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6-Deoxy-l-sorbose (6-DLS) is an imperative rare sugar employed in food, agriculture, pharmaceutical and cosmetic industeries. However, it is a synthetic and very expensive rare sugars, previously synthesized by chemo-enzymatic methods through a long chain of chemical processes. Recently, enzymatic synthesis of rare sugars has attracted a lot of attention due to its advantages over synthetic methods. In this work, a promising approach for the synthesis of 6-DLS from an inexpensive sugar l-fucose was identified. The genes for l-fucose isomerase from Paenibacillus rhizosphaerae (Pr-LFI) and genes for d-tagatose-3-epimerase from Caballeronia fortuita (Cf-DTE) have been used for cloning and co-expression in Escherichia coli, developed a recombinant plasmid harboring pANY1-Pr-LFI/Cf-DTE vector. The recombinant co-expression system exhibited an optimum activity at 50 °C of temperature and pH 6.5 in the presence of Co 2+ metal ion which inflated the catalytic activity by 6.8 folds as compared to control group with no metal ions. The recombinant co-expressed system was stable up to more than 50 % relative activity after 12 h and revealed a melting temperature (T m ) of 63.38 °C exhibiting half-life of 13.17 h at 50 °C. The co-expression system exhibited, 4.93, 11.41 and 16.21 g/L of 6-DLS production from initial l-fucose concentration of 30, 70 and 100 g/L, which equates to conversion yield of 16.44 %, 16.30 % and 16.21 % respectively. Generally, this study offers a promising strategy for the biological production of 6-DLS from an inexpensive substrate l-fucose in slightly acidic conditions with the aid of co-expression system harboring Pr-LFI and CF-DTE genes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

L-Fuculose and D-ribulose are kinds of rare sugars used in food, agriculture, and medicine industries. These are pentoses and categorized into the two main groups, aldo pentoses and ketopentoses. There are 8 aldo- and 4 ketopentoses and only fewer are natural, while others are rare sugars found in a very small amount in nature. These sugars have great commercial applications, especially in many kinds of drugs in the medicine industry. The synthesis of these sugars is very expensive, difficult by chemical methods due to its absence in nature, and could not meet industry demands. The pentose izumoring strategy offers a complete enzymatic tactic to link all kinds of pentoses using different enzymes. The enzymatic production of L-fuculose and D-ribulose through L-fucose isomerase (L-FI) and D-arabinose isomerase (D-AI) is the inexpensive and uncomplicated method up till now. Both enzymes have similar kinds of isomerizing mechanisms and each enzyme can catalyze both L-fucose and D-arabinose. In this review article, the enzymatic process of biochemically characterized L-FI & D-AI, their application to produce L-fuculose and D-ribulose and its uses in food, agriculture, and medicine industries are reviewed., Competing Interests: Declaration of competing interest The authors declare that they do not have any conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

An enzyme, l-ribose isomerase (l-RI), mostly catalyzes the isomerization of l-ribose and l-ribulose. These so-called rare sugars are essential for the treatment of cancer and other viral diseases. In the present study, l-ribose isomerase produced from a bacterium, Mycetocola miduiensis (Mm-LRIse), by using l-ribose as a carbon source. The recombinant l-ribose isomerase gene was cloned and overexpressed from M. miduiensis and purified with an exclusive band of 32 kDa by nickel-affinity chromatography. This gene possessed 267 amino acids protein having an estimated molecular weight of 29,568.17 Da. The native molecular weight of Mm-LRIse estimated by HPLC was 134.84 kDa. The recombinant l-ribose isomerase was highly active in sodium phosphate (50 mM) buffer at 40 °C and pH 7.5, showing the specific activity up to 47.40 U mg -1 . Mm-LRIse showed no significant enhancement in activity with metallic ions except Mn 2+ and Co 2+ . The values of Km, K ca t , K cat /Km and V max of Mm-LRIse against l-ribose substrate were 42.48 mM, 9259.26 min -1 , 217.43 min -1  mM -1 , and 277.78 U mg -1 respectively. At equilibrium, the l-ribulose transformation rate was nearly 32 % (6.34 g L -1 ) using 20 g L -1 of l-ribose. The results revealed that the Mm-LRIse enzyme has a potential for L-ribulose production from l-ribose., Competing Interests: Declaration of Competing Interest The authors declare that they do not have any conflict of interest., (Copyright © 2020 Elsevier Inc. All rights reserved.)

L-fuculose is a rare sugar that is useful for the agriculture and medicine industries. L-fucose isomerase (E.C.5.3.1.25), which is an aldose-ketose isomerase, plays a significant role in producing rare sugars. A recommended L-fucose isomerase gene was cloned from Caldanaerobius polysaccharolyticus and purified with a single band of 65 kDa using nickel-affinity chromatography, with a specific activity of 108.23 U mg -1 . The native molecular mass existed with 214 kDa was a trimer. The purified enzyme showed a maximum activity in 1 mM Mn 2+ at 55 °C and pH 6.5 with a melting temperature (T m ) of 80.3 °C in the presence of one molecule per monomer. L-fucose isomerase from C. polysaccharolyticus (Capo-LfIase) exhibited the highest activity of L-fucose with K m , k cat and K cat /k m values of 94.2 mM, 23854 min -1 and 253.3 min -1 mM -1 , respectively. Capo-LfIase showed more than 50% thermostability after 20 h of incubation at 45, 55, 65, 75 and 85 °C. The 9 putative active site residues of the L-fucose substrate were described using a homology model, and the results showed that Tyr440, Met185, Trp499 and Asn527 are the candidates of metal-binding residues, while Ser393, Glu337, Glu302, His528 and Asp361 would be involved in substrate binding. The conversion rate of L-fuculose from L-fucose was almost 28.2%, with 80 g L -1 L-fucose, and no byproduct was found. To the best of our knowledge, Capo-LfIase produces high yield of L-fuculose from L-fucose by enzymatic methods., Competing Interests: Declaration of Competing Interest The authors declare that they do not have any conflict of interest regarding the publication of this paper., (Copyright © 2019 Elsevier B.V. All rights reserved.)

d-Ribulose and l-fuculose are potentially valuable rare sugars useful for anticancer and antiviral drugs in the agriculture and medicine industries. These rare sugars are usually produced by chemical methods, which are generally expensive, complicated and do not meet the increasing demands. Furthermore, the isomerization of d-arabinose and l-fucose byDd-arabinose and l-fucose by d-arabinose isomerase from bacterial sources for the production of d-ribulose and l-fuculose have not yet become industrial due to the shortage of biocatalysts, resulting in poor yield and high cost of production. In this study, a thermostable d-ribulose- and l-fuculose producing d-arabinose isomerase from the bacterium Thermanaeromonas toyohensis was characterized. The recombinant d-arabinose isomerase from T. toyohensis (Thto-DaIase) was purified with a single band at 66 kDa using His-trap affinity chromatography. The native enzyme existed as a homotetramer with a molecular weight of 310 kDa, and the specific activities for both d-arabinose and l-fucose were observed to be 98.08 and 85.52 U mg -1 , respectively. The thermostable recombinant Thto-DaIase was activated when 1 mM Mn 2+ was added to the reactions at an optimum pH of 9.0 at 75 °C and showed approximately 50% activity for both d-arabinose and l-fucose at 75 °C after 10 h. The Michaelis-Menten constant (K m ), the turnover number (k cat ) and catalytic efficiency (k cat /K m ) for d-arabinose/l-fucose were 111/81.24 mM, 18,466/10,688 min -1 , and 166/132 mM -1  min -1 , respectively. When the reaction reached to equilibrium, the conversion rates of d-ribulose from d-arabinose and l-fuculose from l-fucose were almost 27% (21 g L -1 ) and 24.88% (19.92 g L -1 ) from 80 g L -1 of d-arabinose and l-fucose, respectively., (Copyright © 2019 Elsevier Inc. All rights reserved.)