R. Otto

THE PHOSPHATE POTENTIAL, ADENYLATE ENERGY-CHARGE AND PROTON MOTIVE FORCE IN GROWING-CELLS OF STREPTOCOCCUS-CREMORIS

The phosphate potential (ΔG′p) and the proton motive force (Δp) were recorded simultaneously in Streptococcus cremoris under different conditions. If thermodynamic equilibrium exists the ratio ΔG′p/Δp gives the number of protons translocated by the ATPase per ATP hydrolyzed or synthesized. In resting cells ATP-synthesis could be energized by a valinomycin-induced potassiumdiffusion potential. In the subsequent phase ATP-hydrolysis occurred and the ΔG′p/Δp approached a value close to 2. In growing and fermenting cells the ΔG′p/Δp ratio was considerably higher (almost 5). This indicates that under growing conditions the ΔG′p is not near thermodynamic equilibrium with Δp and that the membrane bound ATPase of S. cremoris acts exclusively as an ATP hydrolase. During logarithmic growth the phosphate potential, the Δp and the adenylate energy charge remain essentially constant at-460 mV,-100 mV and 0.81, respectively. When the growth sustaining substrate is consumed a rapid dissipation of the phosphate potential to about-360 mV and of the Δp to zero mV occurs.

ENERGY-METABOLISM IN STREPTOCOCCUS-CREMORIS DURING LACTOSE STARVATION

The ATP pool of Streptococcus cremoris in a lactose-limited chemostat depletes rapidly when lactose is consumed. The decrease of the intracellular ATP concentration parallels the dissipation of the electrochemical proton gradient. The adenylate energy charge of growing cells is 0.8 but drops rapidly to 0.2 when the cells enter the starvation phase.One of the early events of lactose starvation is a rapid increase of the pools of phosphoenolpyruvate and inorganic phosphate. The accumulation of phosphoenolpyruvate is temporarily and levels off at a much lower value than in growing cells; the accumulation of phosphate is of a more permanent nature. Despite the low PEP concentration starved cells are, after 24 h of incubation in the absence of lactose, still able to take up lactose, to synthesize ATP and to generate quickly an electrochemical proton gradient.

THE RELATION BETWEEN PHOSPHATE POTENTIAL AND GROWTH-RATE OF STREPTOCOCCUS-CREMORIS

During energy limited growth the phosphate potential of the adenine nucleotide pool of Streptococcus cremoris was independent of growth rate. The ratio of the phosphate potential and proton motive force under these conditions was 2.6 to 2.7. Under carbon-limiting conditions the phosphate potential increased with the growth rate which is the result of a decrease of the organic phosphate content of the cells at higher growth rates. The ATP/ADP ratio of energy- and carbon-limited cultures had the same value and it is proposed that this ratio is kept constant by a near equilibrium reaction in the glycolysis.

THE ROLE OF THE ELECTROCHEMICAL GRADIENT OF PROTONS IN SOLUTE TRANSPORT IN BACTERIA

Publisher Summary The chemiosmotic energy-generating processes comprise membrane-bound energy transducing systems such as electron-transfer chains and the Ca 2+ , Mg 2+ -stimulated ATPase complex. These systems act as electrogenic proton pumps that translocate protons across the cytoplasmic membrane from the cytoplasm to the external medium. As a result, an electrochemical gradient of protons is formed that exerts an inwardly directed force on the protons, the proton motive force. This proton motive force is the driving force for several energy-requiring processes in the cytoplasmic membrane such as solute and ion transport (secondary transport), and flagellar movement, reversed electron flow and the transhydrogenase reaction. The main proton motive force generating systems (primary transport systems) are the electron transfer chains and the Ca 2+ , Mg 2+ -stimulated ATPase complex. In aerobic bacteria, the proton motive force can be generated by a respiratory chain in which oxygen functions as the terminal electron acceptor. In phototrophic bacteria, the light-induced cyclic electron transfer systems, and in anaerobic bacteria, electron transfer to electron acceptors other than oxygen can generate a proton motive force. In fermentative bacteria, the proton-motive-force-driven solute transport systems can operate in the reversed direction so that efflux of end products can generate a proton-motive force.

PEPTIDE DEGRADATION IN STREPTOCOCCUS-CREMORIS

The capacity to produce an aldonic acid from aldose sugars is widespread among acinetobacters. The enzyme responsible for this reaction is a membrane-bound, pyrrolo-quinoline quinone (PQQ)dependent aldose dehydrogenase (E.C. 1.1.99,17). Fermentor studies with Acinetobacter calcoacetieus strain LMD 79.41 showed that thealdose dehydrogenase is synthesiT~ constitutively,-Pr~minary results obtained with carbon-limited chemostat cultures of this organism revealed that the cell yield on mixtures of acetate and glucose was significantly higher than on acetate alone (molar growth yield (g. mole2) for acetate 14.6 versus 21.3 for acetate plus glucose). Since glucose is almost quantitatively oxidized to gluconic acid it follows that the aldose dehydrogenase may play a role in energy metabolism of acinetobacters. The activity of the enzyme and the rate of acid production from aldose sugars by whole cells show large differences in various Aeinetobacter strains, cultivated under the same conditions. However, a low activity of aldose dehydrogenase is not necessarily due to a low level of apo-enzyme. In various strains the addition of PQQ to cell suspensions resulted in an instantaneous enhancement of the rate of glucose oxidation. Efficient recombination of apo-enzyme and PQQ in such strains was als0 observed in vitro. It is concluded therefore that the apo-enzyme and coenzyme are not always synchronically synthesized in acinetobacters. Preliminary results show that this may also be true for a number of other genera including Pseudomonas and Rhodopseudomonas.