MesoErythritol Oxidation Dehydrogenase Membrane Bound

meso-Erythritol ^ L-erythrulose

There is some information about the oxidative fermentation of C4 sugar alcohols in the literature, except earlier reports about meso-erythritol oxidation by acetic acid bacteria [57, 58]. Because L-erythrulose is not readily available from commercial sources, it is important to investigate the fermentation profile of L-erythrulose production for the identification of the enzyme responsible for meso-erythritol oxidation, and to purify and characterize the enzyme to provide basic information on L-erythrulose production. According to the recommendation of the US Food and Drug Administration (FDA), dihydroxyacetone should be replaced by L-erythrulose in cosmetics for those who are sensitive to dihydroxyacetone.

Gluconobacter frateurii CHM 43 has been screened and shows high L-erythrulose production from meso-erythritol. NAD (P)-independent enzymes catalyzing meso-erythritol oxidation from the membrane fraction and NAD(P)-dependent enzymes from the cytosolic fraction have been purified from the organism [59]. The purified enzyme from the membrane fraction was identified as a quinoprotein and is responsible for L-erythrulose production, but the NAD(P)-dependent enzyme was independent for the production of L-erythrulose. Growing cells and the membrane fraction of the strain rapidly oxidized meso-erythritol to L-erythrulose irreversibly with almost 100% recovery at 37 °C. L-Erythrulose was also produced efficiently by the resting cells as well. The enzyme responsible for meso-erythritol oxidation was localized on the outer surface of the cytoplasmic membrane of the organism.

The prosthetic group of the enzyme was identified to be PQQ after inactivation of the enzyme by EDTA treatment and the subsequent restoration to original levels by the exogenous addition of PQQ or PQQ and CaCl2. The enzyme was solubilized and purified to homogeneity [59]. The purified enzyme showed a single band in SDS-PAGE with a molecular mass corresponding to 80 kDa. A smaller band at 10-20 kDa that anchors the enzyme to the cytoplasmic membrane was not detected. The optimum pH of meso-erythritol oxidation was found to be pH 5.0. The Michaelis constant of the enzyme for meso-erythritol oxidation was found to be 25 mmol L-1. The enzyme showed broad substrate specificity toward C3-C6 sugar alcohols in which the erythro form of two hydroxy groups existed adjacent to the primary alcohol group according to the Bertrand-Hudson rule (Fig. 1.4).

NAD(P)-dependent meso-erythritol dehydrogenase was purified to a crystalline state. The molecular mass was estimated to be 60 kDa, composed of two identical subunits. Unlike the membrane-bound quinoprotein, the enzyme catalyzes reversible oxidoreduction at an optimum pH of 9.0-10.5 for meso-erythritol oxidation and pH 6.0 for L-erythrulose reduction. It is evident that NAD(P)-dependent enzymes have no function in L-erythrulose production.

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