Proton electrochemical potential

The proton motive force is also known as the proton electrochemical potential. 

Kamo, N., Muratsugu, M., Hongoh, R. and Kobatake, Y., (1979) Membrane potential of mitochondria measured with an electrode sensitive to tetraphenyl phosphonium and relationship between proton electrochemical potential and phosphorylation potential in steady state. Journal of Membrane Biology, 49 (2), 105-121. 

Rottenberg, H. (1998) The generation of proton electrochemical potential gradient by cytochrome c oxidase. Biochim. Biophys. Acta 1364, 1-16. 

The reader should be aware that considerable confusion exists with respect to names and definitions.176 For example, the AGH+ of Eq. 18-8 can also be called the proton electrochemical potential Ap,+, which is analogous to the chemical potential p of an ion (Eq. 6-24) and has units of kj/mol (Eq. 18-10). 

It is now generally accepted that the coupling of electron transport and ATP synthesis is brought about by the action of a proton electrochemical-potential gradient, denoted by the symbol A/uH+. This gradient arises as a consequence of electron transport and is dissipated by ATP synthase to generate ATP from ADP and P,. 

The chemiosmotic model requires that flow of electrons through the electron-transport chain leads to extrusion of protons from the mitochondrion, thus generating the proton electrochemical-potential gradient. Measurements of the number of H+ ions extruded per O atom reduced by complex IV of the electron-transport chain (the H+/0 ratio) are experimentally important because the ratio can be used to test the validity of mechanistic models of proton translocation (Sec. 14.6). 

Fatty acids facilitate the net transfer of protons from intermembrane space into the mitochondrial matrix, hence lowering the proton electrochemical potential gradient and mediating weak uncoupling. Uncoupling proteins generally facilitate the dissipation of the transmembrane electrochemical potentials of H+or Na+produced by the respiratory chain, and result in an increase in the H+and Na+permeability of the coupling membranes. They provide adaptive... 

This chapter is concerned with the proton circuit which in the chemiosmotic scheme links the generators and utilizers of proton electrochemical potential (A/if ) [1,2]. Since the topic has been covered extensively in a recent monograph [3], this chapter will attempt to avoid repetition by concentrating on the ionic circuitry found in association with energy conserving organelles, and no attempt will be made to discuss the structures of the black boxes of the membrane. 

The electrochromic shift of the carotenoids is usually calibrated with K-diffusion potential in the presence of valinomycin. One problem is that the shifts observed in respiring chromatophores (where the proton electrochemical potential is predominantly in the form of a membrane potential) are much larger than those induced by the calibrating diffusion potential, so that an extensive extrapolation is required. Thus, the carotenoids in illuminated chromatophores may indicate a membrane potential in excess of 300 mV, whereas the distribution of CNS, an electrically permeant anion, in the same system only indicates 140 mV [32]. The extent of this discrepancy, and the uncertainty as to whether the carotenoids see the bulk-phase potential, or only the local electrical field within the membrane, limits the confidence with which carotenoids may be used for quantitative as opposed to qualitative potential measurements. 

If the only pathway for proton re-entry into the mitochondrial matrix were through the ATP synthase, then in conditions under which there is no net ATP synthesis, either due to inhibition of the complex itself, for example with oligomycin, or due to the attainment of thermodynamic equilibrium between the ATP pool and the proton electrochemical potential, there should be a complete inhibition of respiration. In practice mitochondria continue to respire at a finite rate even in this State 4 condition, since protons can slowly leak back across the membrane (Fig. 2.2). 

As in an electrical circuit, where the current of electrons flowing through a resistive element is related to the electrical potential difference and the resistance by Ohm s law, the proton current flowing back into the mitochondrial matrix through a leak pathway will be given by the product of the membrane proton conductance and the proton electrochemical potential ... 

Fig. 2.3 also shows that the transfer of energy from the respiratory chain to the proton circuit can be extremely efficient, in that a slight thermodynamic disequilibrium results in a considerable energy flux. The actual disequilibrium between the respiratory chain and the proton electrochemical potential is even less than appears from the drop in the latter, since the redox span across the respiratory chain proton pumps also contracts [24],... 

The three proton pumps of the mitochondrial respiratory chain normally function in parallel with respect to the proton circuit. It is however possible to manipulate the conditions such that the proton electrochemical potential generated by two of the pumps can be used to reverse the third proton pump (but not cytochrome oxidase). 

One way in which this may be achieved is shown in Fig. 2.4. The proton electrochemical potential generated by the downhill flow of electron from Complex III to oxygen can be used to drive Complex I in reverse, protons entering the matrix through this complex and driving electrons uphill from the ubiquinone pool to the NADH/NAD pool, with a redox potential some 300 mV more negative. 

This net proton extrusion results in a net acidification of the external medium and an alkalinization of the matrix (Fig. 2.5), and should be distinguished from the steady-state cycling which occurs during the operation of the proton circuit for ATP synthesis. Clear, alkalinization of the matrix cannot continue indefinitely, and the limitation which is set is essentially thermodynamic - the respiratory chain is incapable of maintaining a proton electrochemical potential in excess of 200-230... 

Respiratory control is a consequence of the thermodynamic back-pressure exerted by the proton electrochemical potential upon the respiratory chain, limiting the rate at which further protons can be extruded. Since the chemiosmotic theory states that the only connection between the respiratory chain and the other proton-utilizing components in the membrane is through the proton electrochemical gradient, the respiratory chain should be incapable of distinguishing between the different ways of producing a given depression in A/tn+. 

Nicholls D.G. (1978) Calcium transport and proton electrochemical potential potential gradient in mitochondria from guinea-pig cerebral cortex and rat heart. Biochem. J. 170 511 —22. 

This approach has been adopted to investigate the function of COX from the proteobacterium Rhodohacter sphaeroides [110], the last enzyme in the respiratory electron transport chain of bacteria, located in the bacterial inner membrane. It receives one electron from each of four ferrocytochrome c molecules, located on the periplasmic side of the membrane, and transfers them to one oxygen molecule, converting it into two water molecules. In the process, it binds four protons from the cytoplasm to make water, and in addition translocates four protons from the cytoplasm to the periplasm, to establish a proton electrochemical potential difference across the membrane. In this ptBLM, the orientation of the protein with respect to the membrane normal depends on the location of the histidine stretch... 

Nishio IN and Whitmarsh J (1993) Dissipation of the proton electrochemical potential in intact chloroplasts. II. The pH gradient monitored by cytochrome/reduction kinetics. Plant Physiol 101 89-96... 

Neglect of the backward transitions (short arrows) is justifiable based oti the large proton electrochemical potential. A//, which drives the process. Neglect of the red arrows is justifiable based on structural considerations showing that at one angle the proton binding site is open to the periplasm but not to the cytoplasm, and at... 

A first model for the organization of respiratory nitrite ammonification and direction of proton exchange associated with electron transfer from formate to nitrate in E. coli had been described earlier [126]. The reduction of nitrate to nitrite by the oxidation of formate is linked to the generation of a proton electrochemical potential (Section 3.1.1., discussion on nitrate reductase). Whether the overall architecture of the nitrate reductase complex from E. coli also holds for other nitrite-ammonifying bacteria, has to be investigated. S. deleyianum and... 

In most cases, the rate of photophosphorylation (PSP) is dependent on the proton electrochemical potential (4aiH ). One of the exceptions is that nigericin and NH Cl at lo concentrations stimulate PSP in steady state but decrease the jaH (1,2,3,, 5). These phenomena are difficult to be explained by the chemiosmotic hypothesis. Our results indicated that there is an alternative mechanism of coupling between the photosynthetic electron transport and ATP formation. 

Ajrfj+ = A P + Z pH where Z = 2.3 RT/ F The proton electrochemical potential (A Xjj+) difference across the coupled (energetically linked) plasma membrane in phototrophic bacteria plays an essential role in photophosphorylation and solute transport into bacterial cells. Therefore, the exact measurements of these quantities are required for studies of the mechanism of energy transduction. It was, for example, shown that R.ruhrum chrom-atophores, associated with a phospholipid-impregnated filter,... 

Fatty acids facilitate the net transfer of protons from intermembrane space into the mitochondrial matrix, hence lowering the proton electrochemical potential gradient and mediating weak uncoupling. Uncoupling... 

By consuming four protons from the interior of the mitochondrion and four electrons from the exterior, a proton-electrochemical potential gradient is formed across the membrane, providing fuel for ATP synthesis and other processes. This process is commonly referred to as proton pumping. 

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