The S213C, I33L, and I33L S213C variants of D-psicose 3-epimerase from Agrobacterium tumefaciens, which were obtained by random and site-directed mutagenesis, displayed increases of 2.5, 5, and 7.5°C in the temperature for maximal enzyme activity, increases of 3.3-, 7.2-, and 29.9-fold in the half-life at 50°C, and increases of 3.1, 4.3, and 7.6°C in apparent melting temperature, respectively, compared with the wild-type enzyme. Molecular modeling suggests that the improvement in thermostability in these variants may have resulted from increased putative hydrogen bonds and formation of new aromatic stacking interactions. The immobilized wild-type enzyme with and without borate maintained activity for 8 days at a conversion yield of 70% (350 g/liter psicose) and for 16 days at a conversion yield of 30% (150 g/liter psicose), respectively. After 8 or 16 days, the enzyme activity gradually decreased, and the conversion yields with and without borate were reduced to 22 and 9.6%, respectively, at 30 days. In contrast, the activities of the immobilized I33L S213C variant with and without borate did not decrease during the operation time of 30 days. These results suggest that the I33L S213C variant may be useful as an industrial producer of D-psicose.D-Psicose (D-allulose), a carbon-3 epimer of D-fructose, is present in small quantities as a nonfermentable constituent of cane molasses (1), as a sugar moiety of the nucleoside antibiotic psicofuranine (4), and as a free sugar in wheat (21) and Itea plants (5). This rare sugar provides suitable sweetness, smooth texture, favorable mouthfeel, and long-time storage stability in food products (23). Psicose is considered to be a potential reduced energy sweetener because the sugar suppresses hepatic lipogenic enzyme activity (18) and does not contribute to calorie production (19).Biological production of psicose from fructose has been studied using D-psicose 3-epimerase from Agrobacterium tumefaciens (10, 13, 16) and D-tagatose 3-epimerases from Pseudomonas cichorii (7,24) and Rhodobacter sphaeroides (26). D-Psicose 3-epimerase from A. tumefaciens has been more effective than the D-tagatose 3-epimerases for the production of psicose. However, D-psicose 3-epimerase is inefficient for the industrial production of psicose because of its short half-life of 63 min at 50°C (10). Thus, improvement in the thermostability of A. tumefaciens D-psicose 3-epimerase is essential for the industrial production of psicose.Random mutagenesis and rational protein design are typical tools for improving enzyme thermostability in the protein engineering field. Random mutagenesis techniques such as errorprone PCR and DNA shuffling can be easily applied for the improvement of enzyme thermostability (8, 9, 12, 28) because these techniques do not require detailed structural information or accurate predictions at the substituted residues (14).In the present study, thermostable variants of D-psicose 3-epimerase from A. tumefaciens were obtained by random and site-directed mutagenesis. The temperature fo...

A recombinant L-fucose isomerase from Caldicellulosiruptor saccharolyticus was purified as a single 68 kDa band with an activity of 76 U mg(-1). The molecular mass of the native enzyme was 204 kDa as a trimer. The maximum activity for L-fucose isomerization was at pH 7 and 75 degrees C in the presence of 1 mM Mn(2+). Its half-life at 70 degrees C was 6.1 h. For aldose substrates, the enzyme displayed activity in decreasing order for L-fucose, with a k (cat) of 11,910 min(-1) and a K (m) of 140 mM, D-arabinose, D-altrose, and L-galactose. These aldoses were converted to the ketoses L-fuculose, D-ribulose, D-psicose, and L-tagatose, respectively, with 24, 24, 85, 55% conversion yields after 3 h.