Soluble DGlucose Dehydrogenase sGDH

D-Glucose ^ D-glucono-8-lactone

As mentioned above, A. calcoaceticus contains s-GDH in addition to m-GDH. s-GDH purified from the soluble fraction of A. calcoaceticus does not cross-react with the antibody for m-GDH [38]. Subsequently, s-GDH and m-GDH were purified separately from A. calcoaceticus and shown to be distinctive in all aspects including optimum pH, kinetics, substrate specificity, ubiquinone reactivity, molecular size, and immunoreactivity [39].

s-GDH of A. calcoaceticus is a monomer consisting of a single polypeptide of 48-55 kDa containing one PQQ molecule. A gene (gdhB) for the enzyme has been cloned and the gene product is estimated to be a polypeptide of 52.8 kDa that contains a 24-amino-acid signal sequence at its N-terminus and thus it is expected to become the mature protein of 50.2 kDa having no hydrophobic regions. Thus, s-GDH seems to be translocated through the cytoplasmic membrane into the periplasmic space.

s-GDH is capable of catalyzing the oxidation of disaccharides, lactose, or maltose, as well as D-glucose. The enzyme is able to donate electrons to several artificial dyes, including PMS, DCIP as well as short-chain ubiquinone homologs, Qj and Q2, but it is unable to react with the longer chain ubiquinones Q6 and Q9 [39]. s-GDH was purified from the soluble fraction of A. calcoaceticus with the overall recovery of 28% and showed a final specific activity of 2210 units mg-1. The substrate specificity of s-GDH is different from that of m-GDH. s-GDH oxidizes D-glucose (100%), D-fucose (28%), D-xylose (15%), D-galactose (30%), maltose (93%), and lactose (72%), while m-GDH is less reactive with disaccharides: D-glucose (100%), D-fucose (119%), D-xylose (81%), D-galactose (73%), D-ribose (54%), maltose (13%), and lactose (5%).

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