Proton impermeability

Several properties of photosynthetic electron transfer and photophosphoryiation in chloroplasts indicate that a proton gradient plays the same role as in mitochondrial oxidative phosphorylation. (1) The reaction centers, electron carriers, and ATP-forming enzymes are located in a proton-impermeable membrane—the thy-lakoid membrane—which must be intact to support photophosphoryiation. (2) Photophosphoryiation can be uncoupled from electron flow by reagents that promote the passage of protons through the thylakoid membrane. 

The protons are released to one side of an otherwise generally proton-impermeable inner mitochondrial membrane to collect the protons in the space between the inner and outer membranes of the mitochondrion. The resulting proton concentration gradient then drives formation of ATP by the quintessential protein-based machine, ATP synthase, as the protons flow back through the inner mitochondrial membrane by means of another special path effecting proton permeability. Thus there are two fundamental questions. The first is, how does electron flow within the membrane achieve unidirectional proton flow across the membrane The second is, how does the return flow of protons result in the formation of ATP, the energy coin of biology ... 

In this chapter, I shall discuss highly ionic hydroxides that are probably the least likely candidates for proton conduction. However, on closer inspection, they reveal an interesting fundamental behaviour that may be important for understanding proton transport in other systems, for instance across bilayer membranes containing inner sections that are considered to be proton impermeable. ... 

An integrated model of acid resistance of the gastric epithelium, where the surface cell provides neutralization and generally a proton impermeable barrier. The parietal cell extrudes bicarbonate into the serosal side to supply this anion to the surface cell, and acid also appears to stimulate surface cell bicarbonate secretion. 

The second type of CO2 fiber-optic chemical sensor is constructed by using ion pairs consisting of a pH indicator anion and an organic quaternary cation. First, a pH indicator dye (DH) and a quaternary ammonium hydroxide (Q OH ) are entrapped into a proton-impermeable but CO2-permeable polymer membrane, which is then immobilized onto the fiber s surface. The mechanism of this CO2 sensor is based on the interaction between the dye molecules (DH) and the quaternary cations (Q+OH ) to form hydrated ion pairs (Q D XH2O). The hydrated ion pair is dissolved in the polymer, where it reacts with CO2 according to the following reaction ... 

Each of the respiratory chain complexes I, III, and IV (Figures 12-7 and 12-8) acts as a proton pump. The inner membrane is impermeable to ions in general but particularly to protons, which accumulate outside the membrane, creating an electrochemical potential difference across the membrane (A iH )-This consists of a chemical potential (difference in pH) and an electrical potential. 

Because the inner mitochondrial membrane is impermeable to protons and other ions, special exchange transporters span the membrane to allow passage of ions such as OH, Pf, ATP , ADP, and metabo-htes, without discharging the electrochemical gradient across the membrane. 

Recent work has shown that bacteria, in common with chloroplasts and mitochondria, are able, through the membrane-bound electron transport chain aerobically, or the membrane-bound adenosine triphosphate (ATP) anerobically, to maintain a gradient of electrical potential and pH such that the interior of the bacterial cell is negahve and alkaline. This potential gradient and the electrical equivalent of the pH difference (1 pH unit = 58 mV at 37°C) give a potential difference across the membrane of 100-180 mV, with the inside negative. The membrane is impermeable to protons, whose extmsion creates the potential described. 

FIG. 14 A model for the uptake of weakly basic compounds into lipid bilayer membrane (inside acidic) in response to the pH difference. For compounds with appropriate pki values, a neutral outside pH results in a mixture of both the protonated form AH (membrane impermeable) and unprotonated form A (membrane permeable) of the compound. The unprotonated form diffuse across the membrane until the inside and outside concentrations are equal. Inside the membrane an acidic interior results in protonation of the neutral unprotonated form, thereby driving continued uptake of the compound. Depending on the quantity of the outside weak base and the buffering capacity of the inside compartment, essentially complete uptake can usually be accomplished. The ratio between inside and outside concentrations of the weakly basic compound at equilibrum should equal the residual pH gradient. 

The inner membrane itself plays an important part in oxidative phosphorylation. As it is impermeable to protons, the respiratory chain—which pumps protons from the matrix into the intermembrane space via complexes 1, 111, and IV—establishes a proton gradient across the inner membrane, in which the chemical energy released during NADH oxidation is conserved (see p. 126). ATP synthase then uses the energy stored in the gradient to form ATP from ADP and inorganic phosphate. Several of the transport systems are also dependent on the H"" gradient. 

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). 

Currently, the most widely applied electrolyte in PEFCs is Nation, manufactured by DuPont, Dow Chemical, Midland, MI, USA and other chemical companies. The Nation polymer electrolyte is a good proton conductor. Besides, it has very low electron conductivity, and is gas impermeable in order to provide the necessary spatial separation between the anode oxidation and the cathode reduction reactions. 

Tight coupling between ATP synthesis and proton translocation is dependent on the impermeability of the membrane to protons, so that the F0 channel and ATP synthesis provide the only way for protons to reenter the mitochondrial matrix. Physical damage to the membranes, or chemicals that allow the dissipation of the proton or electrical potential gradient, will allow alternative pathways for reentry of protons, and will uncouple respiration from ATP synthesis (see Problem 14.5). 

Although there are many advantages to the use of whole cells for biotransformations, there are certain limitations that must be considered. One consideration is the transport of substrates and products across the cell membrane. In life, the cell membrane is a proton-tight barrier to the rest of the world. It is generally impermeable to charged molecules and to water, but may have permeability to hydrophobic molecules. Often cells have... 

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