What Are the Benefits of D Tagatose?

Excessive sugar consumption is associated with chronic diseases such as obesity and diabetes, and is just as harmful to the body as alcohol[1]. In developed countries and regions such as Europe and the United States, sugar taxes are levied on foods such as sugary drinks in order to guide consumers to reduce their sugar intake. In recent years, the market demand for sugar reduction and sugar substitutes has continued to grow; sugar control and sugar reduction meet the important national strategic need of “Healthy China”.

Rare sugars are monosaccharides and their derivatives that exist in nature but in very low concentrations[2]. As a new type of functional sweetener, they have become a hot topic of international research. Rare sugars produce less energy after consumption and have physiological functions such as inhibiting blood sugar rise, anti-obesity, anti-oxidation and anti-inflammation[3-4]. Using rare sugars as a substitute for sugar, which have very low calories, is in line with the concept of “great hygiene, and health” concept [5 - 6]. D-tagatose is a rare sugar that tastes similar to sucrose and is 92% as sweet as sucrose, but only one-third as high in calories.

D-tagatose has a variety of beneficial physiological functions for the human body, such as preventing obesity, lowering blood sugar, anti-caries, anti-oxidation, probiotics, improving intestinal flora, enhancing immunity, and preventing cardiovascular and cerebrovascular diseases. Therefore, D-tagatose is an ideal functional sweetener. In 2001, the U.S. Food and Drug Administration (FDA) Administration (FDA) recognized D-tagatose as a “generally recognized as safe” (GRAS) food [7], and was recommended by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the United Nations Food and Agriculture Organization recommend it as a new type of sweetener that can be used as a food additive [8]. In 2014, in accordance with the Food Safety Law of the People's Republic of China and the Measures for the Administration of the Safety Review of New Food Raw Materials, the National Health and Family Planning Commission approved D-tagatose as a new food ingredient. Therefore, D-tagatose has very broad market prospects.

1. Metabolism and physiological activity of D-tagatose

D-tagatose is an important rare hexose, an isomer of D-galactose, a C-4 epimer of D-fructose or a C-3 epimer of D-sorbitol, as shown in Figure 1.

1. 1. Metabolism of D-tagatose

Although D-tagatose and D-fructose are similar in structure, the body's absorption efficiency of D-tagatose is very low, with only 20% to 25% being absorbed in the small intestine. After D-tagatose is absorbed in the small intestine, the metabolic process is similar to that of D-fructose, but the metabolic rate is only 50% of that of fructose. D-tagatose is converted to D-tagatose-1-phosphate by the action of fructokinase, which is broken down by aldolase into dihydroxyacetone phosphate and glyceraldehyde. Fructokinase shows a lower affinity for D-tagatose than for D-fructose [9]. D-tagatose that is not absorbed in the small intestine enters the large intestine, where it is fermented by intestinal microorganisms and short-chain fatty acids are produced [10–11]. In the large intestine, D-tagatose is metabolised via the galactose branch of the metabolic pathway, the tagatose-6-phosphate metabolic pathway. Under the action of hexokinase, D-tagatose is converted to D-tagatose-6-phosphate, which is then broken down into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by the action of tagatose-6-phosphate kinase and tagatose-1,6-diphosphate aldolase.

1. 2 Physiological activity of D-tagatose

D-tagatose has various physiological functions, such as lowering blood sugar, reducing obesity, promoting intestinal flora, preventing tooth decay, anti-oxidation, and improving fertility (Fig. 3).

1. 2. 1    Lowering blood sugar

D-tagatose can regulate blood sugar levels in both healthy people and type 2 diabetes patients. Compared with traditional oral diabetes drugs, D-tagatose has the advantages of high safety, anti-oxidation activity, and inhibition of weight gain [11]. Studies have shown that giving 75 g of D-tagatose to diabetic patients 30 minutes before a glucose tolerance test can significantly inhibit blood glucose elevation without affecting blood insulin levels[12].

Furthermore, the minimum dose of D-tagatose is 5 g (3 times/d), which can effectively control the level of acetylated hemoglobin in patients with type 2 diabetes[13]. So far, the mechanism of hypoglycemic action of D-tagatose has not been fully elucidated, but the following possible mechanisms have been proposed: D-tagatose is metabolized in the liver similarly to D-fructose, and is converted to D-tagatose-1-phosphate by fructokinase, which induces glucokinase to move from the nucleus into the cytoplasm, enhancing the conversion of glucose to gluco-6-phosphate and entering the glycolytic pathway, while also promoting the conversion of glucose to glycogen [14 -15]. D-tagatose-1-phosphate can also inhibit the activity of glycogen phosphorylase, thereby inhibiting the breakdown of glycogen into glucose. In addition, D-tagatose can inhibit the activity of the main digestive enzymes in the intestine (sucrase and maltase), which has the effect of lowering blood sugar. Therefore, D-tagatose can be added to the diet of diabetic patients as a sugar substitute to help lower blood sugar.

1. 2. 2    Reducing obesity

Weight management plays an important role in reducing the risk of obesity-related diseases. D-tagatose has low calorie properties and can be used as a promising sweetener for weight management. For many people with diabetes, weight loss is a key factor in controlling blood sugar. Long-term consumption of D-tagatose by diabetic patients can significantly reduce body weight [16], and D-tagatose can reduce food intake in healthy men [17]. For pregnant women, weight management is a common problem. Excessive weight gain will increase the health risks of both the mother and the fetus. Therefore, D-tagatose also has potential applications in the mother and child market.

1. 2. 3 Prebiotic function

Prebiotics are organic substances that are not digested or absorbed by the host but selectively promote the metabolism and proliferation of beneficial bacteria in the body, thereby improving host health [18]. D-tagatose is an excellent prebiotic. D-tagatose is resistant to the human gastrointestinal conditions and can effectively stimulate the growth of probiotics, especially Bifid bacterium infantis, thereby inhibiting the growth of pathogenic bacteria [19]; studies have shown that the human body is rich in D-tagatose in a healthy mouth, and because D-tagatose has an inhibitory effect on the growth of bacteria such as Streptococcus gordonii that cause tooth decay [20], it has an anti-caries function. Promoting the proliferation of beneficial bacteria, inhibiting the growth of pathogenic bacteria, and anti-caries functions all reflect the good probiotic function of D-tagatose.

1. 2. 4 Antioxidant

Intracellular free radicals can cause damage to cells, leading to cancer, aging, or other diseases. D-tagatose has the potential to reduce intracellular free radicals that cause cell damage. Studies have shown that compared to an equivalent mass of glucose, mannitol or xylose, D-tagatose can inhibit the oxidative damage caused by the drug furazolidone in mouse liver cells [21]. D-tagatose has weak iron chelating properties. Therefore, it can protect cells from iron-induced cytotoxicity by inhibiting the production of free radicals caused by iron-catalyzed lipid peroxidation and protein carbonylation [22–23].

1. 2. 5 Other functions

D-tagatose also plays a role in organ transplantation, improving fertility, and the growth of newborns [22], and research into its physiological functions has never stopped.

2 D-tagatose biosynthesis methods

The main methods of synthesizing D-tagatose are chemical synthesis and biotransformation. In chemical synthesis, D-galactose is used as the raw material, and under the catalysis of an alkali or alkaline earth metal, an isomerization reaction occurs to form a precipitate of a metal hydroxide and a D-tagatose complex intermediate. The intermediate is then neutralized with acid to obtain D-tagatose. Although the chemical synthesis method is a cost-effective way to produce D-tagatose, it requires high temperatures and pressures during the production process, and it is prone to the formation of impurity sugars such as sorbitol and mannose, which are not conducive to separation and purification. Therefore, in order to overcome these shortcomings, the bioconversion method for producing D-tagatose has received widespread attention and research [24-25]. After years of in-depth research, the biotransformation method for producing D-tagatose can be divided into three categories according to the substrate, which are hexose, lactose and polysaccharide, respectively.

2. 1 Using hexose as a raw material

The rare sugars available on the market are very limited and expensive, which severely restricts their development and application. To overcome this situation, it is necessary to find a way to mass-produce rare sugars. Izumori proposed the Izumoring strategy as a solution to the production of rare sugars [26]. The production of D-tagatose from hexose mainly involves three enzymes: isomerase, diastereoisomerase and dehydrogenase. The isomerase is mainly used to catalyze D-galactose, the diastereoisomerase is used to catalyze D-fructose and D-sorbitol, and the dehydrogenase is used to catalyze galactitol (Figure 4).

2. 1. 1 Using galactitol as a raw material

The production of D-tagatose from galactitol is one of the earliest biosynthetic methods studied. It mainly uses sorbitol dehydrogenase to dehydrogenate galactitol to produce D-tagatose. Early research on the production of D-tagatose mainly used the microorganisms Arthrobacter globiformis and Mycobacterium smegmatis to oxidize galactitol to D-tagatose [27]. Current research mainly uses Oxidobacterium gluconicum to express membrane-bound sorbitol dehydrogenase to oxidize galactitol into D-tagatose and L-xylulose-3-hexulosone [28]. Due to the relatively high price of galactitol and the high production cost, the commercial price of D-tagatose is high, which limits its application.

2. 1. 2 Using D-galactose as a raw material

Because D-galactose is relatively cheaper than galactitol, it has a higher potential for commercial application. Using D-galactose as a raw material, enzymatic synthesis of D-tagatose has gradually become the mainstream synthesis method [29]. The core enzyme used in this method is L-arabinose isomerase (L-arabinose isomer-ase, AI). Due to the structural similarity between D-galactose and L-arabinose, AI can catalyze the conversion of D-galactose to D-tagatose [30]. In order to improve the yield of D-galactose to D-tagatose, many studies have been carried out on this enzyme. For example, AI from various sources has been molecularly modified to improve enzyme activity, catalytic efficiency, pH and temperature stability, etc., or the efficiency of D-galactose conversion to D-tagatose has been improved by enzyme immobilization technology [24, 31]. At present, these studies have gradually reached saturation and are now mainly focused on finding catalytic methods that can increase the yield of D-tagatose. For example, a packed bed reactor using AI microspheres fixed on an alginate carrier was used to produce D-tagatose, which achieved a 50% equilibrium conversion rate and can be used multiple times [32].

2.1.3 Using D-fructose as a raw material

In recent years, the biological synthesis of D-tagatose has generally used D-galactose as a substrate for catalytic synthesis by AI. However, compared with carbohydrates such as glucose, fructose and starch, D-galactose is still relatively expensive to produce, which is not conducive to the large-scale industrial production and application of D-tagatose. Therefore, it is of great research significance to find a method that can use cheap and readily available biomass resources to efficiently synthesize D-tagatose. D-fructose is a kind of monosaccharide that is inexpensive and has a stable source. ideal substrate for the production of D-tagatose, but to date, no naturally occurring enzymes that catalyze the conversion of D-fructose to D-tagatose have been discovered in nature. In 2012, Rodionova et al. [33] discovered a new enzyme family (named “UxaE”) that can catalyze the conversion of D-tagatose aldehyde to D-fructose aldehyde; Shin et al. [ 34] believed that this enzyme may have 4-isomerization activity for D-fructose because its substrate has a similar structure to fructose, and then through rational design and directed evolution, the C4-epimerization activity of this enzyme for D-fructose was improved, and the mutant enzyme was named D-tagatose 4-epimerase. The developed D-tagatose 4-epimerase was applied to the synthesis of D-tagatose. Under optimal reaction conditions, 700 g/L D-fructose was converted to produce 213 g/L D-tagatose within 2 h, for a conversion rate of 30%.

In addition to the use of D-tagatose 4-epimerase to catalyze the production of D-tagatose from fructose, there are also two multi-enzyme catalytic pathways for the production of D-tagatose from D-fructose (Figure 5): 1) D-fructose is converted to D- fructose-6-phosphate, and D-fructose-1,6-bisphosphate aldolase catalyzes the conversion of D-fructose-6-phosphate to D-tagatose-6-phosphate, and finally D-tagatose-6-phosphate is converted to D-tagatose by phosphatase [35]. Since this method requires the participation of a kinase, ATP needs to be added during the reaction, which is not conducive to practical production applications, so this method has been studied relatively little [36]; 2) Yoshihara et al. [37] successfully used the rice mold Rhizopus oryzae MYA-2483 to catalyze the production of D-tagatose from D-allulose, and found that most Mucoraceae fungi have this conversion ability, so D-tagatose-3-epimerase can be used to isomerize D-fructose to D-tagatose. and found that most Mucoraceae fungi have this conversion ability, so D-tagatose-3-epimerase can be used to isomerize D-fructose to D-allulose[38] , and then convert D-allulose to D-tagatose. The large-scale production of D-allulose from D-fructose is feasible, so this method has the potential for commercial production, but there has been little research on it.

2. 2 Using lactose as a raw material

Using lactose as a raw material to produce D-tagatose is catalyzed by the double enzyme β-galactosidase and arabinose isomerase, which has also been a research hotspot in recent years. Whey is usually a by-product rich in lactose produced during the production of dairy products, and in the past it was often treated as waste. With the progress of science and technology, it has now become a valuable food ingredient that can be used in the production of D-tagatose. Using lactose as a substrate, D-tagatose with different product concentrations (12.7-9 6.8 g/L), and the yield of D-tagatose from lactose ranged from 19.4% to 36.7% (Table 1 [39-47]). Therefore, this method is not efficient in terms of carbon source utilization, and the residue will make the separation and purification of D-tagatose difficult. To solve this problem, lactose can be used as a raw material to produce multiple products at the same time. For example, whey powder can be used as a raw material to simultaneously produce D-tagatose and bioethanol, with yields of 23.5% and 26.9%, respectively [48]. This not only improves the utilization rate of the carbon source, but also produces valuable by-products. Due to the other unidentified components in whey powder, which may affect the activity of arabinose isomerase, the yield decreased by 7.8% compared to lactose.

2. 3    Polysaccharides as raw materials

The polysaccharides used to produce D-tagatose are mainly maltodextrin and starch. They are converted to D-tagatose using phosphoribosyltransferase, glucokinase, glucokinase, C4 epimerase, and phosphoribosyl pyrophosphate synthetase [49 -50]. The multi-enzyme catalytic synthesis of D-tagatose using starch or maltodextrin as a substrate is shown in Figure 6. Compared to the multi-enzyme catalytic pathway using fructose as a substrate, the phosphorylase used to catalyze the production of glucose-1-phosphate from maltodextrin and starch does not require ATP, which reduces the production cost of D-tagatose and the instability of the production process. In order to avoid purifying multiple synthetic enzymes, Dai et al. [49] constructed a D-tagatose anabolic pathway in Escherichia coli, achieving the production of 3.38 g/L D-tagatose from 10 g/L maltodextrin sugar. Han et al. [50] constructed a semi-artificial cell factory and achieved the production of 72.2 g/L D-tagatose from 150 g/L maltodextrin, with a yield of 48.1%, which has good application value.

3 Summary and outlook

E-tagatose, as an ideal functional sweetener, has broad application prospects. Against the backdrop of the continued strong market demand for sugar-reducing and sugar-substitute products, D-tagatose is of great significance for meeting people's demand for “less sugar but no less sweet” and improving their quality of life and health. This paper reviews recent research on the physiological functions and biosynthesis of D-tagatose. In order to reduce the production cost of D-tagatose and make it available to every household, the following research is recommended: 1) The research and commercial production of D-tagatose from D-galactose as a raw material is relatively mature, but the high cost of D-galactose limits the promotion and application of D-tagatose. Research can focus on the production of D-tagatose from cheap substrates such as fructose, lactose, malt dextrin and starch to produce D-tagatose, in order to expand the market for D-tagatose; 2) through molecular simulation based on protein structure, (semi-)rationally modify key enzymes to improve their catalytic efficiency and stability and enhance their potential for industrial application; 3) construct food-grade chassis cells and systematically modify the chassis cells to develop a synthetic method for D-tagatose that tolerates high substrate concentrations, has a high conversion rate, and high production intensity, as well as an efficient purification process for D-tagatose; 4) to conduct in-depth research and continue to expand the functions of D-tagatose so that it can fully demonstrate its advantages and be more fully utilized in the fields of food, medicine, agriculture, cosmetics, etc. Carrying out the above research will help industrialize D-tagatose, and it is hoped that this article can provide effective information for the promotion and application of D-tagatose.

Reference:

[1]    LUSTIG R  H , SCHMIDT L A , BRINDIS  C D.  The toxic truth about sugar[J] .  Nature , 2012 , 482(7383) : 27 -29.

[2] HU Y F , WANG Y L , FEI K Q , et al. Efficient synthesis of D-allulose by Bacillus subtilis whole cell transformation [J] .   Journal  of  Food  Science  and Technology ,  2023 , 41(5) : 76 -84.

[3]    HAN Y , PARK H ,  CHOI B R , et al.  Alteration of mi- crobiome profile by D-allulose in amelioration of high-fat- diet-induced  obesity   in   mice  [ J ] .    Nutrients ,   2020 , 12(2) : 352.

[4]    IWASAKI Y , SENDO M , DEZAKI K , et al.  GLP-1 re- lease and vagal afferent activation  mediate the beneficial metabolic and chronotherapeutic effects of D-allulose[J] .  Nature Communications , 2018 , 9 : 113.

[5] CHEN Z ,  QIN  L ,  LI  F ,  et  al.  System  construction  for L-fructose production from glycerol by whole-cell biocatalysis [J] .   Journal  of  Food  Science  and Technology ,  2020 , 38(3) : 60 -68.

[6] ZHAO J Y , GUO Y , FENG T T , et al.  Construction and process  optimization  of  cell  factory  for   allitol  synthesis with multienzymatic cascade system [J] .  Journal of Food Science and Technology , 2022 , 40(3) :88 -97.

[7]    LIU J J , ZHANG G C , KWAK S , et al.  Overcoming the thermodynamic  equilibrium  of  an   isomerization  reaction through   oxidoreductive    reactions   for    biotransformation [J] .  Nature Communications , 2019 , 10 : 1356.

[8]  DU M G , ZHAO D Y , CHENG S S , et al.  Towards effi- cient enzymatic  conversion  of  D-galactose  to  D-tagatose : purification and characterization of L-arabinose isomerase from Lactobacillus brevis[J] .  Bioprocess and Biosystems

Engineering , 2019 , 42(1) : 107 - 116.

[9]    MARTÍNEZ P ,  CARRASCOSA J  M , NUÑEZ  DE C I.  Ketose induced  respiratory  inhibition  in  isolated  hepato- cytes  [ J ] .   Revista  Espanola   De   Fisiologia ,   1987 , 43(2) : 163 - 171.

[10]    LÆRKE H N , JENSEN B B , HØJSGAARD S.  In vitro fermentation pattern of D-tagatose is affected by adapta- tion of the  microbiota  from  the gastrointestinal  tract of pigs[ J] .  The  Journal  of  Nutrition ,  2000 ,  130 (7) : 1772 - 1779.

[11]    LU Y , LEVIN  G V , DONNER T W.  Tagatose , a new antidiabetic  and   obesity  control  drug [ J] .   Diabetes , Obesity ＆ Metabolism , 2008 , 10(2) : 109 - 134.

[12]    DONNER  T   W ,  WILBER  J   F ,  OSTROWSKI   D.  D-tagatose , a novel hexose : acute effects on carbohydrate tolerance  in  subjects  with and  without type  2 diabetes [J] .  Diabetes , Obesity and Metabolism , 1999 , 1(5) : 285 -291.

[13]    ENSOR M , WILLIAMS J , SMITH R , et al.  Effects of three low-doses  of D-tagatose  on  glycemic  control  over six months in subjects with mild type 2  diabetes melli- tus under control with diet and exercise[J] .  Journal of Endocrinology ,  Diabetes  ＆  Obesity ,  2014 ,  2  ( 4 ) : 1057 .

[14]    AGIUS L.  The physiological role of glucokinase binding and translocation in hepatocytes[J] .  Advances  in En- zyme Regulation , 1998 , 38(1) : 303 -331.

[15]    SEOANE  J ,  GÓMEZ-FOIX  A  M ,  O'DOHERTY  R  M , et al.   Glucose  6-phosphate   produced  by  glucokinase , but not hexokinase I , promotes the activation of hepatic glycogen synthase[J] .  Journal of Biological Chemistry , 1996 , 271(39) : 23756 -23760.

[16]    DONNER T W.  The metabolic effects of dietary supple- mentation with D-tagatose in patients with type 2  diabe- tes [J] .  Diabetes , 2006 , 55 : 110.

[17]    BUEMANN  B ,  TOUBRO  S ,  RABEN  A ,  et  al.   The acute   effect   of  D-tagatose   on  food   intake   in   human subjects[J] .    British    Journal    of    Nutrition ,   2000 , 84(2) : 227 -231.

[18]    GIBSON G R , ROBERFROID M B.  Dietary modulation of the human colonic microbiota : introducing the concept of  prebiotics  [ J] .   The   Journal  of  Nutrition ,  1995 , 125(6) : 1401 - 1412.

[19]    LEITE A K F , SANTOS B N , FONTELES T V , et al.  Cashew  apple  juice   containing   gluco-oligosaccharides , dextran ,   and    tagatose    promotes   probiotic    microbial growth[J] .  Food Bioscience , 2021 , 42 : 101080.

[20]    MAYUMI S ,  KUBONIWA  M ,  SAKANAKA A , et  al.  Potential  of  prebiotic  D-tagatose for prevention  of  oral disease[J] .  Frontiers in Cellular and Infection Microbio- logy , 2021 , 11 : 767944.

[21]    PATERNA J C , BOESS F , STÄUBLI A , et al.  Antioxi- dant and  cytoprotective  properties  of  D-tagatose  in  cul- tured murine  hepatocytes[ J] .   Toxicology  and  Applied Pharmacology , 1998 , 148(1) : 117 - 125.

[22]    ROY S , CHIKKERUR J , ROY S C , et al.  Tagatose as a potential nutraceutical : production , properties , biolo- gical roles , and applications[J] .  Journal of Food Sci- ence , 2018 , 83(11) : 2699 -2709.

[23]    BILAL M , IQBAL H  M N , HU H B , et al.  Metabolic engineering pathways for rare  sugars biosynthesis , physio- logical functionalities ,  and  applications : a  review [ J] .  Critical Reviews in Food Science and Nutrition , 2018 , 58(16) : 2768 -2778.

[24]    JAYAMUTHUNAGAI J , GAUTAM P , SRISOWMEYA G , et al.  Biocatalytic production of D-tagatose : a potential rare sugar with versatile  applications[J] .   Critical  Re- views in Food Science and Nutrition , 2017 , 57(16) : 3430 -3437.

[25]    OH D  K.   Tagatose :  properties ,  applications ,  and  bio- technological processes[J] .   Applied  Microbiology  and Biotechnology , 2007 , 76(1) : 1 -8.

[26]    IZUMORI K.   Izumoring : a  strategy for bioproduction of all  hexoses  [ J ] .    Journal    of   Biotechnology ,   2006 , 124(4) : 717 -722.

[27]    MANZONI M , ROLLINI M , BERGOMI S.   Biotransfor- mation of D-galactitol to tagatose by acetic acid bacteria [J] .  Process Biochemistry , 2001 , 36(10) : 971 -977.

[28]    XU Y R , JI L Y , XU S , et al.  Membrane-bound sorbi- tol dehydrogenase is responsible for the unique oxidation of  D-galactitol  to   L-xylo-3-hexulose  and  D-tagatose  in Gluconobacter  oxydans [ J] .   Biochimica   et  Biophysica Acta  ( BBA )-General   Subjects ,  2023 ,   1867  ( 2 ) : 130289.

[29]    ZHANG Y , FAN Y L , HU H J , et al.  D-Tagatose pro- duction  by   Lactococcus  lactis  NZ9000  cells  harboring Lactobacillus plantarum L-arabinose isomerase[J] .  Indian Journal   of   Pharmaceutical   Education   and   Research , 2017 , 51(2) : 288 -294.

[30] MEN Y , ZHU Y M , GUAN Y P , et al.  Screening of food-grade    microorganisms    for     biotransformation    of D-tagatose  and   cloning  and  expression  of  L-arabinose isomerase[J] .  Chinese Journal of Biotechnology , 2012 , 28(5) : 592 -601.

[31]    RAVIKUMAR Y , PONPANDIAN L N , ZHANG  G Y , et  al.   Harnessing  L-arabinose  isomerase for biological production of D-tagatose : recent advances and its appli- cations[ J] .   Trends  in   Food  Science  ＆  Technology , 2021 , 107 : 16 -30.

[32]    RAI S  K ,  KAUR  H ,  SINGH  A ,  et al.  Production of D-tagatose in packed bed reactor containing an immobilized L-arabinose isomerase on  alginate  support [J] .   Biocataly- sis and Agricultural Biotechnology , 2021 , 38 : 102227.

[33]    RODIONOVA I A , SCOTT D A , GRISHIN N V , et al.  Tagaturonate-fructuronate epimerase  UxaE ,  a  novel  en- zyme in the hexuronate catabolic network in Thermotoga maritima  [ J ] .    Environmental    Microbiology ,   2012 , 14(11) : 2920 -2934.

[34]    SHIN K C , LEE T E , SEO M J , et al.  Development of tagaturonate 3-epimerase  into  tagatose 4-epimerase  with a biocatalytic route from fructose to tagatose[J] .  ACS Catalysis , 2020 , 10(20) : 12212 - 12222.

[35]    LEE S  H ,  HONG  S H , KIM  K  R , et al.  High-yield production of pure tagatose from fructose by a three-step enzymatic cascade reaction[J] .  Biotechnology Letters , 2017 , 39(8) : 1141 - 1148.

[36]    DAI Y W.  Biosynthesis of D-tagatose from maltodextrin by multi-enzyme catalytic system [D] .  Wuxi : Jiangnan University , 2021.

[37]    YOSHIHARA K , SHINOHARA Y , HIROTSU T , et al.  Bioconversion of D-psicose to D-tagatose and D-talitol by Mucoraceae fungi[J] .  Journal of Bioscience  and Bioen- gineering , 2006 , 101(3) : 219 -222.

[38]    HU M  K ,  WEI  Y  X ,  ZHANG  R  Z ,  et  al.   Efficient D-allulose  synthesis   under  acidic  conditions  by  auto- inducing expression  of  the  tandem  D-allulose  3-epime- rase genes in Bacillus subtilis[J] .  Microbial Cell Facto- ries , 2022 , 21(1) : 63.

[39]  LI Z Y , ZHANG X , RAO Z M , et al.  One-step synthe- sis of lactose to tagatose by co-expressing β-galactosidase and arabinose isomerase[J] .  Acta Microbiologica Sinica , 2021 , 61(9) : 2907 -2920.

[40]  YUAN  J.    Construction  of  genetic   engineering   strain using  β-galactosidase  and  L-arabinose  isomerase  and biosynthesis  of  D-tagatose  [ D ] .    Zhenjiang :   Jiangsu University , 2021.

[41]  XIE  J  X.   Valorization of  whey to produce  D-tagatose using a double enzyme coupling  system [D] .   Nanjing : Nanjing Forestry University , 2019.

[42] ZHANG L  Q.   Enzymatic synthesis of  D-tagatose from lactose with  the  combination  of  β-galactosidase  and L- arabinose isomerase[D] .  Shanghai : East China Univer- sity of Science and Technology , 2012.

[43]    ZHANG X A , LU R Q , WANG Q A , et al.  Production of  D-tagatose  by whole-cell  conversion  of  recombinant Bacillus  subtilis   in   the   absence  of   antibiotics [ J ] .  Biology , 2021 , 10(12) : 1343.

[44]    ZHANG S S , GUO T T , XIN Y P , et al.  Biotechnologi- cal production of D-tagatose from lactose using metaboli- cally   engineering   Lactiplantibacillus  plantarum  [ J ] .  LWT-Food   Science    and   Technology ,   2021 ,    142 : 110995.

[45]    ZHANG G  Y , ZABED  H M , YUN J H , et al.  Two- stage biosynthesis of D-tagatose from milk whey powder by  an  engineered  Escherichia  coli  strain expressing  L- arabinose  isomerase  from Lactobacillus  plantarum [ J] .  Bioresource Technology , 2020 , 305 : 123010.

[46]    XU Z , XU Z X , TANG  B , et al.  Construction and co- expression of polycistronic plasmids encoding thermophi- lic    L-arabinose     isomerase     and     hyperthermophilic β-galactosidase for  single-step  production  of  D-tagatose [J] .   Biochemical   Engineering  Journal ,  2016 ,   109 : 28 -34.

[47]    ZHAN Y J ,  XU Z , LI S , et al.  Coexpression of β-D-galactosidase and  L-arabinose  isomerase  in  the  produc- tion of D-tagatose : a functional  sweetener[J] .  Journal of Agricultural  and  Food  Chemistry ,  2014 ,  62 ( 11) : 2412 -2417.

[48]    ZHENG Z J , XIE J X , LIU P , et al.  Elegant and effi- cient biotransformation for dual production of D-tagatose and bioethanol from cheese whey powder[J] .  Journal of Agricultural and Food Chemistry , 2019 , 67(3) : 829 -  835.

[49]    DAI Y W , LI C C , ZHENG L H , et al.  Enhanced bio- synthesis of D-tagatose from maltodextrin through modu- lar pathway  engineering  of recombinant Escherichia coli [J] .   Biochemical   Engineering  Journal ,  2022 ,   178 : 108303.

[50]    HAN P P , WANG X Y , LI Y J , et  al.  Synthesis of a healthy  sweetener  D-tagatose  from   starch  catalyzed  by semiartificial cell factories[J] .  Journal of Agricultural and Food Chemistry , 2023 , 71(8) : 3813 -3820.