Thermodynamic and Kinetic Aspects of the Interconversion of Chemical and Osmotic Energies in Mitochondria

Abstract

1. The conversion of osmotic energy into ATP has the following properties: a) Kf, as effluent ion, can be replaced by Rbf, whereas when H+, as influent, is replaced by other cations, the synthesis is abolished. b) Both the effluent cation and the influent H+ seem to provide energy for the synthesis.The minimal sum of ApH + A p K , compatible with the synthesis, is 2.5 units, corresponding to an osmotic potential of 3.45 kcalories per gram-ion equivalent-'. The number of ion equivalents exchanged per ATP synthesized is dependent on the dimension of the osmotic potential.c) The osmotic potential compatible with the ATP synthesis is partially affected by the phosphate potential.2. The conversion of redox into osmotic energy has the following properties:a) The number of Kf ion equivalents exchanged with protons per energy rich bond is dependent on the concentrations of H+ and K+ in the inner and outer aqueous phases, increasing up to a maximal value of 4.b) The nymber of K+ ion equivalents exchanged with protons per energy rich bond increases parallel to the increase of the amount of valinomycin, up to the value of 4.3. On the basis of available data the following thermodynamic parameters are evaluated : a) The relationship between osmotic potential and ATP synthesis. b) The relationship between redox and osmotic energy. c) The osmotic potential barrier for ion translocation. 4. A molecular mechanism for the interconversion of osmotic and chemical energies is discussed.Liver mitochondria, as well as mitochondria from other tissues, behave as a highly organized machinery for the conversion of chemical into osmotic energy and vice versa [l-71. According to the chemiosmotic scheme the translocation of protons across the membrane occurs through the operation of either the respiratory chain or the ATPase. Since the number of hydrogen carriers in the respiratory chain is limited, one of the basic tenets of the chemiosmotic hypothesis is a relatively small and constant number of protons to be translocated per oxygen atom consumed. This implies that a relatively large proton motive force is acting on the protons in order to account for the large A G required for ATP synthesis [6,7]. On the other hand the proton pump hypo-

The Molecular Mechanism of ATP Synthesis by F1F0-ATP Synthase: A Scrutiny of the Major Possibilities

“…However, when the K + transport attains a maximum limit upon saturation, the rate of ATP synthesis also becomes constant. This is in complete agreement with the experimental observations that the K + /ATP ratio increases with an increase in K + concentration and becomes constant at 4 for very high concentrations of K + [61,62]. The dynamically electrogenic but overall electroneutral ion transport implies that, if the transmembrane potential (Dy) is measured with nano/microelectrodes, the potential should be close to zero, as found by direct measurements on giant mitochondria [63,64].…”

“…Since DpH supplies only half the energy requirement for mitochondrial ATP synthase, the rest has to be supplied by a locally present but independent source of Dy. One possible explanation suggested by several research groups is electroneutral transport of ions [61][62][63][64]; however, we postulate a dynamically electrogenic but overall electroneutral ion transport involving membrane-permeable anions (such as succinate), and protons, as discussed. In this way, through cation binding within the electrostatic potential field created by the transport of anions, the enzyme is able to utilize the energy of both the delocalized DpH and the localized Dy.…”

ATP synthesis driven by a protonmotive force inStreptococcus lactis

“…In addition, Thayer and Hinkle (1975) have shown that ATP synthesis in submitochondrial particles can be driven by the appropriate combination of electrical potentials and pH gradients. Similar findings had been reported for intact mitochondria (Azzone & Massari, 1971), although the authors questioned the validity of a chemiosmotic interpretation. More recently, ATP synthesis catalyzed by the mitochondrial ATPase has been demonstrated with ProtonmotJve Force-Driven ATP Synthesis 305 an artificial system, in which the requisite protonmotive force arose from light-induced proton pumping by the "purple membrane" of H. halobiurn (Racker & Stoeckenius, 1974).…”

The Influence of Respiration and ATP Hydrolysis on the Proton‐Electrochemical Gradient across the Inner Membrane of Rat‐Liver Mitochondria as Determined by Ion Distribution

“…While the distribution of a weak acid across the inner membrane has been frequently employed to estimate ApH [14,16,, technical reasons prevent accurate determinations when ApH is positive (matrix compartment more acidic), a condition reported by Mitchell and Moyle [3] for the de-energised mitochondrion. In order to provide an accurate determination of ApH under all conditions, a weak base and a weak acid are present simultaneously in the incubation medium.…”