Oxidative phosphorylation reduction potential

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

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). 

The driving force of oxidative phosphorylation is the electron transfer potential of NADH or FADH2 relative to that of O2. How much energy is released by the reduction of O2 with NADH Let us calculate A G° for this reaction. The pertinent half-reactions are... 

The electron transport chain (ETC) or electron transport system (ETS) shown in Figure 16-1 is located on the inner membrane of the mitochondrion and is responsible for the harnessing of free energy released as electrons travel from more reduced (more negative reduction potential, E to more oxidized (more positive carriers to drive the phosphorylation of ADP to ATP. Complex 1 accepts a pair of electrons from NADH ( = -0.32 V)... 

Energy is released as electrons travel from more reduced (more negative reduction potential, E ) to more oxidized (more positive E carriers to drive the phosphorylation of ADP to ATP. 

The addition of ATP to anaerobic or terminally inhibited mitochondria or submitochondrial particles containing succinate Eo = 0.03 V at pH 7) induces reduction of cjdiochrome bj 16,17,65 see also 6 6). The original concept of the possible mechanism of this phenomenon described by Wilson and Dutton 19) was that the Eo of cytochrome f T changes because of the formation of a high energy derivative which is the primary intermediate for site 2 energy conservation reaction in oxidative phosphorylation. However, there has been another possible mechanism presented in which ATP can induce reduction of cytochrome bx by the decrease in the effective redox potential Ek) of the cytochrome because of reversed electron flow 57) or of the abolition of an accessibility barrier between the substrate and the cytochrome 58). The former explanation would be favored by the chemical hypothesis of oxidative phosphorylation, while the latter is favorable for the chemiosmotic hypothesis. 

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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