Chemiosmotic concept

According to the chemiosmotic concept therefore the central intermediate between the energy-transducing processes in the cytoplasmic membrane is the electrochemical potential gradient of protons, pmf-producing and pmf-consuming processes found in bacteria are listed below. 

Mitchell s chemiosmotic concept states that a proton gradient is the sole coupler of electron transport to ATP synthesis. 

As a matter of fact, the difference in understanding the role of protons in membrane phosphorylation reflects the existence of two different principal approaches to the problem of energy coupling in biomembranes that involve numerous concrete models. The first of them is most clearly expressed in the widely accepted Mitchell chemiosmotic concept [39, 40, 45, 46]. According to Mitchell s postulate, X represents a transmembrane difference in the electrochemical potential of hydrogen ions, A/Ih+- This value can be presented in the form of the sum of two separately measured quantities A/Ih+ = Aq> -h (RT/F) ln([H ]i /[H ], , t), where Aq> = — [Pg.117]

Let us consider the total balance for an ATPsynthase reaction in biomembranes. According to the chemiosmotic concept, this reaction can be formulated as follows ... 

The rate of photophosphorylation depends on a number of factors different from the pmf the observation that the phosphorylation rate is increased in the presence of a lowered pmf is therefore not necessarily in contradiction with the chemiosmotic concept. On the other hand, the PP is assumed to be determined only by the magnitude of the pmf. Therefore, maximum values of the PP can be used to monitor the pmf. Fig. 3 shows the PP attained by a suspension of chloroplasts in the absence and presence of 0.5 mM methylamine. Maximum values in... 

Mitchell, P., 1979. Keilin s re.spiratory chain concept and its chemiosmotic consequences. Science 206 1148-1159. 

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

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. 

A competing model called the chemiosmotic model was suggested by Mitchell in 1961 and won a Nobel prize. The physical events that which Mitchell s theory implies are less consistent with modem concepts of interfacial charge transfer than those of Williams, which do indicate interfacial charge transfer. 

In this review, the chemiosmotic hypothesis [1] at the physiological level, i.e., ATP synthesis in FqF, driven by an electrochemical potential difference of protons (Fig. 5.1) is supported, while the hypothesis [1] at the level of molecular mechanism, i.e., the direct participation of the translocated protons in the dehydration of phosphate during ATP synthesis in F F, is excluded. The solid chemical and physical experiments on the purified Fj, Fq and FqF, and genetic analysis of the F F, established a new concept on the proton motive ATP synthesis. 

Mitchell, P., Keilin s Respiratory Chain Concept and Its Chemiosmotic Consequences, Science, 206 1148-1159, 1979. 

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. 

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.]... 

As the NADH is oxidized, the electrons released are removed by specific carriers, and the protons are transported from cytoplasm to outside the cell. Removal of H+ causes an increase in the nmnber of OH ions inside the membrane. These conditions result in a proton gradient (pH gradient) across the membrane. This gradient of potential energy, termed as proton motive force, can be used to do useful work. This potential energy is captured by the cell by a series of complex membrane-bound enzymes, known as the ATPase in the process called oxidative phosphorylation. In 1961, the concept of proton gradient was first proposed as chemiosmotic theory by Peter Mitchell of England, who won the Nobel Prize for this scientific contribution. 

Mitchell " used Nernstian ideas of electrochemistry to support a concept of the functioning of biological membrane which he called chemiosmotic, as descriptive of a reversible potential difference arising across the membrane as a result of osmotic forces. Mitchell does not consider the transfer of protons as in conduction (or electronic conduction associated with it). He assumes that a carrier system transports H" " from... 

The chemiosmotic theory requires that uncouplers increase the permeability of the coupling membrane to protons, thus collapsing the pH or electrochemical potential, and indeed several recent reports on mitochondria, or on artificial membranes, are in line with the concept... 

The proposed mechanism involves two main aspects. First, the trans-membrane concentration difference of H20-H+ determines the direction and rate of the ballistic proton flux and the corresponding amount of ATP synthesis or hydrolysis. Second, in the direction of ATP synthesis, a threshold electric potential difference is obligatory for compensation of a certain dissipation of the proton s kinetic energy, (e.g. with a total energy of 0.5 proton volt, 0.1 volt compensates for 20% loss). Thus, the ballistic proton mechanism elucidates the thermodynamic concept of a proton-motiveforce in the chemiosmotic hypothesis [42, 43]. It accounts quantitatively for the elementary energetic event it bypasses the problem of trans-membrane proton transportation and it differentiates between independent roles of chemical potential and electrical potential of H20-Fi+ across the membrane [44]. 

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). 

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