Mitchell hypothesis

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

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

Synthesis of ATP by mitochondria is inhibited by oligomycin, which binds to the OSCP subunit of ATP synthase. On the other hand, there are processes that require energy from electron transport and that are not inhibited by oligomycin. These energy-linked processes include the transport of many ions across the mitochondrial membrane (Section E) and reverse electron flow from succinate to NAD+ (Section C,2). Dinitrophenol and many other uncouplers block the reactions, but oligomycin has no effect. This fact can be rationalized by the Mitchell hypothesis if we assume that Ap can drive these processes. 

Energy is provided, for example, by ATP for pumping sodium ions out of and potassium ions into the cell. Another important example of primary active transport is the proton concentration gradient driven ATP synthesis (Mitchell-hypothesis). 

The Mitchell hypothesis was affirmed in 1971 by Kagawa and Packer by reconstituting the ATPase, and then, in 1977, Kagawa and coworkers" showed that phosphorylation occurred when a proton gradient was established. 

In the Mitchell hypothesis, electron transfer is coupled to transmembrane movement of protons, and this transport process results in the creation of an electrochemical potential (proton motive force) at the outside of the membrane. When protons return, they do so through a proton channel in the membrane that leads to the H -ATPase, where synthesis is accomplished. Mitchell (1976) has elaborated on the concept of proticity, i.e., proton flow. One key feature of the chemiosmotic theory is the expectation that H /e = H" /ATP, the value of 2 for both ratios being determined experimentally. Reevaluation of experimental data led Brand and Lehninger (1977) to propose a modification of the chemiosmotic theory which accommodated H" /ATP ratios greater than 2 and H" /e = H" /ATP. Stoichiometric considerations of proton translocation have been reviewed by Papa (1976). 

Slater, E. C., 1967, An evaluation of the Mitchell hypothesis of chemiosmotic coupling in oxidative and photo synthetic coupling, Eur. J. Biochem. 1 317. 

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 21.28 The reconstituted vesicles containing ATP synthase and bacteriorhodopsin used by Stoeckenius and Racker to confirm the Mitchell chemiosmotic hypothesis. 

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

The chemiosmotic hypothesis (also known as the Mitchell hypothe sis) explains how the free energy generated by the transport of elec trons by the electron transport chain is used to produce ATP from ADP + Pj. 

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. 

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

Attempts by 0. E. Brown-Sequard and A. d Arsonval, and S. Merkel have been made to show that the depressed feeling and uneasiness experienced in crowded rooms is not solely due to the diminution of oxygen, and increase of carbon dioxide, but is rather due to the presence of a volatile organic poison in the expired air. This hypothesis was contested by A. Dastre and P. Loye, A. Russo-Giliberti and G. Alessi, D. H. Bergey, S. W. Mitchell and J. S. Billings, G. von Hofmann-Wellenhof, and K. B. Lehmann and F. Jessen. 0. Wurster, and T. Cramer attributed the injurious effects to the presence of nitrates. 

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