Identification of a D-galactitol 2-dehydrogenase from a Rhizobiaceae bacterium for D-tagatose production

Thermodynamically driven biosynthesis of D-tagatose attracts increasing attention because it overcomes the reaction equilibrium caused by aldose isomerization or ketose epimerization. Specific aldose reductase and D-galactitol 2-dehydrogenase may drive the conversion of D-galactose to D-tagatose through galactitol as the intermediate. In this study, a novel D-galactitol 2-dehydrogenase was identified from a Rhizobiaceae bacterium with the NCBI accession No. of MDF1774848. The enzyme was expressed in His-tagged form and purified by one-step nickel-affinity chromatography. The recombinant enzyme showed the highest activity at pH 9.5 and 30 °C. It showed galactitol as the optimal substrate and D-sorbitol as the second substate. It's in vivo D-tagatose production ability was verified in a metabolically engineered Escherichia coli strain. Using a previously constructed galactitol-producing strain as a chassis microbe, genomic integration of this D-galactitol 2-dehydrogenase gene resulted in the efficient production of D-tagatose with a titer of 1.85 g/L.