Oxidative phosphorylation electron-transfer potential

The driving force of oxidative phosphorylation is the difference between the electron transfer potential of NADH or FADH2 relative to that of 02. For the redox couple... 

High-energy electrons and redox potentials are of fundamental importance in oxidative phosphorylation. In oxidative phosphorylation, the electron transfer potential of NADH or FADH2 is converted into the phosphoryl transfer potential of ATP. We need quantitative expressions for these forms of free energy. The measure of phosphoryl transfer potential is already familiar to us it is given by A G° for the hydrolysis of the activated phosphate compound. The corresponding expression for the electron transfer potential is i Q the reduction potential (also called the redox potential or oxidation-reduction potential). 

Conceptual Insights, Energy Transformations in Oxidative Phosphorylation. View this media module for an animated, interactive summary of how electron transfer potential is converted into proton-motive force and, finally, phosphoryl transfer potential in oxidative phosphorylation. 

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. 

Oxidative phosphorylation is the culmination of a series of energy transformations that are called cellular respiration or simply respiration in their entirety. First, carbon fuels are oxidized in the citric acid cycle to yield electrons with high transfer potential. Then, this electron-motive force is converted into a proton-motive force and, finally, the proton-motive force is converted into phosphoryl transfer potential. The conversion of electron-motive force into proton-motive force is carried out by three electron-driven proton pumps—NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and... 

How is the oxidation of NADH coupled to the phosphorylation of ADP It was first suggested that electron transfer leads to the formation of a covalent high-energy intermediate that serves as a high phosphoryl transfer potential compound or to the formation of an activated protein conformation, which then drives ATP synthesis. The search for such intermediates for several decades proved fruitless. 

Two other enzymes that we will encounter later, glycerol phosphate drogenase (p. 528) and fatty acyl CoA dehydrogenase (p. 624), likewise transfer their high-potential electrons from FAD Hi to Q to form ubiquinol (QH f), the reduced state of ubiquinone. These enzymes oxidize glycerol and fats, respectively, providing electrons for oxidative phosphorylation, These enzymes also do not pump protons. 

The electron transport system is the place in the cell where electrons generated by oxidation are transferred. Passage of the electrons through the system generates potential energy that is used to make ATP in oxidative phosphorylation. 

In phase 2 of cellular respiration, the energy derived from fuel oxidation is converted to the high-energy phosphate bonds of ATP by the process of oxidative phosphorylation (see Fig. 2). Electrons are transferred from NADH and FAD(2H) to O2 by the electron transport chain, a series of electron transfer proteins that are located in the inner mitochondrial membrane. Oxidation of NADH and FAD(2H) by O2 generates an electrochemical potential across the inner mitochondrial membrane in the form of a transmembrane proton gradient (Ap). This electrochemical potential drives the synthesis of ATP form ADP and Pi by a transmembrane enzyme called ATP synthase (or FoFjATPase). 

Approximately 90 to 95% of the oxygen we consume is used by the terminal oxidase in the electron transport chain for ATP generation via oxidative phosphorylation. The remainder of the O2 is used directly by oxygenases and other oxidases, enzymes that oxidize a compound in the body by transferring electrons directly to O2 (Fig. 19.12). The large positive reduction potential of O2 makes all of these reactions extremely favorable thermodynamically, but the electronic structure of O2 slows the speed of electron transfer. These enzymes, therefore, contain a metal ion that facilitates reduction of O2. 

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