Bioproduction of a novel sugar 1-deoxy-l-fructose by Enterobacter aerogenes IK7; isomerization of a 6-deoxyhexose to a 1-deoxyhexose

Metabolic engineering pathways for rare sugars biosynthesis, physiological functionalities, and applications-a review

Biomolecules like rare sugars and their derivatives are referred to as monosaccharides particularly uncommon in nature. Remarkably, many of them have various known physiological functions and biotechnological applications in cosmetics, nutrition, and pharmaceutical industries. Also, they can be exploited as starting materials for synthesizing fascinating natural bioproducts with significant biological activities. Regrettably, most of the rare sugars are quite expensive, and their synthetic chemical routes are both limited and economically unfeasible due to expensive raw materials. On the other hand, their production by enzymatic means often suffers from low space-time yields and high catalyst costs due to hasty enzyme denaturation/degradation. In this context, biosynthesis of rare sugars with industrial importance is receiving renowned scientific attention, across the globe. Moreover, the utilization of renewable resources as energy sources via microbial fermentation or microbial metabolic engineering has appeared a new tool. This article presents a comprehensive review of physiological functions and biotechnological applications of rare ketohexoses and aldohexoses, including D-psicose, D-tagatose, L-tagatose, D-sorbose, L-fructose, D-allose, L-glucose, D-gulose, L-talose, L-galactose, and L-fucose. Novel in-vivo recombination pathways based on aldolase and phosphatase for the biosynthesis of rare sugars, particularly D-psicose and D-sorbose using robust microbial strains are also deliberated.

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.

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-De-oxy-l-mannitol (6-de-oxy-l-mannitol or l-rhamnitol)

The crystalline form of 1-de­oxy-l-mannitol, C₆H₁₄O₅, exists as an extensively hydrogen-bonded structure with each mol­ecule acting as a donor and acceptor for five hydrogen bonds. There are no unusual crystal-packing features; the absolute configuration was determined from the use of 6-de­oxy-l-mannose (l-rhamnose) as the starting material.