Isomerization of pentose and hexose sugars by an enzyme reactor packed with cross-linked xylose isomerase crystals

Three size-classes, 20, 40 and 80 μm, of xylose isomerase (E.C. 5.3.1.5., from Streptomyces rubiginosus) crystals were prepared and cross-linked. The cross-linked xylose isomerase crystals (CLXIC) were packed into 7.8 mm×300 mm steel columns. The ability of these CLXIC-reactors to isomerize pentose and hexose sugars was studied. Three new substrates, l-lyxose, l-mannose and d-mannose, were discovered. In addition to the isomerization of pentose sugars, in plug flow mode CLXIC-reactor also partially separated the reaction products. This concomitant enzymatic reaction and separation of the formed products was characteristic to pentose sugars and was seen only weakly with hexose sugars. In case of l-arabinose isomerization to l-ribose, this concomitant reaction and separation led to high relative concentrations of l-ribose at the end of the sugar peak eluting from the CLXIC-column. However, recycling of saturated l-arabinose solution through the CLXIC-reactor turned out to be more efficient method in the production of l-ribose. In the recycling mode, the maximal l-ribose production rate of CLXIC-reactor was 6 mg l-ribose h−1 and volumetric productivity 0.22 g l−1 h−1. High recycling rate, high substrate concentration, high CLXIC-reactor temperature and small crystal size of CLXIC increased the rate of l-ribose formation.