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... 

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... 

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

Experiments with Isolated Chloroplasts Provided the First Direct Evidence for the Chemiosmotic Hypothesis... 

Chemiosmotic hypothesis the concept that electron transport along the electron... 

The aspect of the present position of consensus that I find most remarkable and admirable, is the altruism and generosity with which former opponents of the chemiosmotic hypothesis have not only come to accept it, but have actively promoted it to the status of a theory. 

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... 

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

Ferguson, S. (1997). Bioenergetics after 1960 from the chemiosmotic hypothesis to structure-based molecular mechanisms. Foundations of Modem Biochemistry, Vol. 3, pp. 3-22. JAI Press, Greenwich, CT. 

CONTENTS Acknowledgements, Margery G. Ord and Lloyd A. Stocken. Introduction. Bioenergetics After 1960 From the Chemiosmotic Hypothesis to Structure-Based Molecular Mechanisms, Stuart J. Ferguson. Changing Views of Photosynthesis, F.R. Whatley. Muscle Contraction and Relaxation,... 

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 chemiosmotic hypothesis does not use the notion of a general, highly active intermediate product. 

The mechanism based on the chemiosmotic hypothesis of ATP synthesis can also be related to this class of mechanism, because it promotes the separate use of the highly active intermediate particle (H+ ion) with the maximum effectiveness and in each of conjugated reactions. 

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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