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 1.1.1.56) 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.

Heal Yourself With Qi Gong

Heal Yourself With Qi Gong

Qigong also spelled Ch'i Kung is a potent system of healing and energy medicine from China. It's the art and science of utilizing breathing methods, gentle movement, and meditation to clean, fortify, and circulate the life energy qi.

Get My Free Ebook


Post a comment