Oxidative phosphorylation chemiosmotic hypothesis

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

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 hypothesis of chemiosmotic mechanism of the oxidative phosphorylation the concentration gradient of H+ ions, formed by the electron transfer energy, is required for speeding up ATP synthesis from ADP and phosphate according to the mechanism based on quick withdrawal of formed H20 molecules dissociated to H+ and OH ions. 

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). 

Fig. 1.3. A scheme of oxidative phosphorylation according to the chemiosmotic coupling hypothesis.

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. 

The answer is b. (Murray, pp 123-148. Scriver, pp 2367-2424. Sack, pp 159-115. Wilson, pp 287-3111) The chemiosmotic hypothesis of Mitchell describes the coupling of oxidative phosphorylation and electron transport. The movement of electrons along the electron transport chain allows protons to be pumped from the matrix of the mitochondria to the cytoplasmic side. The protons are pumped at three sites in the electron transport chain to produce a proton gradient. When protons flow back through proton channels of the asymmetrically oriented ATPase of the inner mitochondrial membrane, ATP is synthesized. 

The addition of ATP to anaerobic or terminally inhibited mitochondria or submitochondrial particles containing succinate Eo = 0.03 V at pH 7) induces reduction of cjdiochrome bj 16,17,65 see also 6 6). The original concept of the possible mechanism of this phenomenon described by Wilson and Dutton 19) was that the Eo of cytochrome f T changes because of the formation of a high energy derivative which is the primary intermediate for site 2 energy conservation reaction in oxidative phosphorylation. However, there has been another possible mechanism presented in which ATP can induce reduction of cytochrome bx by the decrease in the effective redox potential Ek) of the cytochrome because of reversed electron flow 57) or of the abolition of an accessibility barrier between the substrate and the cytochrome 58). The former explanation would be favored by the chemical hypothesis of oxidative phosphorylation, while the latter is favorable for the chemiosmotic hypothesis. 

In 1961, Peter Mitchell proposed the now widely accepted chemiosmotic coupling hypothesis to explain ATP synthesis as a result of electron transport (ETS) and oxidative phosphorylation. It consists of the following principles ... 

Our understanding of oxidative phosphorylation is based on the chemiosmotic hypothesis, which proposes that the energy for ATP synthesis is provided by an electrochemical gradient across the inner mitochondrial membrane. This electrochemical gradient is generated by the components of the electron transport chain, which pump protons across the inner mitochondrial membrane as they sequentially accept and donate electrons (see Fig. 21.1). The final acceptor is O2, which is reduced to H2O. 

P. Mitchell and I. Moyle, Chemiosmotic hypothesis of oxidative phosphorylation, Nature, 213 (1967) 137-139. 

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.

Throughout this discussion of oxidative phosphorylation, we have assumed that the coupling mechanisms involve the formation of high-energy intermediates. This chemical hypothesis is not accepted by all the chemiosmotic hypothesis of oxidative phosphorylation was proposed by Mitchell in 1961, and in 1966 Boyer [148] proposed a new hypothesis involving conformation coupling. 

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. 

Much investigative effort has been directed towards the elucidation of the coupling of the two aspects of oxidative phosphorylation. Historically, three mechanisms have been proposed the chemical coupling hypothesis, the chemiosmotic hypothesis and conformational coupling hypothesis. 

The energetics of the translocation process and the role of GTP therein have fascinated investigators for many years. Brot (1977) and Spirin (1978) have reviewed this process and the proposed means by which translocation is carried out. Spirin has suggested that, while the ribosome binds peptidyl-tRNA on the acceptor site (i.e., prior to translocation), the whole system is akin to a high-energy intermediate in which eventual translocation is thermodynamically ensured. This provocative hypothesis merits careful reading in view of its similarity to the role of the membrane in the chemiosmotic hypothesis for oxidative phosphorylation (Chapter 9). 

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