Proton flux

Berry, S., and Rnmberg, B., 1996. H" /ATP coupling ratio at the unmodulated CFiCFq-ATP synthase determined by proton flux measurements. 

The Langmuir-Blodged (LB) technique allows one to form a monolayer at the water surface and to transfer it to the surface of supports. Formation of the BR monolayer at the air/water interface, however, is not a trivial task, for it exists in the form of membrane fragments. These fragments are rather hydrophilic and can easily penetrate the subphase volume. In order to decrease the solubility, the subphase usually contains a concentrated salt solution. The efficiency of the film deposition by this approach (Sukhorukov et al. 1992) was already shown. Nevertheless, it does not allow one to orient the membrane fragments. Because the hydrophilic properties of the membrane sides are practically the same, fragments are randomly oriented in opposite ways at the air/water interface. Such a film cannot be useful for this work, because the proton pumping in the transferred film will be automatically compensated i.e., the net proton flux from one side of the film to the other side is balanced by a statistically equal flux in the opposite direction. 

What drives the inter-conversion of the three states is the rotation of the y subunit. As the proton flux causes rotation of the y subunit, say by 120°, the three-P subunits will change... 

Shimizu, S., Eguchi, Y., Kanuike, W., Funahashi, Y., Mignon, A., Lacronique, V., Matsuda, H., and Tsujimoto, Y., 1998, Bcl-2 prevents apoptotic mitochondrial dysfunction by regulating proton flux, Proc.Natl.Acad.Sci. U.S.A. 95 1455-1459. 

The EOD coefficient, is the ratio of the water flux through the membrane to the proton flux in the absence of a water concentration gradient. As r/d,3g increases with increasing current density during PEMFC operation, the level of dehydration increases at the anode and normally exceeds the ability of the PEM to use back diffusion to the anode to achieve balanced water content in the membrane. In addition, accumulation of water at the cathode leads to flooding and concomitant mass transport losses in the PEMFC due to the reduced diffusion rate of O2 reaching the cathode. 

Proton current is determined by Ohm s law and by the continuity of proton flux. 

In addition to the differences in phospholipid content between microbial and host cell membranes, it has been demonstrated that disparity exists between the transmembrane potentials of both organisms. The transmembrane potential is defined by the proton flux between the inner and outer bilayers of the cytoplasmic membrane and ranges from —90 to —110 mV in normal mammalian cells in contrast to transmembrane potentials of —130 to —150mV for logarithmic phase microbes. The differences in these electrochemical gradients have been postulated to drive the influx of peptides into the cell and thus act as a crucial barrier for defining host defense peptide selectivity. ... 

Weber and Newman do the averaging by using a capillary framework. They assume that the two transport modes (diffusive for a vapor-equilibrated membrane and hydraulic for a liquid-equilibrated one) are assumed to occur in parallel and are switched between in a continuous fashion using the fraction of channels that are expanded by the liquid water. Their model is macroscopic but takes into account microscopic effects such as the channel-size distribution and the surface energy of the pores. Furthermore, they showed excellent agreement with experimental data from various sources and different operating conditions for values of the net water flux per proton flux through the membrane. 

Figure 18. Pseudo-2-D simulation results at 0.4 A/cm where the feed gases are dry and countercurrent, (a) Water partial pressure profiles at four positions in the fuel-cell sandwich as a function of distance along the channel the positions are at the anode and cathode gas channels (I and IV) and catalyst layers (II and III), respectively. Also plotted is the value of fS, the net flux of water per proton flux, as a function of position. The data are from Janssen. (Reproduced with permission from ref 55. Copyright 2001 The Electrochemical Society, Inc.) (b) Membrane water content as a function of position both along the gas channel and through the thickness of the membrane for the same simulation conditions as above. The data are from Weber and Newman. (Reproduced with permission from ref 55 and 134. Copyright 2004 The Electrochemical Society, Inc.)...

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

How is a concentration gradient of protons transformed into ATP We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 lcJ) per mole of electron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13-1). Mitochondrial oxidative phosphorylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation ... 

The performance of calixarenes as cation carriers through H20-organic solvent H20 liquid membranes has also been studied.137 In basic metal hydroxide solutions, the monodeprotonated phenolate anions complex and transport the cations, while [18]crown-6 does not, under the same conditions. Low water solubility, neutral complex formation and potential coupling of cation transport to reverse proton flux have been cited as desirable transport features inherent in these molecules.137... 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

To determine the basis for this regular variation in rate of proton leakage, several characteristics of the mitochondria were measured, including inner membrane surface area per unit of matrix volume and fatty acid composition of mitochondrial membrane phospholipids. The largest share (about 70%) of the variation in proton flux rate appears to be due to dififer-... 

The allometric variation in respiration rate, mitochondrial area, rates of uncoupled proton flux across the inner mitochondrial membrane, and plasma membrane sodium pump activity found in mammals appears consistent with selection for higher mass-specific rates of heat generation in small mammals. However, the... 

The term d [Hreference] /dt is computed from the proton fluxes of the reference reactions ... 

Figure 18.2. Essence of Oxidative Phosphorylation. Oxidation and ATP synthesis are coupled by transmembrane proton fluxes.

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