Potential transmembrane electrochemical

An exported protein is thus pushed through the membrane by a SecA protein located on the cytoplasmic surface, rather than being pulled through the membrane by a protein on the periplasmic surface. This difference may simply reflect the need for the translocating ATPase to be where the ATP is. The transmembrane electrochemical potential can also provide energy for translocation of the protein, by an as yet unknown mechanism. 

Action on the plasma membrane is the first and most fundamental of the bewildering array of deleterious effects of the cinnamic and benzoic acids. They reduce the transmembrane electrochemical potential with the immediacy and extent of that action depending on the concentration and lipid solubility of the compound.35,37,45,60 Rate of uptake also is concentration and pH-dependent, with transfer into and across the membrane greatest with lower pH conditions and higher external concentrations.60 Phenolic acid-induced depolarization of membranes causes a nonspecific efflux of both anions and cations accompanying the increased cell membrane permeability, and these membrane effects correlate with an inhibition of ion uptake. The phenolic acids suppress absorption of phosphate, potassium, nitrate, and magnesium ions, and overall changes in tissue... 

Fatty acids facilitate the net transfer of protons from intermembrane space into the mitochondrial matrix, hence lowering the proton electrochemical potential gradient and mediating weak uncoupling. Uncoupling proteins generally facilitate the dissipation of the transmembrane electrochemical potentials of H+or Na+produced by the respiratory chain, and result in an increase in the H+and Na+permeability of the coupling membranes. They provide adaptive... 

Fig. 9. The coupling of electron and proton flow in succinate iquinone oxidoreduc-tases in aerobic (a,c) and anaerobic respiration (b,d), respectively. Positive and negative sides of the membrane are as described for Fig. 1. (a) and (b) Electroneutral reactions as catalyzed by C-type SQR enzymes (a) and D-type E. coli QFR (b). (c) Utilization of a transmembrane electrochemical potential Ap as possibly catalyzed by A-type and B-type enzymes, (d) Electroneutral fumarate reduction by B-type QFR enzymes with a proposed compensatory E-pathway. ...

Fig. 4. Energetics and modes of energy coupling of endergonic and exergonic partial reactions involved in methane formation from CO2 and H2. CH2=H4MPT, methylene-HaMPT CH3-S-C0M, methyl-coenzymeM transmembrane electrochemical potential of H, Ap,Na, transmembrane...

Note that this cyclic electron-transfer process produces no net oxidation or reduction. However, in the process, protons acquired from the cytoplasm are translocated across the plasma membrane to establish a transmembrane electrochemical potential gradient. The dissipation of such a proton gradient then provides the necessary energy to drive ATP synthesis. A similar simplified cyclic electron-transport diagram has been shown earlier in Chapter 3 as Fig. 12 (C) on p. 81, in coimection with a discussion of a LHl-RC-Cyt6c, supercomplex of Rb. sphaeroides. More detailed discussion of the cytochromeic] and bff complexes and ATP synthesis will be presented in Chapters 35 and 36, respectively. 

Figure 1.8. Energetics of O2 reduction by mammalian CcO. Subscripts signify the location of charged species in the transmembrane electrochemical gradient the intermembrane space (IMS) the heme/Cu site or the matrix (Figure 1.1). The potentials are approximate and referenced to the normal hydrogen electrode at pH 7.

Cytochrome bd oxidases are aerobic terminal oxidases unrelated to HCOs. They function as quinol oxidases and are widely distributed in Gram-negative bacteria (see footnote on page 4) and possibly some Archaea Like cytochromes ebbs, cytochromes bd are also suggested to be essential for microaer-obiosis and may protect anaerobic processes from O2. Cytochromes bd are not known to operate as proton pumps. The enzymes generate a transmembrane electrochemical potential of 180 mV relying solely on substrate protons, e.g., by... 

Illumination inhibits respiratory electron transport in the photosynthetic bacterium Rhodobacter sphaeroides [1,2]. It is generally accepted that this is caused by an increase in the transmembrane electrochemical potential difference [2], although there is some evidence for a kinetic interaction via those components of the respiratory and photosynthetic electron transport chains which are common to them both [3]. 

The driving force for electron transport is determined by both the redox potential difference and the electrochemical potential gradient [4, 5]. Consequently, the rate of respiratory electron transport, in either the light or the dark, is determined by the imbalance between these factors. If the redox potential drop between the electron donor and the ultimate electron acceptor is greater than the transmembrane electrochemical gradient opposing it, then electron transport may operate in the forward direction. 

A.Baccarini-Melandri, R.Casadio and B.A.Melandri, Thermodynamics and kinetics of photophosphorylation in bacterial chromatophores and their relation with the transmembrane electrochemical potential difference of protons, Eur.J.Biochem. 78 389 (1977). 

Potentiometry is a method in which the electrochemical cell potential is measured at equilibrium at which the current is zero. The properties of the interface region differ from the bulk properties. A potential is established at the phase boundaries, e.g., between the solution and the electrode surface. The potential of electrochemical cells is the sum of all interface potentials including electrode/electrolyte interface and liq-uid/Uquid interface (i.e., the two electrolyte solutions of different compositions that are in contact with each other). Ideally the measured potential should depend only on the potential between the interfaces of interest for analytical purpose. This is typically accomplished by keeping all other interfaces constant through a suitable electrode construction. Potentiometric sensors (e.g., ion selective electrodes) usually consist of a manbrane that contains ion exchangers, lipophilic salts, and plasticizers, and the transmembrane potential gives the activity of the analyte ion in solution. 

In addition to the references included in this section, another paper worthy of study is that of Skulachev (1977), in which the author has proposed that the formation of transmembrane electrochemical potentials along with ATP constitute the energy currency for life processes. This particular view is fortified by studies Skulachev details in which organisms devoid of ATPase were able to carry out energy-... 

Skulachev, V. P., 1977, Transmembrane electrochemical potential as a convertible energy source for the living cell, FEBS Lett. 74 1. 

IS THE TRANSMEMBRANE ELECTROCHEMICAL POTENTIAL A COMPETENT INTERMEDIATE IN MEMBRANE ASSOCIATED ATP SYNTHESIS ... 

PS photosystem CF coupling factor Aa520 light-induced absorbance variation at 520 nm PQ, PQH2 plastoquinone, plastoquinol AyH transmembrane electrochemical potential difference for proton ApH transmembrane pH difference A transmembrane electrical potential difference. 

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