Biochemical Characterization of a Novel Galactitol 2-Dehydrogenase from a <i>Ciceribacter</i> sp. L1K22 with High Catalytic Efficiency for <scp>d</scp> -tagatose Production

Conventional d -tagatose production via isomerization suffers from thermodynamic equilibrium limitations. An alternative two-step pathway employing aldose reductase and galactitol 2-dehydrogenase enables thermodynamically driven conversion of d -galactose to d -tagatose through a galactitol intermediate. In this study, a novel galactitol 2-dehydrogenase from Ciceribacter sp. L1K22 (WP_208553890.1) was identified and characterized. The recombinant His-tagged enzyme, purified by nickel-affinity chromatography, displayed peak activity at pH 9.5 and 35 °C. It showed specific activities of 36.7 U/mg toward galactitol and 48.8 U/mg toward d -sorbitol. Kinetic analysis for galactitol revealed a K m of 13.4 mM, k cat of 1,889.0 min –1, and catalytic efficiency ( k cat / K m ) of 141.0 mM –1 min –1 . When expressed in a galactitol-producing Escherichia coli chassis via pET-22b (+), this enzyme drove efficient d -tagatose biosynthesis, attaining a titer of 8.80 g/L. These findings establish the Ciceribacter sp. L1K22 enzyme as an effective biocatalyst for thermodynamically favorable d -tagatose production.