The Chemiosmotic Theory

But the chemiosmotic theory gives membranous structure a role, Accounts for uncoupler action and respiratory control. 

Mitochondrial inner membranes have redox complexes inlaid And their topological features direct a proton cascade. 

Electrons and protons in symport move outward on enzymic track Followed by charge separation as electrons alone cross back. 

In the simplest of formulations all reductions on matrical side Need protons as well as electrons (hydrogen atoms implied) 

Whilst reductions at external surface (for example of cytochrome c) Take electrons alone to pass inwards leaving protonic charges free. 

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P. Mitchell (Bodmin, Cornwall) contributions to the understanding of biological energy transfer through the formulation of the chemiosmotic theory. 

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

The respiratory chain contains components organized in a sided manner (transverse asymmetry) as required by the chemiosmotic theory. 

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.

The Chemiosmotic Theory Can Account for Respiratory Control and the Action of Uncoupiers... 

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. 

Nicholls DG. 1982. Bioenergetics An Introduction to the Chemiosmotic Theory. London Academic Press. 

In Mitchell s earliest physicochemical formulations of the chemiosmotic theory, any involvement of the membrane across which the proton gradient was established received little attention. Between 1961... 

For a proton motive force to develop, the inner mitochondrial membrane must have a very low permeability to protons so that they do not simply flow back down their concentration gradient and dissipate the high-energy state. Indeed, support for the chemiosmotic theory was first provided by the fact that the rate of the leak of protons back across the membrane is very low, although it can occur under special conditions. When it occurs, it is known as uncoupling (Box 9.2). 

This mechanism was first described as the chemiosmotic theory of ATP generation, or the Mitchell hypothesis. 

A prediction of the chemiosmotic theory is that, because the role of electron transfer in mitochondrial ATP synthesis is simply to pump protons to create the electrochemical potential of the proton-motive force, an artificially created proton gradient should be able to replace electron transfer in driving ATP synthesis. This has been experimentally confirmed (Fig. 19-20). Mitochondria manipulated so as to impose a difference of proton concentration and a separation of charge across the inner membrane synthesize ATP in the absence of an oxidizable substrate the proton-motive force alone suffices to drive ATP synthesis. 

A fundamental postulate of the chemiosmotic theory is the presence of an oriented ATP synthase that utilizes the Gibbs energy difference of the proton gradient to drive the synthesis of ATP (Fig. 18-9). 

A consequence of the chemiosmotic theory is that there is no need for an integral stoichiometry between protons pumped and ATP formed or for an integral P / O ratio. There are bound to be inefficiencies in coupling, and Ap is also used in ways other than synthesis of ATP. 

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

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. 

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. 

The chemiosmotic theory postulates that protons moving back into the matrix via an ATP-synthase drive the formation of ATP. Evidence for this is that an electrochemical potential gradient for protons can support the formation of ATP in the absence of electron-transfer reactions. A transient pH gradient that pulls protons into the matrix can be set up by first incubating mitochondria at pH 9, so that the inside becomes alkaline, and then quickly lowering the pH of the suspension medium to 7 (fig. 14.21). 

One of the principles underlying the chemiosmotic theory is that the mitochondrial inner membrane is relatively impermeable to ions. This raises the question of how Pi, adenine nucleotides, and substrates, such as pyruvate and citrate, move into and out of mitochondria. 

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... 

Nicholls, D.G. Ferguson, S.J. (1992). Bioenergetics an introduction to the chemiosmotic theory. 2nd ed. Academic Press. 

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. 

Diagrammatic representation of a section through the inner mitochondrial membrane, illustrating features of the chemiosmotic theory in the sequence in which they are mentioned in the song. 

Figure 2 The chemiosmotic theory of respiration. The mitochondrial or bacterial membrane (yellow) provides resistance to proton conduction. The respiratory chain generates a proton electrochemical gradient across the membrane by redox-coupled proton translocation (Figure 1). This gradient is used as the driving force for synthesis of ATP, as catalyzed by the H+-ATP synthase in the same membrane...

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