Ribitol Dehydrogenase Membrane Bound

Ribitol ^ L-ribulose

Ribitol oxidation to L-ribulose in acetic acid bacteria is catalyzed by the membrane-bound PQQ-dependent polyol dehydrogenase, similar to GLDH or ARDH. Membrane-bound ribitol dehydrogenase in acetic acid bacteria catalyzes L-ribulose formation, while an NAD-dependent ribitol dehydrogenase is independent of oxidative fermentation [50]. To identify the enzyme responsible for penti-tol oxidation by acetic acid bacteria, two different ribitol-oxidizing enzymes, one NAD(P)-dependent in the cytosolic fraction and the other NAD(P)-independent in the membrane fraction, were examined with respect to the oxidative fermentation. The cytoplasmic NAD-dependent ribitol dehydrogenase (EC was crystallized from Gluconobactersuboxydans IFO 12528 and found to be an enzyme of molecular mass 100 kDa and 5 S as the sedimentation constant, composed of four identical subunits of 25 kDa each.

The enzyme catalyzed a shuttle reversible oxidoreduction between ribitol and D-ribulose in the presence of NAD and NADH, respectively. Xylitol and L-arabitol were well oxidized by the enzyme with reaction rates comparable to ribitol oxidation. D-Ribulose, L-ribulose, and L-xylulose were well reduced by the enzyme in the presence of NADH as cosubstrates. The optimum pH of pentitol oxidation was found to be at alkaline pH such as 9.5-10.5 and ketopentose reduction was optimum at pH 6.0.

NAD-Dependent ribitol dehydrogenase seemed to be specific to oxidoreduction between pentitols and ketopentoses. D-Sorbitol and D-mannitol were not oxidized with the NAD-dependent enzyme. However, no D-ribulose accumulation was observed outside the cells during the growth of the organism on ribitol. L-Ribulose was only accumulated in the culture medium instead, as the direct oxidation product catalyzed by the membrane-bound NAD (P)-independent ribitol dehydrogenase. Thus, the physiological role of NAD-dependent ribitol dehydrogenase seems to be to catalyze ribitol oxidation to D-ribulose in the cytoplasm.

Phosphorylated D-ribulose is involved in the pentose phosphate pathway. L-Ribulose outside the cells could be incorporated into the cytoplasm in several ways when the use of L-ribulose as carbon and energy source becomes necessary for cell survival. From a series of simple experiment, membrane-bound PQQ-dependent sugar alcohol dehydrogenase was concluded to be the enzyme responsible for L-ribulose production from ribitol in oxidative fermentation by acetic acid bacteria.

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