Isomerization of deoxyhexoses: green bioproduction of 1-deoxy-d-tagatose from l-fucose and of 6-deoxy-d-tagatose from d-fucose using Enterobacter agglomerans strain 221e

Using galactitol dehydrogenase coupled with water-forming NADH oxidase for efficient enzymatic synthesis of L-tagatose

L-Tagatose, a promising building block in the production of many value-added chemicals, is generally produced by chemical routes with a low yield, which may not meet the increasing demands. Synthesis of l-tagatose by enzymatic oxidation of d-galactitol has not been applied on an industrial scale because of the high cofactor costs and the lack of efficient cofactor regeneration methods. In this work, an efficient and environmentally friendly enzymatic method containing a galactitol dehydrogenase for d-galactitol oxidation and a water-forming NADH oxidase for regeneration of NAD⁺ was first designed and used for l-tagatose production. Supplied with only 3 mM NAD⁺, subsequent reaction optimization facilitated the efficient transformation of 100 mM of d-galactitol into l-tagatose with a yield of 90.2 % after 12 h (obtained productivity: 7.61 mM.h^-1). Compared with the current chemical and biocatalytic methods, the strategy developed avoids by-product formation and achieves the highest yield of l-tagatose with low costs. It is expected to become a cleaner and more promising route for industrial biosynthesis of l-tagatose.

The rare sugar D-tagatose protects plants from downy mildews and is a safe fungicidal agrochemical

The rare sugar D-tagatose is a safe natural product used as a commercial food ingredient. Here, we show that D-tagatose controls a wide range of plant diseases and focus on downy mildews to analyze its mode of action. It likely acts directly on the pathogen, rather than as a plant defense activator. Synthesis of mannan and related products of D-mannose metabolism are essential for development of fungi and oomycetes; D-tagatose inhibits the first step of mannose metabolism, the phosphorylation of D-fructose to D-fructose 6-phosphate by fructokinase, and also produces D-tagatose 6-phosphate. D-Tagatose 6-phosphate sequentially inhibits phosphomannose isomerase, causing a reduction in D-glucose 6-phosphate and D-fructose 6-phosphate, common substrates for glycolysis, and in D-mannose 6-phosphate, needed to synthesize mannan and related products. These chain-inhibitory effects on metabolic steps are significant enough to block initial infection and structural development needed for reproduction such as conidiophore and conidiospore formation of downy mildew.

Growth inhibition by 1-deoxy-d-allulose, a novel bioactive deoxy sugar, screened using Caenorhabditis elegans assay

The biological activities of deoxy sugars (deoxy monosaccharides) have remained largely unstudied until recently. We compared the growth inhibition by all 1-deoxyketohexoses using the animal model Caenorhabditis elegans. Among the eight stereoisomers, 1-deoxy-d-allulose (1d-d-Alu) showed particularly strong growth inhibition. The 50% inhibition of growth (GI₅₀) concentration by 1d-d-Alu was estimated to be 5.4 mM, which is approximately 10 times lower than that of d-allulose (52.7 mM), and even lower than that of the potent glycolytic inhibitor, 2-deoxy-d-glucose (19.5 mM), implying that 1d-d-Alu has a strong growth inhibition. In contrast, 5-deoxy- and 6-deoxy-d-allulose showed no growth inhibition of C. elegans. The inhibition by 1d-d-Alu was alleviated by the addition of d-ribose or d-fructose. Our findings suggest that 1d-d-Alu-mediated growth inhibition could be induced by the imbalance in d-ribose metabolism. To our knowledge, this is the first report of biological activity of 1d-d-Alu which may be considered as an antimetabolite drug candidate.

Reductive Deamination by Benzyne for Deoxy Sugar Synthesis Through a Domino Reaction

Benzyne was developed as a reducing agent in the key step of converting amino sugars and ketoses into deoxy sugars, which occur widely in natural products. Many deoxy sugars exhibit antibiotic and anticancer activities, and furthermore, they play essential biological roles. By treatment with CS2 and then Ac2O, amino sugars and ketoses were converted into the corresponding 1,3-thiazolidine-2-thiones. In the key step, these intermediates were treated with 2-trimethylsilylphenyl triflate (2.0 equiv.) and CsF (3.0 equiv.) in MeCN at 25 °C to produce acyclic enol acetates in 60-63 % yields. Saponification of the enol acetates with NaOMe/MeOH followed by intramolecular cyclization afforded the target 2-deoxy sugars. The key step of the reductive deamination involved a domino 1,2-elimination/[3+2]-cycloaddition/retro [3+2]-ring-opening sequence. The generality of this new method was proven by the use of various substrates, including pentoses, hexoses, monosaccharides, disaccharides, aldoses, and ketoses.

Triacetonide of Glucoheptonic Acid in the Scalable Syntheses of d-Gulose, 6-Deoxy-d-gulose, l-Glucose, 6-Deoxy-l-glucose, and Related Sugars

Ease of separation of petrol-soluble acetonides derived from the triacetonide of methyl glucoheptonate allows scalable syntheses of rare sugars containing the l-gluco or d-gulo structural motif with any oxidation level at the C6 or C1 position of the hexose, usually without chromatography: meso-d-glycero-d-guloheptitol available in two steps is an ideal entry point for the study of the biotechnological production of heptoses.

Two-step enzymatic synthesis of 6-deoxy-L-psicose

Rare sugars offer a plethora of applications in the pharmaceutical, medicinal, and industries, as well as in synthetic chemistry. However, studies of rare sugars have been hampered by their relative scarcity. In this work, we describe a two-step strategy to efficiently and conveniently prepare 6-deoxy-L-psicose from L-rhamnose. In the first reaction step, the isomerization of L-rhamnose (6-deoxy-L-mannose) to L-rhamnulose (6-deoxy-L-fructose) catalyzed by L-rhamnose isomerase (RhaI), and the epimerization of L-rhamnulose to 6-deoxy-L-psicose catalyzed by D-tagatose 3-epimerase (DTE) were coupled with selective phosphorylation reaction by fructose kinase from human (HK), which selectively phosphorylate 6-deoxy-L-psicose at C-1 position. 6-deoxy-L-psicose 1-phosphate was purified by a silver nitrate precipitation method. In the second step, the phosphate group of the 6-deoxy-L-sorbose 1-phosphate was hydrolyzed with acid phosphatase (AphA) to produce 6-deoxy-L-psicose in 81% yield with respect to L-rhamnose. This method allows that the 6-deoxy-L-psicose to be obtained from readily available starting materials with high purity and without having to undergo isomer separation.

Biosynthesis of rare hexoses using microorganisms and related enzymes

Rare sugars, referred to as monosaccharides and their derivatives that rarely exist in nature, can be applied in many areas ranging from foodstuffs to pharmaceutical and nutrition industry, or as starting materials for various natural products and drug candidates. Unfortunately, an important factor restricting the utilization of rare sugars is their limited availability, resulting from limited synthetic methods. Nowadays, microbial and enzymatic transformations have become a very powerful tool in this field. This article reviews the biosynthesis and enzymatic production of rare ketohexoses, aldohexoses and sugar alcohols (hexitols), including D-tagatose, D-psicose, D-sorbose, L-tagatose, L-fructose, 1-deoxy-L-fructose, D-allose, L-glucose, L-talose, D-gulose, L-galactose, L-fucose, allitol, D-talitol, and L-sorbitol. New systems and robust catalysts resulting from advancements in genomics and bioengineering are also discussed.

Enzymes for the biocatalytic production of rare sugars

Carbohydrates are much more than just a source of energy as they also mediate a variety of recognition processes that are central to human health. As such, saccharides can be applied in the food and pharmaceutical industries to stimulate our immune system (e.g., prebiotics), to control diabetes (e.g., low-calorie sweeteners), or as building blocks for anticancer and antiviral drugs (e.g., l-nucleosides). Unfortunately, only a small number of all possible monosaccharides are found in nature in sufficient amounts to allow their commercial exploitation. Consequently, so-called rare sugars have to be produced by (bio)chemical processes starting from cheap and widely available substrates. Three enzyme classes that can be used for rare sugar production are keto–aldol isomerases, epimerases, and oxidoreductases. In this review, the recent developments in rare sugar production with these biocatalysts are discussed.

Synthesis of rare sugars with L-fuculose-1-phosphate aldolase (FucA) from Thermus thermophilus HB8

We report herein a one-pot four-enzyme approach for the synthesis of the rare sugars d-psicose, d-sorbose, l-tagatose, and l-fructose with aldolase FucA from a thermophilic source (Thermus thermophilus HB8). Importantly, the cheap starting material DL-GP (DL-glycerol 3-phosphate), was used to significantly reduce the synthetic cost.

2-Azido-3,4;6,7-di-O-isopropyl-idene-α-d-glycero-d-talo-heptopyran-ose

In the title compound, C(13)H(21)N(3)O(6), the six-membered ring adopts a twist-boat conformation with the azide group in the bowsprit position. The azide group is disordered over two sets of sites in a 0.642 (10):0.358 (10) ratio. The crystal structure consists of O-H⋯O hydrogen-bonded trimer units. The absolute configuration was determined from the use of d-mannose as the starting material.

2,6-Dide-oxy-2,6-imino-l-glycero-d-ido-heptitol

The title mol­ecule, C₇H₁₅NO₅, the major product from selective enzymatic oxidation followed by hydrogeno­lysis of the corresponding azido­heptitol, was found by X-ray crystallography to exisit in a chair conformation with three axial hydroxyl groups. One of the hydroxymethyl groups is disordered over two sets of sites in a 0.590 (3):0.410 (3) ratio. In the crystal, O—H⋯O, O—H⋯(O,O), O—H⋯N and N—H⋯O hydrogen bonding occurs.

α-d-Tagatopyran-ose

The title compound, C(6)H(12)O(6), also known as d-Tagatose, occurs in its furanose and pyranose forms in solution, but only the α-pyran-ose form crystallizes out. In the crystal, the molecules form hydrogen bonded chains propagating in [100] linked by O-H⋯O interactions. Further O-H⋯O bonds cross-link the chains.

2-De-oxy-2,3-O-isopropyl-idene-2,4-di-C-methyl-β-l-arabinose

X-ray crystallography unequivocally confirmed the stereochemistry of the C atom at position 2 in the carbon scaffold of the title mol-ecule, C(10)H(18)O(4). The pyran-ose ring exists in a chair conformation with the methyl group on the C atom in the 2 position in an equatorial configuration. The absolute stereochemistry was determined from the starting material. The crystal structure consists of O-H⋯O hydrogen-bonded chains of mol-ecules running parallel to the b axis.

(4R)-4-(2-Allyl-2H-1,2,3-triazol-4-yl)-1,2-O-isopropylidene-L-threose

X-ray crystallography unequivocally confirmed the structure of the title compound, C₁₂H₁₇N₃O₄, as (4R)-4-(2-allyl-2H-1,2,3-triazol-4-yl)-1,2-O-isopropyl­idene-l-threose. The absolute configuration was determined by the use of d-glucorono-3,6-lactone as the starting material. The crystal structure consists of hydrogen-bonded chains of mol­ecules running parallel to the a axis. There are no unusual packing features.

tert-Butyl 2-deoxy-4,5-O-isopropylidene-D-gluconate

The relative configuration of tert-butyl 2-de­oxy-4,5-O-iso­propyl­idene-d-gluconate, C₁₃H₂₄O₆, an inter­mediate in the synthesis of 2-de­oxy sugars, was determined by X-ray crystallography, and the crystal structure consists of chains of O—H⋯O hydrogen-bonded mol­ecules running parallel to the a axis. There are two mol­ecules in the asymmetric unit. The absolute configuration was inferred from the use of d-erythrono­lactone as the starting material.

1-Deoxy-D-galactitol (L-fucitol)

1-De­oxy-d-galactitol, C₆H₁₄O₅, exists in the crystalline form as hydrogen-bonded layers of mol­ecules running parallel to the ac plane, with each mol­ecule acting as a donor and acceptor of five hydrogen bonds.

6-Azido-6-deoxy-α-L-galactose (6-azido-L-fucose) monohydrate

Although 6-azido-6-de­oxy-l-galactose in aqueous solution is in equilibrium between the open-chain, furan­ose and pyran­ose forms, it crystallizes solely as 6-azido-6-de­oxy-α-l-galactopyran­ose monohydrate, C₆H₁₁N₃O₅·H₂O, with the six-membered ring adopting a chair conformation. The structure exists as hydrogen-bonded chains, with each mol­ecule acting as a donor and acceptor of five hydrogen bonds. There are no unusual crystal packing features and the absolute configuration was determined from the use of 1-azido-1-de­oxy-d-galactitol as the starting material.

6-De-oxy-α-l-talopyran-ose

X-ray crystallography showed that the title compound, C₆H₁₂O₅, crystallizes in the α-pyran­ose form with the six-membered ring in a chair conformation. The crystal structure exists as a three-dimensional hydrogen-bonded network of mol­ecules with each mol­ecule acting as a donor and aceptor for four hydrogen bonds. The absolute configuration was determined by the use of l-fucose as starting material.

1-Deoxy-D-arabinitol

Addition of methyl lithium to d-erythrono-1,4-lactone followed by acid deprotection was shown, by X-ray crystallography, to give 1-de­oxy-d-arabinitol, C₅H₁₂O₄, rather than 1-de­oxy-d-ribitol as the major product. The crystal structure exists as hydrogen-bonded chains of mol­ecules running parallel to the c axis which are further linked together by hydrogen bonds. Each mol­ecule is a donor and an acceptor for four hydrogen bonds.