Sorbitol Dehydrogenase Membrane Bound

D-Sorbitol ^ L-sorbose

D-Sorbitol oxidation to L-sorbose in acetic acid bacteria is catalyzed by the membrane-bound PQQ-dependent polyol dehydrogenase in addition to FAD-dependent D-sorbitol dehydrogenase (SLDH) (see Section 1.4.5). The introduction of D-sorbitol oxidation to L-sorbose by microbial bioconversion (L-sorbose fermentation) stimulated the vitamin C industry [60]. The chemical method for L-ascorbate synthesis developed by Leichstein in 1937 leads to racemic sorbose after oxidation of D-sorbitol, while L-sorbose fermentation gives only L-sorbose with almost 100% yield, apparently two times higher than the chemical method. Many strains of "sorbose bacterium" have been isolated and used for L-sorbose production.

Together with acetate production this process is a typical and classical microbial bioconversion with acetic acid bacteria. However, the enzymatic mechanism of L-sorbose formation remained to be clarified. In 1982, SLDH was purified for the first time from the membrane fraction of Gluconobacter suboxydans var. a IFO 3254 [67]. The enzyme contains a covalently bound FAD as the coenzyme, as described below (see Section 1.4.5). Until recently, before we started to survey the enzymatic properties of polyol dehydrogenases involved in D-sorbitol metabolism in acetic acid bacteria, SLDH from G. suboxydans var. a IFO 3254 was the sole described enzyme [50, 68, 69]. Through investigations on quinoproteins such as GLDH, ARDH, meso-erythritol dehydrogenase, and ribitol dehydrogenase, it has been concluded that these enzymes are responsible for D-sorbitol oxidation to L-sorbose as described in this chapter.

Recently, we have screened thermotolerant acetic acid bacteria that can grow at 37 °C. Gluconobacterfrateurii CHM 54 was isolated and applied to L-sorbose production [66]. In most cases, strict temperature control is required for oxidative fermentation. A hot summer readily allows indoor temperatures to rise above 30 °C in many countries. This is a serious challenge not only to oxidative fermentation but also to other fermentation industries, since a huge amount of expense is required for cooling. A temperature increase by 2-3 °C causes a serious failure in both fermentation rate and fermentation efficiency. In submerged cultures, a large amount of heat is generated during fermentation and cooling costs become even more expensive. If favorable strains of acetic acid bacteria that can work optimally at 37-40 °C were available, such strains would be able to accept loose temperature control and the cooling expenses would be reduced considerably.

The thermotolerant strain of G. frateurii CHM 54 was able to produce L-sorbose at higher temperatures. When D-sorbitol oxidation was done at higher temperatures with the thermotolerant strain, both the fermentation rate as well as fermentation efficiency were superior to that achieved using non-thermotolerant mesophilic strains [66].

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