Reaction investigation of lactulose-producing cellobiose 2-epimerases under operational relevant conditions

Publication Type

Journal contribution (peer reviewed)

Authors

Kuschel, B.; Claaßen, W.; Mu, W.; Jiang, B.; Stressler, T.; Fischer, L.

Year of publication

2016

Published in

Journal of Molecular Catalysis B: Enzymatic

DOI

10.1016/j.molcatb.2016.11.022

The cellobiose 2-epimerases (CEs) from Caldicellulosiruptor saccharolyticus (CsCE) and Dictyoglomus turgidum (DtCE) are the two CEs best investigated which are able to catalyse an epimerisation and an isomerisation reaction on lactose. However, both enzymes are thermophilic and have, so far, only been characterised under their respective optimal conditions. The CE from Caldicellulosiruptor saccharolyticus has recently been applied for epilactose and lactulose production in milk at lower temperatures. The kinetic parameters under supposedly industrial relevant conditions and the direct comparability of CsCE and DtCE still remain unknown and were, therefore, the object of this study. In general, CsCE and DtCE displayed similar reaction progresses and similar kinetic parameters. Surprisingly, the CsCE was observed to catalyse the isomerisation reaction of lactulose into lactose and epilactose twice as efficiently (k_cat/K_m) as DtCE. For the first time, the reaction progress for lactose, epilactose and lactulose as substrates was observed for both CsCE and DtCE over 48 h at 50 °C, pH 7.5. These conversions led to the same final sugar concentrations of about 30.5%_ts lactose, 14.5%_ts epilactose and 55%_ts lactulose with both CEs. During the course of the conversions, the interrelated ratios of lactose and epilactose remained steady at about 70:30, respectively. In conclusion, both CEs were of comparable aptitude for industrial application under the conditions tested and most probably catalyse the same course of reaction.