Oxidative phosphorylation proton pumps

The inner membrane itself plays an important part in oxidative phosphorylation. As it is impermeable to protons, the respiratory chain—which pumps protons from the matrix into the intermembrane space via complexes 1, 111, and IV—establishes a proton gradient across the inner membrane, in which the chemical energy released during NADH oxidation is conserved (see p. 126). ATP synthase then uses the energy stored in the gradient to form ATP from ADP and inorganic phosphate. Several of the transport systems are also dependent on the H"" gradient. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

The reaction catalyzed by F-type ATPases is reversible, so a proton gradient can supply the energy to drive the reverse reaction, ATP synthesis (Fig. 11-40). When functioning in this direction, the F-type ATPases are more appropriately named ATP synthases. ATP synthases are central to ATP production in mitochondria during oxidative phosphorylation and in chloroplasts during photophosphorylation, as well as in eubacteria and archaebacteria. The proton gradient needed to drive ATP synthesis is produced by other types of proton pumps powered by substrate oxidation or sunlight. As noted above, we return to a detailed description of these processes in Chapter 19. 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

The electrochemical potential across the membrane drives protons back into the cell through a membrane ATP synthase complex very similar to that of mitochondria and chloroplasts. Thus, when 02 is limited, halobacteria can use light to supplement the ATP synthesized by oxidative phosphorylation. Halobacteria do not evolve 02, nor do they carry out photoreduction of NADP+ their phototransducing machinery is therefore much simpler than that of cyanobacteria or plants. Nevertheless, the proton-pumping mechanism used by this simple protein may prove to be prototypical for the many other, more complex, ion pumps. Bacteriorhodopsin ... 

Correct answer = D. Thirteen of the approximately 100 polypeptides required for oxidative phosphorylation are coded for by mitochondrial DNA, including the electron transport components cytochrome c and coenzyme Q. Oxygen directly oxidizes cytochrome oxidase. Succinate dehydrogenase directly reduces FAD. Cyanide inhibits electron flow, proton pumping, and ATP synthesis. 

Chemolithotrophic organisms often grow slowly, making study of their metabolism difficult.310 Nevertheless, these bacteria usually use electron transport chains similar to those of mitochondria. ATP is formed by oxidative phosphorylation, the amount formed per electron pair depending upon the number of proton-pumping sites in the chain. This, in turn, depends upon the electrode potentials of the reactions involved. For example, H2, when oxidized by 02, leads to passage of electrons through the entire electron transport chain with synthesis of 3 molecules of ATP per electron pair. On the other hand, oxidation by 02 of nitrite, for which E° (pH 7) = +0.42 V, can make use only of the site III part of the chain. Not only is the yield of ATP less than in the oxidation of H2 but also there is another problem. Whereas reduced pyridine... 

Figure 18-20 Electron transport system for oxidation of the nitrite ion to the nitrate ion by Nitrobacter. Only one site of proton pumping for oxidative phosphorylation is available. Generation of NADH for biosynthesis requires two stages of reverse electron transport.

Formation of ATP by photosynthetic systems is often called photophosphorylation. Although photophosphorylation and oxidative phosphorylation are very similar, they do differ in a few details. The respiratory chain pumps protons... 

Nitrate respiration can support the synthesis of ATP, while proton pumping has been quantified for several physiological substrates. Stoichiometries of about 4H+/NO, and 2H+/N03" have been found for L-malate and formate, and succinate, D-lactate and glycerol respectively. There is evidence that about one mole of ATP is synthesized by oxidative phosphorylation per mole of nitrate reduced.1440... 

Let us start by detecting the principal difference between the two possible forms of the use of membrane catalyst closed and open. In the case of the closed surface, active sites (H+ ions) generated during cell nutrition oxidation are located on both sides of the membrane in almost equal amounts. This does not mean equality of their concentrations in cytoplasm and the matrix. Hence, the same pair of H+ ions is consumed in both ATP synthesis (oxidative phosphorylation) and H20 production (respiration). In the case in where the mitochondrial surface is open, H+ ions would also be consumed in both reactions, the only difference being that now they begin competing for H+ ions. In reality, oxidative phosphorylation is unable to compete for H+ ions with respiration. The so-called proton pump , which promotes... 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

Oxidative phosphorylation occurs in the mitochondria of all animal and plant tissues, and is a coupled process between the oxidation of substrates and production of ATP. As the TCA cycle runs, hydrogen ions (or electrons) are carried by the two carrier molecules NAD or FAD to the electron transport pumps. Energy released by the electron transfer processes pumps the protons to the intermembrane region, where they accumulate in a high enough concentration to phosphorylate the ADP to ATP. The overall process is called oxidative phosphorylation. The cristae have the major coupling factors F, (a hydrophilic protein) and F0 (a hydrophobic lipoprotein complex). F, and F0 together comprise the ATPase (also called ATP synthase) complex activated by Mg2+. F0 forms a proton translocation pathway and Fj... 

More than 95% of the metabolic energy is used in the form of adenosine triphosphate (ATP). Its concentration in a myocyte is about 10 mM. ATP is synthesized by oxidative phosphorylation in the mitochondria. There, acetyl CoA is broken down to CO2 and hydrogen atoms. Electrons are pumped out to form a proton gradient across the mitochondrial membrane. The protons reenter the mitochondria and combine with oxygen, eventually forming water. 

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... 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

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