Glucose and galactose metabolism in Gluconabacter liquefaciens

Abstract

Glucose-grown cells of Gluconobacter liquefaciens oxidize glucose, gluconate and 2-ketogluconate practically completely to 2,5-diketogluconate by particulate enzymes, localized in the protoplasmic membrane. The bulk of the 2,5-diketgluconate (and 5-ketogluconate) enters the cytoplasm and is metabolized after reduction to gluconate by soluble enzymes. The gluconate is then phosphorylated and metabolized by the enzymes of the pentosephosphate cycle. The particulate enzymes do not participate in the metabolism of this gluconate because of their localization. A small part of the 2,5-diketogluconate is slowly ozidized by the particles to rubiginol (3,5=dihydroxy-4-ketopyran), which gives upon decomposition the brown colouration, which is characteristics for this strain grown in a glucose-chalk medium. The formation of 2,5-diketogluconate and of brown pigments only occurs with glucose-grown cells and is dependent on the induction of the adaptive enzyme 2-ketogluconooxydase in the protoplasmic membrane.

By growth on a galactose medium a 2-keto-3-deoxygalactonokinase is induced. Galactose is broken down with galactonate, 2-keto-3-deoxygalactonate and its phosphate ester as intermediates. Pyruvate and triosephosphate are the ultimate reaction products of this system (Fig. 14). In galactose-grown cells no 2-ketogluconooxydase is present in the protoplasmic membrane. No 2,5-diketogluconate and no brown pigments are formed. In these cells glucose and gluconate are oxidized after phosphorylation.