Periplasmic oxidation of glucose into gluconate and 2-ketogluconate in occurs via glucose dehydrogenase (GDH) and gluconate dehydrogenase (GaDH), respectively. Since, as is shown here, in the presence of glucose, gluconate and 2-ketogluconate are not further metabolized intracellularly the physiological function of this periplasmic route was studied. It was found that periplasmic oxidation of glucose could function as an alternative production route of ATP equivalents. Instantaneous activation of either GDH or GaDH reduced the rate of degradation of glucose via glycolysis and the tricarboxylic acid (TCA) cycle Furthermore, aerobic, magnesium- and phosphate-limited chemostat cultures with glucose as the carbon source showed high GDH plus GaDH activities in contrast to nitrogen-and sulphate-limited cultures. However, when fructose, which is not degraded by GDH, was the carbon source, specific oxygen consumption rates under these four conditions were essentially the same. The latter observation suggests that high transmembrane phosphate gradients which are supposedly present under phosphate-limited conditions do not cause high energetic demands due to futile cycling of phosphate ions. In addition, dissipation of the transmembrane phosphate gradient of phosphate-limited cells immediately increased the rate of intracellular glucose degradation. It is concluded that under phosphate-limited conditions (i) extensive futile cycling of phosphate ions is absent and (ii) low concentrations of phosphate ions limit intracellular degradation of glucose. Glyceraldehyde-3-phosphate dehydrogenase (GADPH) activities of cell-free extracts of glucose-grown cells harvested from aerobic chemostat cultures limited in various nutrients showed that at least a tenfold overcapacity in GAPDH activity was present under phosphate-limited conditions with respect to the steady-state carbon fluxes through this enzyme. The physiological significance of this adaptation and the possible role of GDH and GaDH are discussed.


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