Proton flow back into matrix

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

The mechanics of converting proton flow back into mitochondria into high-energy phosphate bonds is performed by the FoFj ATPase. This complex enzyme system spans the inner mitochondrial membrane and appears as particles labeled EP in Figure 17.3. Structural details are given in Figure 17.7. The enzyme consists of two sections the F0 section is embedded in the membrane and forms a channel through which protons are permitted to enter the F, section. The latter is located on the matrix side of the membrane and is attached to the F0 section. The ATPase catalyzes the formation of ATP from ADP and P,. The reverse reaction, ATP —> ADP + P, is carried out when Fj is separated from F0 hence the term ATPase. 

In the final step the energy of the reservoir is used to make ATP. This last step is carried out by the enzyme complex ATP synthase. As protons flow back into the mitochondrial matrix through a pore in the ATP synthase complex, the enzyme catalyzes the synthesis of ATP. 

Chemiosmotic coupling is the mechanism most widely used to explain the manner in which electron transport and oxidative phosphorylation are coupled to one another. In this mechanism, the proton gradient is directly linked to the phosphorylation process. The way in which the proton gradient leads to the production of ATP depends on ion channels through the inner mitochondrial membrane these channels are a feature of the structure of ATP synthase. Protons flow back into the matrix through proton channels in the Fq part of the ATP synthase. The flow of protons is accompanied by formation of ATP, which occurs in the Fj unit. 

Proton transport via complexes I, III, and IV takes place vectorially from the matrix into the intermembrane space. When electrons are being transported through the respiratory chain, the concentration in this space increases—i. e., the pH value there is reduced by about one pH unit. For each H2O molecule formed, around 10 H ions are pumped into the intermembrane space. If the inner membrane is intact, then generally only ATP synthase (see p. 142) can allow protons to flow back into the matrix. This is the basis for the coupling of electron transport to ATP synthesis, which is important for regulation purposes (see p. 144). 

As in an electrical circuit, where the current of electrons flowing through a resistive element is related to the electrical potential difference and the resistance by Ohm s law, the proton current flowing back into the mitochondrial matrix through a leak pathway will be given by the product of the membrane proton conductance and the proton electrochemical potential ... 

When protons do flow back into the matrix, the free energy arising from the gradient (21 kJ/mol of protons) is dissipated, with some of it being used to drive the synthesis of ATP. 

In eukaryotes, oxidative phosphorylation occurs in mitochondria, while photophosphorylation occurs in chloroplasts to produce ATP. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2 in all aerobic organisms. After, carbon fuels (nutrients) are oxidized in the citric acid cycle, electrons with electron-motive force is converted into a proton-motive force. Photophosphorylation involves the oxidation of H2O to O2, with NADP as electron acceptor. Therefore, the oxidation and the phosphorylation of ADP are coupled by a proton gradient across the membrane. In both organelles, mitochondria and chloroplast electron transport chains pump protons across a membrane from a low proton concentration region to one of high concentration. The protons flow back from intermembrane to the matrix in mitochondria, and from thylakoid to stroma in chloroplast through ATP synthase to drive the synthesis of adenosine triphosphate. Therefore, the adenosine triphosphate is produced within the matrix of mitochondria and within the stroma of chloroplast. 

Fig. 19-17), the electrochemi- 192°-1992 cal energy inherent in the difference in proton concentration and separation of charge across the inner mitochondrial membrane—the proton-motive force—drives the synthesis of ATP as protons flow passively back into the matrix through a proton pore associated with ATP synthase. To emphasize this crucial role of the proton-motive force, the equation for ATP synthesis is sometimes written... 

Flow of Protons Back into the Matrix Drives the Formation of ATP... 

According to the chemiosmotic theory, flow of electrons through the electron-transport complexes pumps protons across the inner membrane from the matrix to the intermembrane space. This raises the pH in the matrix and leaves the matrix negatively charged with respect to the intermembrane space and the cytosol. Protons flow passively back into the matrix through a channel in the ATP-synthase, and this flow drives the formation of ATP. 

Protons, which are present in the intermembrane space in great excess, can pass through the inner membrane and back into the matrix down their concentration gradient only through special channels. (The inner membrane itself is impermeable to ions such as protons.) As the thermodynamically favorable flow of protons occurs through a channel, each of which contains an ATP synthase activity, ATP synthesis occurs. 

Figure 13.11 The chemiosmotic hypothesis. Electron transport pumps protons (H ) out of the matrix. The formation of ATP accompanies the flow of protons through Fi-ATPase back into the matrix.

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