Oxidative phosphorylation chemiosmotic theory

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. 

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

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

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... 

In the chemiosmotic theory for oxidative phosphorylation (Chap. 14), electron flow in the electron-transport chain is coupled to the generation of a proton concentration gradient across the inner mitochondrial membrane. Derive an expression for the difference in electrochemical potential for a proton across the membrane. 

Mitochondrial oxidative phosphorylation is the process by which ATP is synthesized in mitochondria during the passage of electrons along a chain of carriers. Chemical and chemiosmotic theories for this process have been propounded by their respective adherents, and mechanisms in bioenergetics have been reviewed, but experimental evidence to support these theories has been difficult to obtain with living systems. However, this year, reduced lipoic acid and unsaturated fatty acids have been shown to function as cofactors in the energy-linked synthesis of ATP in mitochondria, and this observation has prompted much activity in this field which will be discussed more fully in Section 6 of this Chapter. 

Oxidative phosphorylation is the mechanism by which electron transport is coupled to the synthesis of ATP. According to the chemiosmotic theory, the creation of a proton gradient that accompanies electron transport is coupled to ATP synthesis. 

As the NADH is oxidized, the electrons released are removed by specific carriers, and the protons are transported from cytoplasm to outside the cell. Removal of H+ causes an increase in the nmnber of OH ions inside the membrane. These conditions result in a proton gradient (pH gradient) across the membrane. This gradient of potential energy, termed as proton motive force, can be used to do useful work. This potential energy is captured by the cell by a series of complex membrane-bound enzymes, known as the ATPase in the process called oxidative phosphorylation. In 1961, the concept of proton gradient was first proposed as chemiosmotic theory by Peter Mitchell of England, who won the Nobel Prize for this scientific contribution. 

According to Mitchell s chemiosmotic theory [52,53] active transport, oxidative phosphorylation and ATP synthesis are driven by a protonmotive force which is made up of two components an electrical potential gradient and a pH gradient across the cell membrane. Agents which conduct ions across the membrane destroy the pH gradient and thereby reduce the protonmotive force. It is possible to measure the pH gradient and to monitor its attenuation by... 

If valid, the chemiosmotic theory indicates that the ultimate step of one of the most essential reactions of live coupling of oxidation and phosphorylation is not simply molecular, but is also vectorial because of the special orientation of the molecules. The separate distribution of the products generates a physical force capable of driving the phosphorylation of ATP, and there is no need to postulate the existence of high-energy intermediates. In conclusion, it seems likely that an adequate and comprehensive interpretation of the mechanism of coupling may well require cooperation of biochemists and physicists. 

The above is a very simplified account of the chemiosmotic theory and a number of the details of oxidative phosphorylation remain to be elucidated, in particular the molecular mechanism of proton pumping and the exact mechanism of action of the ATPase. However, the basic principles of the theory are now widely accepted. This theory is also able to account for photosynthetic phosphorylation in chloroplasts and for the synthesis of ATP by bacteria. 

Of all the intracellular organelles, the mitochondrion has been the most extensively studied with respect to the compartmentation of compounds within its boimdaries. In part, this results from the ease of separation of mitochondria from mammalian tissues (most notably the liver), as well as from the key role mitochondria play in a number of metabolic processes. The mitochondrial membrane is capable of transporting metabolites on specific transporters and of segregating metabolites from the cytosol. It is important to note that some metabolites apparently move across the mitochondrial membrane in an unspecific or non-carrier-linked manner. For example, ketone bodies, water, CO2, and oxygen appear to freely diffuse into and out of mitochondria. In the following sections we will discuss specific aspects of the transport mechanisms, followed by a more general discussion of their role in regulating major metabolic pathways. We will start with the most important result of intracellular compartmentation—oxidative phosphorylation—as viewed by the chemiosmotic theory. 

---