Proton transport/transfer

The reduction of oxygen in Complex IV is accompanied by transport of protons across the inner mitochondrial membrane. Transfer of four electrons through this complex drives the transport of approximately four protons. The mechanism of proton transport is unknown but is thought to involve the steps from state P to state O (Figure 21.20). Four protons are taken up on the matrix side for every two protons transported to the cytoplasm (see Figure 21.17). 

FIGURE 21.21 A model for the electron transport pathway in the mitochondrial inner membrane. UQ/UQH9 and cytochrome e are mobile electron carriers and function by transferring electrons between the complexes. The proton transport driven by Complexes I, III, and IV is indicated. 

Besides these generalities, little is known about proton transfer towards an electrode surface. Based on classical molecular dynamics, it has been suggested that the ratedetermining step is the orientation of the HsO with one proton towards the surface [Pecina and Schmickler, 1998] this would be in line with proton transport in bulk water, where the proton transfer itself occurs without a barrier, once the participating molecules have a suitable orientation. This is also supported by a recent quantum chemical study of hydrogen evolution on a Pt(lll) surface [Skulason et al., 2007], in which the barrier for proton transfer to the surface was found to be lower than 0.15 eV. This extensive study used a highly idealized model for the solution—a bilayer of water with a few protons added—and it is not clear how this simplification affects the result. However, a fully quantum chemical model must necessarily limit the number of particles, and this study is probably among the best that one can do at present. 

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... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Unfortunately, cell performance is not proportional to catalyst layer porosity. In order to achieve maximum fuel cell performance, the CL should have an optimal porosity [24]. With higher catalyst layer porosity, the mass transfer rate increases, while the electron and proton transport rates decrease. Gamburzev and Appleby [25] documented fuel cell performance with pore formers in the CL and found that optimum pore-former content was about 33%. 

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

Cytochrome h(6)f PDB 1VF5 Electron transfer Proton transport... 

Figure 7.26 Electron transfer and proton transport in cytochrome b(6)f cofactors (PDB 1VF5). Visualized nsing The PyMOL Molecnlar Graphics System and ChemDraw Ultra, version 10.0. (Printed with permission of Delano Scientific, LLC and CambridgeSoft Corporation.) (See color plate)...

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 properties are as follows, (i) The activity of the protein (i.e. the inward transport of protons) is inhibited by ATP. (ii) The activity of the protein is increased by the presence of long-chain fatty acids, since they relieve the ATP inhibition, (iii) When mitochondria, isolated from brown adipose tissue, are incubated in the presence of fatty acids, there is a sharp increase in the rates of electron transfer, substrate utilisation and oxygen consumption, whereas the rate of ATP generation remains low. These studies indicate that the rate of proton transport, by the uncoupling protein, depends on the balance between the concentrations of ATP and fatty acids, (iv) In adipocytes isolated from brown adipose tissue, the rate of oxygen consumption (i.e. electron transfer) is increased in the presence of catecholamines. 

It seems reasonable to note that the micro-jet stream generated by the ultrasonic cavitation promotes mass transport. Such an effect was discussed for proton transport in aqueous solutions (Atobe et al. 1999). Understandably, a proton moves in the solution as a hydrated particle. Nevertheless, we should pay attention on the similarity between proton and electron, in the sense that both are essentially quantum particles. A solvated electron, therefore, can be considered as a species that is similar to a hydrated proton. Hence, the micro-jet stream can promote electron transfer. 

Our failed — at least insofar as precise transition state characterization is concerned — attempt was nonetheless instructive i) contrary to what found in ref 30, the surface version of reaction (1) does show a barrier, although small, thus Supporting the view that both desolvation of the Cl ion and weak hydrogen bonding to the nitrate group contribute to the barrier ii) reaction (1) appears to be faster than (1), if it is verified that the proton has transferred away from the adsorption site. Concerning the latter point, as noted in Sec.II., there is experimental support for the view that such a transport does not occur. On the other hand, there is other experimental support for the view that it does, so that it seems fair to say that the situation remains ambiguous from an experimental viewpoint. 

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. 

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

Anomalous response to the NADH oxidase in animal cells, such as amiloride-insensitive proton transport, may be based on activation of the H+-ATPase or direct electron transport-linked proton transfer. Further definition of the components of the NADH oxidase and the characteristics of electron transport are needed. In addition, the presence of a poorly characterized glutathione oxidase in the plasma membrane opens an alternative for oxidation-reduction control of proton transport. At this stage no evidence has been found for control of HCOj/Cl" exchange or organic acid transport by the plasma membrane oxidase. 

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