D-Mannitol ^ D-fructose
D-Mannitol oxidation to D-fructose in acetic acid bacteria is catalyzed by the membrane-bound PQQ-dependent polyol dehydrogenase, similar to GLDH or ARDH. The oxidation of D-mannitol to D-fructose was reported in many ketogenic acetic acid bacteria, as in the case of D-sorbitol oxidation. Fulmer and Underkofler looked at the possibility of using this oxidation for the industrial producton of D-fructose using A. suboxydans (G. suboxydans at present) . More detailed study of polyol dehydrogenase from acetic acid bacteria was carried out by Arcus and
Edson . Two different enzymes were indicated in the organism. One enzyme was an NAD(P)-independent particulate dehydrogenase active on D-mannitol and other polyols at pH 5.0, with a substrate specificity following Bertrand-Hudson's rule (Fig. 1.4). The other enzyme was NAD(P)-dependent. As shown in other cases in this chapter, NAD(P)-dependent dehydrogenases have no use for oxida-tive fermentation. Thus, only the membrane-bound enzyme contributes to D-mannitol oxidation to D-fructose.
In our recent study, several strains of thermotolerant acetic acid bacteria that can grow at 37-40 °C were screened. Typical mesophilic strains cannot grow at these temperatures. Among the thermotolerant strains, Gluconobacter frateurii CHM 16 was chosen as the best biocatalyst for D-fructose production with the highest conversion efficiency as well as the highest yield . Purification of the enzyme catalyzing D-mannitol oxidation was not tried, due to the issues described above. If we tried this, we would get a polyol dehydrogenase showing broad substrate specificity as seen with other examples in this chapter.
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