Phosphorylation, chemiosmotic

Mechanism of Oxidative Phosphorylation Chemiosmotic Coupling (Figure 15.15)... 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

Mitchell, P. (1961). Coupling of phosphorylation to electron and hydrogen transfer by a chemiosmotic type of mechanism. Nature 191, 144-148. 

THE CHEMIOSMOTIC THEORY EXPLAINS THE MECHANISM OF OXIDATIVE PHOSPHORYLATION... 

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Mitchell, P. (1966). Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation. Glynn Research, Bodmin, Cornwall, U.K. 

In many instances, substrate phosphorylation is not coupled to oxidation of the electron donor source by the bacteria therefore, growth will result from oxidative phosphorylation with electrons energizing the plasma membrane for ATP production according to the chemiosmotic system. A list of bacteria displaying dissimilatory reduction where growth is coupled to reduction of metaPmetalloid electron acceptors is given in Table 16.4. 

Chemiosmotic theory provides the intellectual framework for understanding many biological energy transductions, including oxidative phosphorylation and photophosphorylation. 

Before the general acceptance of the chemiosmotic model for oxidative phosphorylation, the assumption was that the overall reaction equation would take the following form ... 

Figure 18-13 Principal features of MitchelTs chemiosmotic theory of oxidative phosphorylation.

The Chemiosmotic Theory Proposes That Phosphorylation Is Driven by Proton Movements... 

The fact that uncouplers are lipophilic weak acids (see above) explains their ability to collapse transmembrane pH gradients. Their lipophilic character allows uncouplers to diffuse relatively freely through the phospholipid bilayer. Because they are weak acids, uncouplers can release a proton to the solution on one side of the membrane and then diffuse across the membrane to fetch another proton. The chemiosmotic theory thus provides a simple explanation of the effects of uncouplers on oxidative phosphorylation. 

Observations in chloroplasts played a key role in the development of the chemiosmotic theory of oxidative phosphorylation, which we discussed in chapter 14. Andre Jagendorf and his colleagues discovered that if chloroplasts are illuminated in the absence of ADP, they developed the capacity to form ATP when ADP was added later, after the light was turned off. The amount of ATP synthesized was much greater than the number of electron-transport assemblies in the thylakoid membranes, so the energy to drive the phosphorylation could not have been stored in an energized... 

Mitchell postulated the chemiosmotic hypothesis for the mechanism of oxidative phosphorylation. 

Since biochemists clearly understood that H+ ion was involved in oxidative phosphorylation, the alternative ATP formation concept occurred as a counter to chemical conjugation. This concept was called the chemiosmotic hypothesis of the oxidative phosphorylation mechanism. This hypothesis was developed by Mitchell, the famous English biochemist [20], who turned is attention to the blind sides of the chemical conjugation concept. 

The hypothesis of chemiosmotic mechanism of the oxidative phosphorylation the concentration gradient of H+ ions, formed by the electron transfer energy, is required for speeding up ATP synthesis from ADP and phosphate according to the mechanism based on quick withdrawal of formed H20 molecules dissociated to H+ and OH ions. 

Let us analyze the ATP synthesis reaction (3.50), which, with respect to inorganic phosphate ion charge, requires one or two H+ ions for oxidation reaction. Figure 3.4 clearly illustrates that the H+-ATP-synthase responsible for oxidative phosphorylation consumes active H30+ particles (H+ ion) from both parts of the reaction system (matrix and cytoplasm). Specifying the work of H+-ATP-synthase, it should be noted that H+ ions delivered from the cytoplasm to the membrane and ADP and P substrates participate in phosphorylation reaction proceeding on the internal surface of the membrane. In this case, water molecules are one of the products of oxidative phosphorylation. It does not release to the volume, but dissociates to H + and OH ions immediately on the membrane. Then according to the chemiosmotic mechanism OH anion is desorbed to cytoplasm and H+ ion to the matrix, where its occurrence as the active particle is associated with water production at the final stage of the respiration process. 

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 is the name given to the synthesis of ATP (phosphorylation) that occurs when NADH and FADH2 are oxidized (hence oxidative) by electron transport through the respiratory chain. Unlike substrate level phosphorylation (see Topics J3 and LI), it does not involve phosphorylated chemical intermediates. Rather, a very different mechanism was proposed by Peter Mitchell in 1961, the chemiosmotic hypothesis. This proposes that energy liberated by electron transport is used to create a proton gradient across the mitochondrial inner membrane and that it is this that is used to drive ATP synthesis. Thus the proton gradient couples electron transport and ATP synthesis, not a chemical intermediate. The evidence is overwhelming that this is indeed the way that oxidative phosphorylation works. The actual synthesis of ATP is carried out by an enzyme called ATP synthase located in the inner mitochondrial membrane (Fig. 3). 

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