Utilization of d -Lactate as an Energy Source Supports the Growth of Gluconobacter oxydans

d -Lactate was identified as one of the few available organic acids that supported the growth ofGluconobacter oxydans 621H in this study. Interestingly, the strain usedd -lactate as an energy source but not as a carbon source, unlike other lactate-utilizing bacteria. The enzymatic basis for the growth ofG. oxydans 621H ond -lactate was therefore investigated. Although two putative NAD-independentd -lactate dehydrogenases, GOX1253 and GOX2071, were capable of oxidizingd -lactate, GOX1253 was the only enzyme able to support thed -lactate-driven growth of the strain. GOX1253 was characterized as a membrane-bound dehydrogenase with high activity towardd -lactate, while GOX2071 was characterized as a soluble oxidase with broad substrate specificity towardd -2-hydroxy acids. The latter used molecular oxygen as a direct electron acceptor, a feature that has not been reported previously ind -lactate-oxidizing enzymes. This study not only clarifies the mechanism for the growth ofG. oxydans ond -lactate, but also provides new insights for applications of the important industrial microbe and the noveld -lactate oxidase.