Electron transport equilibria

The mechanism of ATP synthesis discussed here assumes that protons extruded during electron transport are in the bulk phase surrounding the inner mitochondrial membrane (intermembrane and extramitochondrial spaces). An alternative view is that there are local proton circuits within or close to the respiratory chain and complex V, and that these protons may not be in free equilibrium with the bulk phase (Williams, 1978), although this has not been supported experimentally (for references see Nicholls and Ferguson, 1992). The chemiosmotic mechanism is both elegant and simple and explains all the known facts about ATP synthesis and its dependence on the structural integrity of the mitochondria, although the details may appear complex. This mechanism will now be discussed in more detail. 

Mozumder (1996) has discussed the thermodynamics of electron trapping and solvation, as well as that of reversible attachment-detachment reactions, within the context of the quasi-ballistic model of electron transport. In this model, as in the usual trapping model, the electron reacts with the solute mostly in the quasi-free state, in which it has an overwhelmingly high rate of reaction, even though it resides mostly in the trapped state (Allen and Holroyd, 1974 Allen et ah, 1975 Mozumder, 1995b). Overall equilibrium for the reversible reaction with a solute A is then represented as... 

The energetics of the electron transport steps makes the process work. Overall there s a lot of free energy lost in the tranfer of electrons from NADH to oxygen—the overall reaction is very favorable, with an equilibrium constant that s overwhelmingly large. At the three sites where ATPs are made (labeled I, II, and III), the reaction is the most downhill. 

N. K. Dutta, Radiative Transitions in GaAs and Other III-V Compounds R. K. Ahrenkiel, Minority-Carrier Lifetime in III-V Semiconductors T. Furuta, High Field Minority Electron Transport in p-GaAs M. S. Lundstrom, Minority-Carrier Transport in III-V Semiconductors R A. Abram, Elfects of Heavy Doping and High Excitation on the Band Structure of GaAs D. Yevick and W. Bardyszewski, An Introduction to Non-Equilibrium Many-Body Analyses of Optical Processes in III-V Semiconductors... 

Electron attachment rates have been measured for numerous solutes. Many of these studies were limited to three solvents cyclohexane, 2,2,4-trimethylpentane, and tetrame-thylsilane (TMS), and those rates are discussed here. What to expect in other liquids can be inferred from these results. Considerable insight has been gained into certain reactions. Equilibrium reactions of electrons are particularly interesting since they provide information not only on energy levels, as mentioned above, but also on the partial molar volume of trapped electrons. This has led to a better understanding of the mechanism of electron transport. 

The number of receptor sites and the position of the equilibrium (Eq. 1) as reflected in KT, will clearly influence the nature of the dose response, although the curve will always be of the familiar sigmoid type (Fig. 2.4). If the equilibrium lies far to the right (Eq. 1), the initial part of the curve may be short and steep. Thus, the shape of the dose-response curve depends on the type of toxic effect measured and the mechanism underlying it. For example, as already mentioned, cyanide binds very strongly to cytochrome a3 and curtails the function of the electron transport chain in the mitochondria and hence stops cellular respiration. As this is a function vital to the life of the cell, the dose-response curve for lethality is very steep for cyanide. The intensity of the response may also depend on the number of receptors available. In some cases, a proportion of receptors may have to be occupied before a response occurs. Thus, there is a threshold for toxicity. With carbon monoxide, for example, there are no toxic effects below a carboxyhemoglobin concentration of about 20%, although there may be... 

An interesting experiment is to allow oxidative phosphorylation to proceed until the mitochondria reach state 4 and to measure the phosphorylation state ratio Rp, which equals the value of [ATP] / [ADP][PJ that is attained. This mass action ratio, which has also been called the "phosphorylation ratio" or "phosphorylation potential" (see Chapter 6 and Eq. 6-29), often reaches values greater than 104-105 M 1 in the cytosol.164 An extrapolated value for a zero rate of ATP hydrolysis of log Rf) = 6.9 was estimated. This corresponds (Eq. 6-29) to an increase in group transfer potential (AG of hydrolysis of ATP) of 39 kj/mol. It follows that the overall value of AG for oxidation of NADH in the coupled electron transport chain is less negative than is AG. If synthesis of three molecules of ATP is coupled to electron transport, the system should reach an equilibrium when Rp = 106 4 at 25°C, the difference in AG and AG being 3RT In Rp = 3 x 5.708 x 6.4 = 110 kj mol-1. This value of Rp is, within experimental error, the same as the maximum value observed.165 There apparently is an almost true equilibrium among NADH, 02 and the adenylate system if the P/O ratio is 3. 

The twist conformer, which has a lower ionization potential, rapidly scavenges the chair form of the cation radical. Being endothermic, the backward transfer is relatively slow, and equilibrium is reached in 20-30 ns. Thus, the electron transport can be described as a series of periods of very fast hole migration between the chair forms and intermittent migration with participation of the twist forms. 

Keywords. Electron transport low-conductance transport non-equilibrium transport perturbation theory density functional theory scanning tunneling microscopy. 

Fig. 5. (a) Bulk electronic concentration at the metal—oxide interface and electron-hole concentration at the oxide—oxygen interface associated with equilibrium interfacial reactions, (b) Electronic energy-level diagram illustrating the dielectric (or semiconducting) nature of the oxide, with the possibility of electron transport (e.g. by tunneling or thermal emission) from the metal to fill O levels at the oxide—oxygen interface to create a potential difference, VM, across the oxide. 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

For simplicity, only electron transport across the interface is considered. At zero applied voltage, the equilibrium current fluxes in... 

Interpretation of the validity of the near-equilibrium concept is dependent on the accuracy of intramitochondrial free NAD/NADH measurements and the difference between extra- and intramitochondrial phosphorylation potentials. In a series of studies, Wilson and associates [40,41,225,226] have presented evidence in support of their hypothesis. Utilizing rat liver mitochondria, Forman and Wilson [225] compared the mass action ratios to calculated equilibrium constants under conditions promoting either forward (net ATP synthesis) or reversed (net ATP hydrolysis) electron transport. Since the mass action ratios calculated under various conditions were similar to the calculated K, these findings were said to support a near-equi-... 

Normally the concentrations of ATP, ADP, NAD+, and NADH are relatively constant in the mitosol and are thus unlikely to be very effective as allosteric regulators under most circumstances. On the other hand the availability of NAD and FAD as substrate will affect the rate not only of the reactions in the table, but also the near-equilibrium dehydrogenases. NAD+ availability in turn is determined by the activity of the electron transport system, whose activity is closely coupled to the availability of ADP. Thus high [ATP] will slow the TCA cycle since high [ATP] means low [ADP], which will slow the ETS resulting in low [NAD ] ... 

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