Electron transport chemiosmotic coupling

Oxidative phosphorylation is the mechanism by which electron transport is coupled to the synthesis of ATP. According to the chemiosmotic theory, the creation of a proton gradient that accompanies electron transport is coupled to ATP synthesis. 

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. 

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). 

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. 

At least two sites can be identified by chemiosmotic principles [12] that is, sites at which transthylakoid proton movement is coupled to electron transport. One is in the reduction of the non-heme iron protein by plastoquinone (i.e. between plastoquinone and Cyt f, in agreement with the former technique), and a second at the water oxidation reaction. Since water oxidation has been shown to occur on the inside of the thylakoid vesicles, each water molecule oxidized leaves two protons intravesicularly, resulting, by chemiosmotic principles, in the creation of a high-energy state. Two coupling sites should result in a maximal H /c2 of 4, in agreement with the above discussed conclusions from ATP/e2 measurements. 

CH3-S-C0M is reduced to methane via the heterodisulfide of H-S-CoM and H-S-HTP. The reduction of the heterodisulfide has been shown to be coupled with ATP synthesis according to a chemiosmotic mechanism (see above). The electrons required for the reduction are derived from the oxidation of enzyme-bound CO ([CO]) which is oxidized to CO2 via CO-DH. It is assumed that electron transport from [CO] to the heterodisulfide is coupled with the generation of an electrochemical proton potential which then drives ATP synthesis. Possible eleetron transport components, a cytochrome b and a membrane-bound hydrogenase, have been identified [232]. Probably two H" -translocating sites are present in electron transport from CO to the heterodisulfide the oxidation of CO to CO2 and H2, and the reduction of the heterodisulfide (or methyl-CoM) by H2. Both H2 and... 

The anisotropic organization of electron carriers across the membrane accounts for the vectorial transport of protons from the inside to the outside of the membrane, which occurs with the passage of electrons. The coupling of this proton gradient to a proton-translocating ATP synthase (also known as ATP synthetase) accounts for the chemiosmotic coupling in oxidative phosphorylation. 

Three hypotheses have been proposed to explain how the mechanism of energy conservation is coupled to electron transport (the energy transduction system) the chemical, conformational, and chemiosmotic hypotheses. 

According to Mitchell s chemiosmotic theory, photophosphorylation is driven by energy derived from electron transfer coupled to proton translocation. The results of postillumination discussed in the previous section further supports the notion that a proton gradient is the driving force for phosphorylation. It is therefore possible in principle that a similar proton gradient produced by artificial means might also be able to drive phosphorylation in a chloroplast membrane, entirely in the dark, i. e., without the aid of photo-induced electron transport. Such a scheme was indeed realized by the so-called acid-bath ATP-forma-tion demonstrated by Jagendorf and Uribe " in 1966. 

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. 

In 1961, Peter Mitchell, a British biochemist, proposed a mechanism by which the free energy generated during electron transport drives ATP synthesis. Now widely accepted, Mitchell s model, referred to as the chemiosmotic coupling theory (Figure 10.11), has the following principal features ... 

Mitchell suggested that the free energy release associated with electron transport and ATP synthesis is coupled by the protonmotive force created by the ETC. (The term chemiosmotic emphasizes that chemical reactions can be coupled to osmotic gradients.) An overview of the chemiosmotic model as it operates in the mitochrondion is illustrated in Figure 10.12. 

See also Electron Transport, P/O Ratio, Chemiosmotic Coupling, Integrity of Mitochondrial Membranes, Uncoupling ETS and Oxidative Phosphorylation, The FIFO Complex, Oxidation as a Metabolic Energy Source (from Chapter 12)... 

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 ... 

Several mechanisms have been proposed to account for the coupling of electron transport and ATP production. The mechanism that served as the point of departure in all discussions is chemiosmotic coupling, which was later modified to include a consideration of conformational coupling. 

FIGURE 20.15 The creation of a proton gradient in chemiosmotic coupling. The overall effect of the electron transport reaction series is to move protons (H ) out of the matrix into the intermembrane space, creating a difference in pH across the membrane. 

Chemiosmotic coupling is the mechanism most widely used to explain the manner in which electron transport and oxidative phosphorylation are coupled to one another. In this mechanism, the proton gradient is directly linked to the phosphorylation process. The way in which the proton gradient leads to the production of ATP depends on ion channels through the inner mitochondrial membrane these channels are a feature of the structure of ATP synthase. Protons flow back into the matrix through proton channels in the Fq part of the ATP synthase. The flow of protons is accompanied by formation of ATP, which occurs in the Fj unit. 

In Chapter 20, we saw that a proton gradient across the inner mitochondrial membrane drives the phosphorylation of ADP in respiration. The mechanism of photophosphorylation is essentially the same as that of the production of ATP in the respiratory electron transport chain. In fact, some of the strongest evidence for the chemiosmotic coupling of phosphorylation to electron transport has been obtained from experiments on chloroplasts rather than mitochondria. Chloroplasts can synthesize ATP from ADP and P in the dark if they are provided with a pH gradient. 

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