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

Abstract

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