Proton transport processes

Recent results by Zawodzinski et al. [97] show that several PFSA membranes exhibit similar electroosmotic behavior, i.e., a drag coefficient of close to 1.0 H2O/H+ over a wide range of water contents for a membrane equilibrated with vapor-phase water. The lack of dependence of the drag coefficient on membrane nanostructure suggests that the drag coefficient is determined by basic elements of the proton transport process which are similar for all membranes, such as proton solvation and local water structure. 

Figure 2.4 illustrates electron and proton transport processes. Electrons are initially energized by sunlight hitting photosystem II (PSII see figure legend) and transported to photosystem I (PSI). In PSI, sunhght energy is again imparted and the electrons are transferred by ferredoxin, another electron carrier, to NADPH. Electron transport from PSII to PSI is via plastoquinone (PQ), cytochrome b6/cytochrome f complex, and plastocyanin (blue arrows in Fig. 2.4). During electron transport, protons are taken up by plastoquinone (similar to...

Strong evidence for this assumption has been provided by Sachs and coworkers [58,66,78,87]. They found that the rate of proton uptake depends on the nature of the cation present and that the sequence of the stimulating effect of these cations is the same as for the ATPase reaction. The only exception is T1, which strongly stimulates the ATPase but inhibits proton transport [71]. The substrate specificity for the proton transport is also the same as for the (K +H" )-ATPase activity. Most inhibitors of the enzyme reaction, described in Section 3g, also inhibit the proton transport process. 

The main principles of membrane phosphorylation are the same in chloroplasts, mitochondria, and photosynthetic bacteria. In this section, in order to analyze the role of protonmotive force in the processes of energy transduction in biomembranes, we will focus our attention on the consideration of proton-transport processes in chloroplasts. In thylakoids the ApH is the main component of transmembrane difference in electrochemical potentials of hydrogen ions, AjuH+ = Acp — 2.3(RT/F)ApH. The conductivities of the thylakoid membrane for the majority of cations (Mg ", Na ), existing... 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

Let us systematically delineate the transport pathways of the nondissociated and protonated species of the P-blockers by applying Eq. (82). The insignificance of the mass transfer resistance of the ABL on the overall transport process, as evidenced by the lack of influence of stirring on Pe, indicates that the passive diffusional kinetics are essentially controlled by the cell monolayer and filter. Therefore, Eq. (82) simplifies to... 

For weak acids, e.g., salicylic acid, the dependency on a pH gradient becomes complex since both the passive diffusion and the active transport process will be dependent on the proton concentration in the apical solution [61, 63, 98, 105] and a lowering of the pH from 7.4 to 6.5 will increase the apical to basolateral transport more than 20-fold. Similarly, for weak bases such as alfentanil or cimetidine, a lowering of the pH to 6.5 will decrease the passive transport towards the basolateral side [105]. The transport of the ionizable compound will, due to the pH partition hypothesis, follow the pKa curve. 

Pethig, R. (1985). Ion, electron, and proton transport in membranes a review of the physical processes involved. In Modern Bioelectrochemistry, eds. Gutmann, F. and Keyzer, H., Plenum, New York, pp. 199-239. 

Additionally, amino acids may be reclaimed as dipeptides. The transport mechanisms for dipeptides are less specific than those for individual amino acids but require the dipeptide to carry a net positive charge so there is cotransport of protons, rather than of Na+ as for free amino acids. A potential advantage of dipeptide transport process is the favourable cell-lumen concentration gradient, which exists for peptides compared with free amino acids. 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

The important processes occurring in a catalyst layer include interfacial ORR at the electrochemically active sites, proton transport in the electrolyte phase, electron conduction in the electronic phase (i.e., Pt/C), and oxygen diffusion through the gas phase, liquid water, and electrolyte phase. 

Figure 9.13 Examples of mitochondrial transport systems for anions. 0 The anb port system transfers malate into but oxo-glutarate out of the mitochondrion. The symport system transfers both pyruvate and protons into the mitochondrion across the inner membrane. Both transport processes are electroneutral.

One view to explain different P/O ratios for different classes of organisms is to consider variability in both the molecular mechanism as well as the stoichiometry of proton transport and ATP synthesis with the source of the enzyme [67]. However, considering our molecular mechanism and the energetics of the oxidative phosphorylation process, we believe that a universality in the mechanistic, kinetic and thermodynamic characteristics of the system is operative. 

Fig. 2.7 Schematic representation of electrochemical processes (reduction) for organic solids able to experience coupled proton transport/electron transport processes...

FIGURE 11-40 Reversibility of F-type ATPases. An ATP-driven proton transporter also can catalyze ATP synthesis (red arrows) as protons flow down their electrochemical gradient. This is the central reaction in the processes of oxidative phosphorylation and photophosphorylation, both described in detail in Chapter 19. 

Although the primary role of the proton gradient in mitochondria is to furnish energy for the synthesis of ATP, the proton-motive force also drives several transport processes essential to oxidative phosphorylation. The inner mitochondrial membrane is generally impermeable to charged species, but two specific systems transport ADP and Pj into the matrix and ATP out to the cytosol (Fig. 19-26). 

A second membrane transport system essential to oxidative phosphorylation is the phosphate translocase, which promotes symport of one H2PO4 and one H+ into the matrix. This transport process, too, is favored by the transmembrane proton gradient (Fig. 19-26). Notice that the process requires movement of one proton from the P to the N side of the inner membrane, consuming some of the energy of electron transfer. A complex of the ATP synthase and both translocases, the ATP synthasome, can be isolated from... 

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

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