrial application. A number of genome-sequencing projects have been completed or are underway allowing comparative studies. These reveal a significant amount of genome rearrangement within the genus, the presence of large genomic islands encompassing all the hemicellulose utilization genes and a genomic island incorporating a set of long chain alkane monooxygenase genes. With G+C contents of 45-55%, thermostability appears to derive in part from the ability to synthesize protamine and spermine, which can condense DNA and raise its Tm.

L-Arabinose isomerase and its use for biotechnological production of rare sugars

L-Arabinose isomerase (AI), a key enzyme in the microbial pentose phosphate pathway, has been regarded as an important biological catalyst in rare sugar production. This enzyme could isomerize L-arabinose into L-ribulose, as well as D-galactose into D-tagatose. Both the two monosaccharides show excellent commercial values in food and pharmaceutical industries. With the identification of novel AI family members, some of them have exhibited remarkable potential in industrial applications. The biological production processes for D-tagatose and L-ribose (or L-ribulose) using AI have been developed and improved in recent years. Meanwhile, protein engineering techniques involving rational design has effectively enhanced the catalytic properties of various AIs. Moreover, the crystal structure of AI has been disclosed, which sheds light on the understanding of AI structure and catalytic mechanism at molecular levels. This article reports recent developments in (i) novel AI screening, (ii) AI-mediated rare sugar production processes, (iii) molecular modification of AI, and (iv) structural biology study of AI. Based on previous reports, an analysis of the future development has also been initiated.