Characterization of 3 phylogenetically distinct membrane-bound d-gluconate dehydrogenases of Gluconobacter spp. and their biotechnological application for efficient 2-keto-d-gluconate production

We identified a novel flavoprotein-cytochrome c complex d-gluconate dehydrogenase (GADH) encoded by gndXYZ of Gluconobacter oxydans NBRC 3293, which is phylogenetically distinct from previously reported GADHs encoded by gndFGH and gndSLC of Gluconobacter spp. To analyze the biochemical properties of respective GADHs, Gluconobacter japonicus NBRC 3271 mutant strain lacking membranous d-gluconate dehydrogenase activity was constructed. All GADHs (GndFGH, GndSLC, and GndXYZ) were successfully overexpressed in the constructed strain. The optimal pH and K-M value at that pH of GndFGH, GndSLC, and GndXYZ were 5, 6, and 4, and 8.82 +/- 1.15, 22.9 +/- 5.0, and 11.3 +/- 1.5 m m, respectively. When the mutants overexpressing respective GADHs were cultured in d-glucose-containing medium, all of them produced 2-keto-d-gluconate, revealing that GndXYZ converts d-gluconate to 2-keto-d-gluconate as well as other GADHs. Among the recombinants, the gndXYZ-overexpressing strain accumulated the highest level of 2-keto-d-gluconate, suggesting its potential for 2-keto-d-gluconate production.