Mitchell’s chemiosmotic hypothesis

Mitchell s chemiosmotic hypothesis. The ratio of protons transported per pair of electrons passed through the chain—the so-called HV2 e ratio—has been an object of great interest for many years. Nevertheless, the ratio has remained extremely difficult to determine. The consensus estimate for the electron transport pathway from succinate to Og is 6 H /2 e. The ratio for Complex I by itself remains uncertain, but recent best estimates place it as high as 4 H /2 e. On the basis of this value, the stoichiometry of transport for the pathway from NADH to O2 is 10 H /2 e. Although this is the value assumed in Figure 21.21, it is important to realize that this represents a consensus drawn from many experiments. 

Peter Mitchell s chemiosmotic hypothesis revolutionized our thinking about the energy coupling that drives ATP synthesis by means of an electrochemical gradient. How much energy is stored in this electrochemical gradient For the transmembrane flow of protons across the inner membrane (from inside [matrix] to outside), we could write... 

In 1961, Peter Mitchell proposed a novel coupling mechanism involving a proton gradient across the inner mitochondrial membrane. In Mitchell s chemiosmotic hypothesis, protons are driven across the membrane from the matrix to the intermembrane... 

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

Two major ATP synthesizing reactions in living organisms are oxidative phosphorylation and photophosphorylation. Both reactions take place in H -ATPase (FqF,), which is driven by an electrochemical potential difference of protons across the biomembrane, as predicted by Mitchell [1]. In Racket s laboratory, ATPases related to oxidative phosphorylation were prepared, but their relationship to Mitchell s chemiosmotic hypothesis [1] was not described [2], Later, an insoluble ATPase (H -ATPase) was shown to translocate protons across the membrane when it was reconstituted into liposomes [3], H -ATPase was shown to be composed of a catalytic moiety called F, (coupling factor 1) [4], and a membrane moiety called Fq [5], which confers inhibitor sensitivity to F,. F was shown to be a proton channel, which translocates down an electrochemical potential gradient across the membrane when Fg is reconstituted into liposomes (Fig. 5.1) [6]. Thus, -ATPase was called FqFj or ATP synthetase. 

His early observations provided some of the most important experimental evidence in support of Mitchell s chemiosmotic hypothesis. 

Proton transfer reactions are one of principal chemical processes and they are of primary importance in chemistry and biology. Protolytic reactions are the main type of acid-base ( heterolytic ) reactions. Mitchell s chemiosmotic hypothesis especially stimulated the investigation of proton transfer reactions in biochemistry. 

Greville, G. D., 1969b, A scrutiny of Mitchell s chemiosmotic hypothesis of respiratory chain and photosynthetic phosphorylation, in Current Topics in Bio energetics, Vol. 3 (D. Sanadi, ed.). Academic Press, New York. 

Mitchell s Nobel lecture, outlining the evolution of the chemiosmotic hypothesis. 

The chemiosmotic hypothesis had the great virtue of predicting the following consequences which could be tested (1) electron-transport driven proton pumps with defined stoichiometries and (2) a separate ATP synthase, which could be driven by a pH gradient or membrane potential. Mitchell s hypothesis was initially greeted with skepticism but it encouraged many people, including Mitchell and his associate Jennifer Moyle, to test these predictions, which were soon found to be correct.178... 

The chemiosmotic coupling hypothesis, proposed by P. Mitchell, is the most attractive explanation, and many experimental observations now support this idea. Simply stated, Mitchell s hypothesis suggests that electron transfer is accompanied by transport of protons across the membrane. 

Figure 1. Chemiosmotic coupling in oxidative phosphorylation. The model is drawn according to Mitchell s hypothesis for mitochondria, but might also apply to other systems of phosphorylation.

This brings me to the latest chapter in the field of oxidative phosphorylation and the current controversies on the mechanism of oxidative phosphorylation. Slater s formulations based on the mode of glyceraldehyde-3-phosphate dehydrogenase action became known as the chemical hypothesis. In 1961 Mitchell proposed a chemiosmotic hypothesis. ... 

I admit that when I first read his proposition, I was not impressed. In 1965 I published a book on Mechanisms in Bioenergetics and did not even mention the chemiosmotic hypothesis. Phil Handler who wrote a generous review of my book in Science objected to my failure to discuss Mitchell s hypothesis. By the time his review appeared I knew that his criticism was justified because Peter Mitchell had visited me in New York in 1965. This was another important event in my scientific life. Not that I really understood most of what Mitchell said during these days of intensive discussions, but I opened my mind to a new way of thinking. I am now convinced that the basic formulations of his chemiosmotic hypothesis are correct, namely that the function of the respiratory chain is to translocate protons and that the return of those protons via the oligomycin-sensitive ATPase is responsible for ATP formation. Thus the problem of the mecham sm of coupling of oxidation and phosphorylation is basically solved. 

While the chemiosmotic hypothesis does not embrace the need for the mediation of a chemical coupling compound(s) or for conformational coupling interaction(s) between the redox system and the ATP-synthesizing system, Mitchell notes that there is every reason to believe that conformational interactions may be involved within the translocation system itself, in a manner consistent with the role of conformational changes in enzymic catalysis, as elaborated in Chapters 3 and 4 of this book. 

Figure 1. Topography of the inner mitochondrial membrane. This figure shows the folding of the respiratory chain in the inner mitochondrial membrane and its association with the ATPase of the FI. The figure is from a review by Racker (1970) and presents the author s reconstruction of the membrane in a way that has appeal in terms of the chemiosmotic hypothesis of Mitchell.

Mitchell, P. Keilin s Respiratory Chain Concept and Its Chemiosmotic Consequences. Science 206, 1148-1159 (1979). [A Nobel Prize lecture by the scientist who first proposed the chemiosmotic coupling hypothesis.]... 

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