Hidden Reaction: Mesophilic cellobiose 2-epimerases produce lactulose

Publication Type

Journal contribution (peer reviewed)

Authors

Kuschel, B.; Seitl, I.; Glück, C.; Mu, W.; Jiang, B.; Stressler, T.; Fischer, L.

Year of publication

2017

Published in

Journal of Agricultural and Food Chemistry

DOI

10.1021/acs.jafc.6b05599

Lactulose (4-O-β-D-galactopyranosyl-D-fructofuranose) is a prebiotic sugar derived from the milk sugar lactose (4-O-β-D-galactopyranosyl-D-glucopyranose). In our study we observed for the first time, that known cellobiose 2-epimerases (CEs; EC 5.1.3.11) from mesophilic microorganisms were generally able to catalyze the isomerization reaction of lactose into lactulose. Commonly, CEs catalyze the C2-epimerization of D-glucose and D-mannose moieties at the reducing end of β-1,4-glycosidic-linked oligosaccharides. Thus, epilactose (4-O-β-D-galactopyranosyl-D-mannopyranose) is formed with lactose as substrate. So far, only three CEs, exclusively from thermophilic microorganisms, have been reported to additionally catalyze the isomerization reaction of lactose into lactulose. The specific isomerization activity of the seven CEs in this study ranged between 8.7 ± 0.1 to 1300 ± 37 pkat/mg. The results indicate that very likely all CEs are able to catalyze both the epimerization as well as the isomerization reaction, whereby the latter is performed at comparative much lower reaction rate.