D-allose has attracted a great deal of attention in recent years due to its many pharmaceutical activities, which include anti-cancer, anti-tumor, anti-inflammatory, anti-oxidative, anti-hypertensive, cryoprotective, and immunosuppressant activities. D-allose has been produced from D-psicose using D-allose-producing enzymes, including L-rhamnose isomerase, ribose-5-phosphate isomerase, and galactose-6-phosphate isomerase. In this article, the properties, applications, and metabolism of D-allose are described, and the biochemical properties of D-allose-producing enzymes and their D-allose production are reviewed and compared. Moreover, several methods for effective D-allose production are suggested herein.

A putative L-rhamnose isomerase (RhaA) from Thermotoga maritima was purified with a specific activity of 55 U/mg by His-Trap affinity chromatography. The native enzyme was estimated as a 46 kDa tetramer by gel filtration chromatography. The half-lives of the enzyme at 75, 80, 85, 90 and 95°C were 773, 347, 187, 118, and 65 h, respectively, indicating that it is the most thermostable of all RhaAs. Under the optimum conditions of pH 8.0, 85°C, and 1 mM Mn(2+), RhaA with 100 U enzyme/ml converted 500 L-xylulose/l to 225 g/l L-lyxose after 3 h, and converted 500 L-fructose/l to 175 g/l L-mannose after 5 h.

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Aims:  Characterization of substrate specificity of a d‐lyxose isomerase from Serratia proteamaculans and application of the enzyme in the production of d‐lyxose and d‐mannose. Methods and Results:  The concentrations of monosaccharides were determined using a Bio‐LC system. The activity of the recombinant protein from Ser. proteamaculans was the highest for d‐lyxose among aldoses, indicating that it is a d‐lyxose isomerase. The native recombinant enzyme existed as a 54‐kDa dimer, and the maximal activity for d‐lyxose isomerization was observed at pH 7·5 and 40°C in the presence of 1 mmol l−1 Mn2+. The Km values for d‐lyxose, d‐mannose, d‐xylulose, and d‐fructose were 13·3, 32·2, 3·83, and 19·4 mmol l−1, respectively. In 2 ml of reaction volume at pH 7·5 and 35°C, d‐lyxose was produced at 35% (w/v) from 50% (w/v) d‐xylulose by the d‐lyxose isomerase in 3 h, while d‐mannose were produced at 10% (w/v) from 50% (w/v) d‐fructose in 5 h. Conclusions:  We identified the putative sugar isomerase from Ser. proteamaculans as a d‐lyxose isomerase. The enzyme exhibited isomerization activity for aldose substrates with the C2 and C3 hydroxyl groups in the left‐hand configuration. High production rates of d‐lyxose and d‐mannose by the enzyme were obtained. Significance and Impact of the Study:  A new d‐lyxose isomerase was found, and this enzyme had higher activity for d‐lyxose and d‐mannose than previously reported enzymes. Thus, the enzyme can be applied in industrial production of d‐lyxose and d‐mannose.

Aims:  To characterize of a thermostable recombinant α‐l‐arabinofuranosidase from Caldicellulosiruptor saccharolyticus for the hydrolysis of arabino‐oligosaccharides to l‐arabinose. Methods and Results:  A recombinant α‐l‐arabinofuranosidase from C. saccharolyticus was purified by heat treatment and Hi‐Trap anion exchange chromatography with a specific activity of 28·2 U mg−1. The native enzyme was a 58‐kDa octamer with a molecular mass of 460 kDa, as measured by gel filtration. The catalytic residues and consensus sequences of the glycoside hydrolase 51 family of α‐l‐arabinofuranosidases were completely conserved in α‐l‐arabinofuranosidase from C. saccharolyticus. The maximum enzyme activity was observed at pH 5·5 and 80°C with a half‐life of 49 h at 75°C. Among aryl‐glycoside substrates, the enzyme displayed activity only for p‐nitrophenyl‐α‐l‐arabinofuranoside [maximum kcat/Km of 220 m(mol l−1)−1 s−1] and p‐nitrophenyl‐α‐l‐arabinopyranoside. This substrate specificity differs from those of other α‐l‐arabinofuranosidases. In a 1 mmol l−1 solution of each sugar, arabino‐oligosaccharides with 2–5 monomer units were completely hydrolysed to l‐arabinose within 13 h in the presence of 30 U ml−1 of enzyme at 75°C. Conclusions:  The novel substrate specificity and hydrolytic properties for arabino‐oligosaccharides of α‐l‐arabinofuranosidase from C. saccharolyticus demonstrate the potential in the commercial production of l‐arabinose in concert with endoarabinanase and/or xylanase. Significance and Impact of the Study:  The findings of this work contribute to the knowledge of hydrolytic properties for arabino‐oligosaccharides performed by thermostable α‐l‐arabinofuranosidase.

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A putative recombinant enzyme from Dictyoglomus turgidum was characterized and immobilized on Duolite A568 beads. The native enzyme was a 46 kDa tetramer. Its activity was highest for L-rhamnose, indicating that it is an L-rhamnose isomerase. The maximum activities of both the free and immobilized enzymes for L-rhamnose isomerization were at pH 8.0 and 75 °C in the presence of Mn(2+). Under these conditions, the half-lives of the free and immobilized enzymes were 28 and 112 h, respectively. In a packed-bed bioreactor, the immobilized enzyme produced an average of 130 g L-rhamnulose l(-1) from 300 g L-rhamnose l(-1) after 240 h at pH 8.0, 70 °C, and 0.6 h(-1), with a productivity of 78 g l(-1) h(-1) and a conversion yield of 43 %. To the best of our knowledge, this is the first report describing the enzymatic production of L-rhamnulose.