Bioproduction of D-Tagatose from D-Galactose Using Phosphoglucose Isomerase from Pseudomonas aeruginosa PAO1

Highly efficient production and simultaneous purification of d-tagatose through one-pot extraction-assisted isomerization of d-galactose

A one-pot extraction-assisted d-galactose-to-d-tagatose isomerization strategy was proposed based on the selective extraction of d-tagatose by phenylborate anions. 4-Vinylphenylboronic acid was selected with high extraction efficiency and selectivity towards d-tagatose. The extracted sugars could be desorbed through a two-staged stripping process with the purity of d-tagatose significantly increased. In-situ extraction-assisted d-galactose-to-d-tagatose isomerization was implemented for the first time ever reported, and the effect of boron-to-sugar ratio (boron: sugar) was investigated. The conversion yield of d-tagatose at 60 °C increased from ∼ 39 % (boron: sugar = 0.5) to ∼ 56 % (boron: sugar = 1) but then decreased to ∼ 44 % (boron: sugar = 1.5). With temperature increased to 70 °C, the conversion yield of d-tagatose was further improved to ∼ 61 % (boron: sugar = 1.5), with the minimized formation of byproducts. Moreover, high purity (∼83 %) and concentrated d-tagatose solution (∼40 g/L) was obtained after sequential desorption. The proposed extraction-assisted isomerization strategy achieved improving the yield and purity of d-tagatose, proving its feasibility in industrial applications.

Enzymes, In Vivo Biocatalysis, and Metabolic Engineering for Enabling a Circular Economy and Sustainability

Since the industrial revolution, the rapid growth and development of global industries have depended largely upon the utilization of coal-derived chemicals, and more recently, the utilization of petroleum-based chemicals. These developments have followed a linear economy model (produce, consume, and dispose). As the world is facing a serious threat from the climate change crisis, a more sustainable solution for manufacturing, i.e., circular economy in which waste from the same or different industries can be used as feedstocks or resources for production offers an attractive industrial/business model. In nature, biological systems, i.e., microorganisms routinely use their enzymes and metabolic pathways to convert organic and inorganic wastes to synthesize biochemicals and energy required for their growth. Therefore, an understanding of how selected enzymes convert biobased feedstocks into special (bio)chemicals serves as an important basis from which to build on for applications in biocatalysis, metabolic engineering, and synthetic biology to enable biobased processes that are greener and cleaner for the environment. This review article highlights the current state of knowledge regarding the enzymatic reactions used in converting biobased wastes (lignocellulosic biomass, sugar, phenolic acid, triglyceride, fatty acid, and glycerol) and greenhouse gases (CO₂ and CH₄) into value-added products and discusses the current progress made in their metabolic engineering. The commercial aspects and life cycle assessment of products from enzymatic and metabolic engineering are also discussed. Continued development in the field of metabolic engineering would offer diversified solutions which are sustainable and renewable for manufacturing valuable chemicals.

Phosphate sugar isomerases and their potential for rare sugar bioconversion

Phosphate sugar isomerases, catalyzing the isomerization between ketopentose/ketohexose phosphate and aldopentose/aldohexose phosphate, play an important role in microbial sugar metabolism. They are present in a wide range of microorganisms. They have attracted increasing research interest because of their broad substrate specificity and great potential in the enzymatic production of various rare sugars. Here, the enzymatic properties of various phosphate sugar isomerases are reviewed in terms of their substrate specificities and their applications in the production of valuable rare sugars because of their functions such as low-calorie sweeteners, bulking agents, and pharmaceutical precursor. Specifically, we focused on the industrial applications of D-ribose-5-phosphate isomerase and D-mannose-6-phosphate isomerase to produce D-allose and L-ribose, respectively.

A Three-Step Process for the Bioconversion of Whey Permeate into a Glucose-Free D-Tagatose Syrup

We have developed a sustainable three-stage process for the revaluation of cheese whey permeate into D-tagatose, a rare sugar with functional properties used as sweetener. The experimental conditions (pH, temperature, cofactors, etc.) for each step were independently optimized. In the first step, concentrated whey containing 180–200 g/L of lactose was fully hydrolyzed by β-galactosidase from Bifidobacterium bifidum (Saphera®) in 3 h at 45 °C. Secondly, glucose was selectively removed by treatment with Pichia pastoris cells for 3 h at 30 °C. The best results were obtained with 350 mg of cells (previously grown for 16 h) per mL of solution. Finally, L-arabinose isomerase US100 from Bacillus stearothermophilus was employed to isomerize D-galactose into D-tagatose at pH 7.5 and 65 °C, in presence of 0.5 mM MnSO4. After 7 h, the concentration of D-tagatose was approximately 30 g/L (33.3% yield, referred to the initial D-galactose present in whey). The proposed integrated process takes place under mild conditions (neutral pH, moderate temperatures) in a short time (13 h), yielding a glucose-free syrup containing D-tagatose and galactose in a ratio 1:2 (w/w).

Identification and biochemical characterization of a novel cold-adapted 1,3-α-3,6-anhydro-L-galactosidase, Ahg786, from Gayadomonas joobiniege G7

Agar is a major polysaccharide of red algal cells and is mainly decomposed into neoagarobiose by the co-operative effort of β-agarases. Neoagarobiose is hydrolyzed into monomers, D-galactose and 3,6-anhydro-L-galactose, via a microbial oxidative process. Therefore, the enzyme, 1,3-α-3,6-anhydro-L-galactosidase (α-neoagarobiose/neoagarooligosaccharide hydrolase) involved in the final step of the agarolytic pathway is crucial for bioindustrial application of agar. A novel cold-adapted α-neoagarooligosaccharide hydrolase, Ahg786, was identified and characterized from an agarolytic marine bacterium Gayadomonas joobiniege G7. Ahg786 comprises 400 amino acid residues (45.3 kDa), including a 25 amino acid signal peptide. Although it was annotated as a hypothetical protein from the genomic sequencing analysis, NCBI BLAST search showed 57, 58, and 59% identities with the characterized α-neoagarooligosaccharide hydrolases from Saccharophagus degradans 2-40, Zobellia galactanivorans, and Bacteroides plebeius, respectively. The signal peptide-deleted recombinant Ahg786 expressed and purified from Escherichia coli showed dimeric forms and hydrolyzed neoagarobiose, neoagarotetraose, and neoagarohexaose into 3,6-anhydro-L-galactose and other compounds by cleaving α-1,3-glycosidic bonds from the non-reducing ends of neoagarooligosaccharides, as confirmed by thin-layer chromatography and mass spectrometry. The optimum pH and temperature for Ahg786 activity were 7.0 and 15 °C, respectively, indicative of its unique cold-adapted features. The enzymatic activity severely inhibited with 0.5 mM ethylenediaminetetraacetic acid was completely restored or remarkably enhanced by Mn²⁺ in a concentration-dependent manner, suggestive of the dependence of the enzyme on Mn²⁺ ions. K_m and V_max values for neoagarobiose were 4.5 mM and 1.33 U/mg, respectively.

D-Tagatose Is a Promising Sweetener to Control Glycaemia: A New Functional Food

The objective of the current research was to review and update evidence on the dietary effect of the consumption of tagatose in type 2 diabetes, as well as to elucidate the current approach that exists on its production and biotechnological utility in functional food for diabetics. Articles published before July 1, 2017, were included in the databases PubMed, EBSCO, Google Scholar, and Scielo, including the terms "Tagatose", "Sweeteners", "Diabetes Mellitus type 2", "Sweeteners", "D-Tag". D-Tagatose (D-tag) is an isomer of fructose which is approximately 90% sweeter than sucrose. Preliminary studies in animals and preclinical studies showed that D-tag decreased glucose levels, which generated great interest in the scientific community. Recent studies indicate that tagatose has low glycemic index, a potent hypoglycemic effect, and eventually could be associated with important benefits for the treatment of obesity. The authors concluded that D-tag is promising as a sweetener without major adverse effects observed in these clinical studies.