Universal free-energy landscape produces efficient and reversible electron bifurcation

For decades, it was unknown how electron-bifurcating systems in nature prevented energy-wasting short-circuiting reactions that have large driving forces, so synthetic electron-bifurcating molecular machines could not be designed and built. The underpinning free-energy landscapes for electron bifurcation were also enigmatic. We predict that a simple and universal free-energy landscape enables electron bifurcation, and we show that it enables high-efficiency bifurcation with limited short-circuiting (the EB scheme). The landscape relies on steep free-energy slopes in the two redox branches to insulate against short-circuiting using an electron occupancy blockade effect, without relying on nuanced changes in the microscopic rate constants for the short-circuiting reactions. The EB scheme thus unifies a body of observations on biological catalysis and energy conversion, and the scheme provides a blueprint to guide future campaigns to establish synthetic electron bifurcation machines.

A protonmotive force drives ATP synthesis in bacteria

When cells of Streptococcus lactis or Escherichia coli were suspended in a potassium-free medium, a membrane potential (negative inside) could be artificially generated by the addition of the potassium ionophore, valinomycin. In response to this inward directed protonmotive force, ATP synthesis catalyzed by the membrane-bound ATPase (EC 3.6.1.3) was observed. The formation of ATP was not found in S. lactis that had been treated with the ATPase inhibitor, N,N'-dicyclohexylcarbodiimide, nor was it observed in a mutant of E. coli lacking the ATPase. Inhibition of ATP synthesis in S. lactis was also observed when the membrane potential was reduced by the presence of external potassium, or when cells were first incubated with the proton conductor, carbonylcyanidefluoromethoxyphenylhydrazone. These results are in agreement with predictions made by the chemiosmotic hypothesis of Mitchell.

Depolarization of the plasma membrane of Neurospora during active transport of glucose: evidence for a proton-dependent cotransport system

Intracellular microelectrodes were used to measure the effects of glucose transport on membrane voltage and membrane resistance in Neurospora crassa. Sudden activation of glucose uptake, via the high-affinity, derepressible system II, results in a very large depolarization of the plasma membrane. At saturating concentrations of glucose, the depolarization averages 120 mV; it is diphasic in time, with an initial shift at rates of 100-200 mV/sec followed by a slow, spontaneous, partial repolarization. Changes in intracellular ATP concentration are small and could account for only 10 mV of the initial depolarization, while the rest appears to depend upon the transport process itself. A plot of peak depolarization against the extracellular glucose concentration gives a saturation curve which is half-maximal at 42 muM, in good agreement with the K(1/2) reported for glucose transport via system II. The nonmetabolized analogue 3-O-methyl-D-glucose also causes depolarization, and in addition leads to a pulsed alkalinization of the medium occurring at approximately the same rate as 3-O-methyl-D-glucose uptake. The membrane resistance falls only slightly during glucose depolarization, a fact which requires the transport system itself to have a high internal resistance, or the membrane current-voltage relationship in glucose-starved cells to be quite nonlinear. All of the data support Mitchell's notion that sugar and hydrogen ions are contransported under the influence of the membrane potential, and lead to values for H(+):glucose stoichiometry of 0.8 to 1.4.

Proton-coupled accumulation of galactoside in Streptococcus lactis 7962

When cells of the anaerobe Streptococcus lactis 7962 are deprived of their normal fermentable energy source, active transport of galactosides is completely abolished although the membrane carriers are still capable of facilitating the equilibration of sugars across the cell membrane. In these nonmetabolizing cells it was possible to test the Mitchell hypothesis of obligatory coupling of proton movement with sugar transport. This hypothesis was supported by alkalinization of the medium observed when thiomethylgalactoside was added to a lightly buffered suspension of S. lactis cells. Conversely, addition of protons resulted in active transport of thiomethylgalactoside. Accumulation of thiomethylgalactoside to a concentration more than 20-times that in the external medium was induced by suddenly exposing cells to a medium at pH 6; no accumulation of thiomethylgalactoside was observed with cells exposed to pH 8.Active transport of thiomethylgalactoside occurred in the absence of energy metabolism when S. lactis cells were treated with valinomycin. This ionophore allowed intracellular K(+) to flow out, thus imposing a membrane potential (inside negative). This potential resulted in a proton uptake and an associated active transport of galactoside. The membrane potential was measured from the distribution ratio (inside/outside) of K(+) in the presence of valinomycin. The pH gradient was measured from the distribution ratio of [(14)C]methylamine. The protonmotive force, calculated from the membrane potential and the pH gradient, was found to be directly related to the accumulation of galactoside, in accordance with the chemiosmotic hypothesis.

Role of an electrical potential in the coupling of metabolic energy to active transport by membrane vesicles of Escherichia coli

Membrane vesicles from E. coli can oxidize D-lactate and other substrates and couple respiration to the active transport of sugars and amino acids. The present experiments bear on the nature of the link between respiration and transport. Respiring vesicles were found to accumulate dibenzyldimethylammonium ion, a synthetic lipid-soluble cation that serves as an indicator of an electrical potential. The results suggest that oxidation of D-lactate generates a membrane potential, vesicle interior negative, of the order of -100 mV. In vesicles lacking substrate, an electrical potential was created by induction of electrogenic efflux of K(+) with the aid of the K(+) ionophores, valinomycin and monactin. These conditions induced transient accumulation by the vesicles of [(14)C]proline and other metabolites. Experiments with inhibitors and ionophores indicate that neither ATP nor the respiratory chain is involved; the electrical potential generated by K(+) efflux is coupled directly to the transport systems. The results verify two predictions derived from Mitchell's chemiosmotic hypothesis: respiring vesicles generate an electrical potential of the proper polarity and magnitude; and a membrane potential is in itself sufficient to drive the active transport of amino acids and other metabolites.