Cloning and characterization of a galactitol 2-dehydrogenase from Rhizobium legumenosarum and its application in d-tagatose production

• It reports characterization of a galactitol dehydrogenase (RlGDH) from Rhizobium leguminosarum.
• RlGDH catalyzes the oxidation of polyvalent alcohols and polyols.
• RlGDH is distinguished from other GDHs by its higher specific activity for galactitol.
• RlGDH can be a good option for d-tagatose production from galactitol.

Galactitol 2-dehydrogenase (GDH) belongs to the protein subfamily of short-chain dehydrogenases/reductases and can be used to produce optically pure building blocks and for the bioconversion of bioactive compounds. An NAD+-dependent GDH from Rhizobium leguminosarum bv. viciae 3841 (RlGDH) was cloned and overexpressed in Escherichia coli. The RlGDH protein was purified as an active soluble form using His-tag affinity chromatography. The molecular mass of the purified enzyme was estimated to be 28 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 114 kDa by gel filtration chromatography, suggesting that the enzyme is a homotetramer. The enzyme has an optimal pH and temperature of 9.5 and 35 °C, respectively. The purified recombinant RlGDH catalyzed the oxidation of a wide range of substrates, including polyvalent aliphatic alcohols and polyols, to the corresponding ketones and ketoses. Among various polyols, galactitol was the preferred substrate of RlGDH with a Km of 8.8 mM, kcat of 835 min−1 and a kcat/Km of 94.9 min−1 mM−1. Although GDHs have been characterized from a few other sources, RlGDH is distinguished from other GDHs by its higher specific activity for galactitol and broad substrate spectrum, making RlGDH a good choice for practical applications.