Oxidative phosphorylation proton pumping across membranes

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. 

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 ... 

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. 

Again, it is convenient to select certain special cases to simplify the measurements. One obvious choice is the steady state of zero net proton movement across the membrane ( h 0)- This state is usually reached rapidly, because of the small internal volume of the mitochondria soon after an addition the maximal attainable proton gradient will have been established by the proton pumps and net movement of protons ceases. The steady-state condition (7 = 0) allows us to mathematically eliminate one of the thermodynamic forces. Since A/Ih is technically the most difficult to measure, we choose to eliminate this force. The elimination leads to the following relation between the rates of phosphorylation and oxidation ... 

Fig. 21.1. Oxidative phosphorylation. Blue arrows show the path of electron transport from NADH to O2. As electrons pass through the chain, protons are pumped from the mitochondrial matrix to the intermembrane space, thereby establishing an electochemical potential gradient, Ap, across the inner mitochondrial membrane. The positive and negative charges on the membrane denote the membrane potential (Aijr). Ap drives protons into the matrix through a pore in ATP synthase, which uses the energy to form ATP from ADP and Pi.

Our understanding of oxidative phosphorylation is based on the chemiosmotic hypothesis, which proposes that the energy for ATP synthesis is provided by an electrochemical gradient across the inner mitochondrial membrane. This electrochemical gradient is generated by the components of the electron transport chain, which pump protons across the inner mitochondrial membrane as they sequentially accept and donate electrons (see Fig. 21.1). The final acceptor is O2, which is reduced to H2O. 

Figure 8.6. Oxidative phosphorylation of the mitochondria, showing the four complexes of the electron transport chain that pump protons across the inner mitochondrial membrane into the cytoplasmic side to produce a higher concentration of proton within the inner membrane cytosolic space and showing the ATP synthase (at right) that uses the proton flow,...

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. 

The mosaic nonequilibrium thermodynamics formulation of oxidative phosphorylation uses the chemiosmotic model as a basis, besides assuming that the membrane has certain permeability to protons, and that the ATP synthase is a reversible pump coupled to the hydrolysis of ATP. It is assumed that the reversibility of the reactions allows the coupled transfer of electrons in the respiratory chain for the synthesis of ATP, and the proton gradient across the inner mitochondrial... 

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