A stable immobilized d-psicose 3-epimerase for the production of d-psicose in the presence of borate

Maximal activity of the immobilized d-psicose 3-epimerase from Agrobacterium tumefaciens on Duolite A568 beads was achieved at pH 9.0 and 55 °C with borate, and at pH 8.5 and 50 °C without borate. The half-lives of the immobilized enzyme at 50 °C with and without borate were increased 4.2- and 128-fold compared to that of the free enzyme without borate, respectively. The immobilized enzyme with borate produced 441 g l−1 psicose from 700 g l−1 fructose at pH 9.0 and 55 °C, whereas 193 g l−1 psicose was produced without borate at pH 8.5 and 50 °C after 120 min in a batch reaction. The immobilized enzyme in a packed-bed bioreactor without borate was produced continuously 325 g l−1 psicose from 500 g l−1 fructose at a dilution rate of 1.62 h−1 over a 236 h period with productivity of 527 g l−1 h−1 while that without borate produced 146 g l−1 psicose at 4.15 h−1 over a 384-h period with productivity of 606 g l−1 h−1. The operational half-lives of the enzyme with and without borate in the bioreactor were 601 and 645 h, respectively. In the present study, psicose was produced stably with high productivity using the immobilized d-psicose 3-epimerase in the presence of borate.