Purification and characterization of d-allulose 3-epimerase derived from Arthrobacter globiformis M30, a GRAS microorganism.

An enzyme that catalyzes C-3 epimerization between d-fructose and d-allulose was found in Arthrobacter globiformis strain M30. Arthrobacter species have long been used in the food industry and are well-known for their high degree of safety. The enzyme was purified by ion exchange and hydrophobic interaction chromatographies and characterized as a d-allulose 3-epimerase (d-AE). The molecular weight of the purified enzyme was estimated to be 128 kDa with four identical subunits. The enzyme showed maximal activity and thermostability in the presence of Mg2+. The optimal pH and temperature for enzymatic activity were 7.0-8.0 and 70°C, respectively. The enzyme was immobilized to ion exchange resin whereupon it was stable for longer periods than the free enzyme when stored at below 10°C. In the column reaction, the enzyme activity also maintained stability for more than 4 months. Under these conditions, 215 kg of d-allulose produced per liter immobilized enzyme, and this was the highest production yield of d-allulose reported so far. These highly stable properties suggest that this enzyme represents an ideal candidate for the industrial production of d-allulose.

Crystal structure of ß-d,l-allose.

The title compound, C6H12O6, a C-3 position epimer of glucose, was crystallized from an equimolar mixture of d- and l-allose. It was confirmed that d-allose (l-allose) formed ß-pyran-ose with a (4) C 1 ((1) C 4) conformation in the crystal. In the crystal, molecules are linked by O-H⋯O hydrogen bond, forming a three-dimensional framework. The cell volume of the racemic ß-d,l-allose is 739.36 (3) Å(3), which is about 10 Å(3) smaller than that of chiral ß-d-allose [V = 751.0 (2) Å(3)].