Proton-motive force

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

A proton-motive force of approximately —250 mV is needed to achieve ATP synthesis. This proton-motive force, A, is composed of a membrane potential, A P, and a pH gradient, ApH (Chapter 21). The proton-motive force is defined as the free energy difference, AG, divided by S, Paraday s constant ... 

In chloroplasts, the value of AT is typically —50 to —100 mV, and the pH gradient is equivalent to about 3 pH units, so that — (2.3 i T/S ) ApH = —200 mV. This situation contrasts with the mitochondrial proton-motive force, where the membrane potential contributes relatively more to bsp than does the pH gradient. 

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

FIGURE 22.21 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CFiCFo ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetrical with respect to the thylakoid membrane so that vectorial discharge and uptake of ensue, generating the proton-motive force. 

Noncyclic photophosphorylation has been the focus of our discussion and is represented by the scheme in Figure 22.21, where electrons activated by quanta at PSII and PSI flow from HgO to NAJDP, with concomitant establishment of the proton-motive force driving ATP synthesis. Note that in noncyclic photophosphorylation, Og is evolved and NADP is reduced. 

Assuming that the concentrations of ATP, ADP, and P in chloroplasts are 3 mM, 0.1 mM, and 10 mM, respectively, what is the AG for ATP synthesis under these conditions Photosynthetic electron transport establishes the proton-motive force driving photophosphorylation. What redox potential difference is necessary to achieve ATP synthesis under the foregoing conditions, assuming an electron pair is transferred per molecule of ATP generated ... 

Cells need a certain amount of energy for maintenance. The maintenance energy is, for instance, needed for maintaining the proton motive force which is, among other purposes, used for maintaining the ion gradients across the cell membrane. Furthermore, energy is needed for the turnover of proteins and mRNA, for repair and for movement (if mobile). 

Currently, five different molecular classes of mdr efflux pumps are known [5], While pumps of the the ATP-binding cassette (ABC) transporter superfamily are driven by ATP hydrolysis, the other four superfamilies called resistance-nodulation-division (RND), major facilitator superfamily (MFS), multidrug and toxic compound extrusion (MATE), and small multidrag resistance transporter (SMR) are driven by the proton-motive force across the cytoplasmic membrane. Usually a single pump protein is located within the cytoplasmic membrane. However, the RND-type pumps which are restricted to Gram-negative bacteria consist of two additional components, a periplasmic membrane fusion protein (MFP) which connects the efflux pump to an outer... 

Complex V catalyzes the synthesis of ATP from ADP and Pj utilizing the energy of the proton motive force across the inner membrane (Senior, 1988,1990). 

Figure 9. Proposed cyclic mechanism for ATP synthesis by complex V involving all three catalytic sites of F,. In this scheme only the a and p subunits of F, are shown these are connected by a short stalk to F, in the inner membrane. Proton translocation through Fq driven by the proton motive force (AP) causes sequential conformational changes in each of the p-subunits and ATP synthesis as described in the text hexagons, high-affinity sites semicircles, low affinity sites parallelepipeds, intermediate-affinity sites (with no movement of F,).

Hafher, R.P., Brown, G.C.. Brand, M.D. (1990). Analysis of the control of respiration rate, phosphorylation rate, proton leak rate and proton motive force in isolated mitochondria using the top-down approach of metabolic control theory. Eur. J. Biochem. 188,313-319. 

Stumpf, D.A., Haas, R., Eguren, L.A., Parks, J.K., Eilert, R.E. (1982). Proton motive force in muscle mitochondria. Muscle and Nerve 5, 14-19. 

Plasmid- or transposon-encoded tetracycline efflux proteins have been described in a number of bacteria. These efflux profeins are fhoughf to span fhe cytoplasmic membrane and are dependenf on the proton-motive force for their action, ft is thought that the efflux proteins bind tetracyclines and initiate proton transfer, although no functional domains have been identified. Eight distinct tetracycline efflux profeins have been idenfified thus far. 

A detailed investigation by Groenewegen et al. (1990) has examined the uptake of 4-chlorobenzoate by a coryneform bacterium that degraded this compound. The uptake was inducible and occurred in cells grown with 4-chlorobenzoate but not with glucose. A proton motive force (Ap)-driven mechanism was almost certainly involved, and uptake could not take place under anaerobic conditions unless an electron acceptor such as nitrate was present. 

Figure 18.2 Summary of respiratory energy flows. Foods ate converted into the reduced form of nicotinamide adenine dinucleotide (NADH), a strong reductant, which is the most reducing of the respiratory electron carriers (donors). Respiration can he based on a variety of terminal oxidants, such as O2, nitrate, or fumarate. Of those, O2 is the strongest, so that aerobic respiration extracts the largest amount of free energy from a given amount of food. In aerobic respiration, NADH is not oxidized directly by O2 rather, the reaction proceeds through intermediate electron carriers, such as the quinone/quinol couple and cytochrome c. The most efficient respiratory pathway is based on oxidation of ferrocytochrome c (Fe ) with O2 catalyzed by cytochrome c oxidase (CcO). Of the 550 mV difference between the standard potentials of c)Tochrome c and O2, CcO converts 450 mV into proton-motive force (see the text for further details).

ATP-proton motive force interconversion Electron transport Entner-Doudoroff Fermentation Glycolysis/gluconeogenesis Pentose phosphate pathway Pyruvate dehydrogenase Sugars TCA cycle Methanogenesis Polysaccharides Other... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

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