Aldopentoses as new substrates for the membrane-bound, pyrroloquinoline quinone-dependent glycerol (polyol) dehydrogenase of Gluconobacter sp.

Membrane-bound, pyrroloquinoline quinone (PQQ)-dependent glycerol dehydrogenase (GLDH, or polyol dehydrogenase) of Gluconobacter sp. oxidizes various secondary alcohols to produce the corresponding ketones, such as oxidation of D-sorbitol to L-sorbose in vitamin C production. Substrate specificity of GLDH is considered limited to secondary alcohols in the D-erythro configuration at the next to the last carbon. Here, we suggest that L-ribose, D- and L-lyxoses, and L-tagatose are also substrates of GLDH, but these sugars do not meet the substrate specificity rule of GLDH. The oxygen consumption activity of wild-type Gluconobacter frateurii cell membranes depends on several kinds of sugars as compared with that of the membranes of a GLDH-negative variant. Biotransformation of those sugars with the membranes was examined to determine the reaction products. A time course measuring the pH in the reaction mixture and the increase or decrease in substrates and products on TLC suggested that oxidation products of L-lyxose and L-tagatose were ketones with unknown structures, but those of L-ribose and D-lyxose were acids. The oxidation product of L-ribose was purified and revealed to be L-ribonate by HRMS and NMR analysis. Biotransformation of L-ribose with the membranes and also with the whole cells produced L-ribonate in nearly stoichiometric amounts, indicating that the specific oxidation site in L-ribose is recognized by GLDH. Since purified GLDH produced L-ribonate without any intermediate-like compounds, we propose here a reaction model where the first carbon in the pyranose form of L-ribose is oxidized by GLDH to L-ribonolactone, which is further hydrolyzed spontaneously to produce L-ribonate.