Electrochemical transmembrane potential gradient

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. ... 

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... 

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. 

The free energy gained from the quinol oxidation inthe cytochrome-6c, complex allows further proton transfer from the cytoplasm to the periplasm. The 6c,-complex also mediates ET to the periplasmic side. There, soluble cytochromes accept the electrons and transport them back to the RC to reduce D+. The electron transfer is cycUc and therefore does not cause transmembrane potential. This potential is generated by the electrogenic proton translocation in the cytochrome-6c, complex. The electrochemical proton gradient is utilized by the ATP-synthase to form adenosine triphosphate from adenosine diphosphate and phosphate. 

In phase 2 of cellular respiration, the energy derived from fuel oxidation is converted to the high-energy phosphate bonds of ATP by the process of oxidative phosphorylation (see Fig. 2). Electrons are transferred from NADH and FAD(2H) to O2 by the electron transport chain, a series of electron transfer proteins that are located in the inner mitochondrial membrane. Oxidation of NADH and FAD(2H) by O2 generates an electrochemical potential across the inner mitochondrial membrane in the form of a transmembrane proton gradient (Ap). This electrochemical potential drives the synthesis of ATP form ADP and Pi by a transmembrane enzyme called ATP synthase (or FoFjATPase). 

Fig. 15.1. Electrophysiology of excitatory and inhibitory neurotransmitters. The tendency of an ion to move across the membrane depends on the difference in its electrochemical gradient on either side of the membrane. The electrochemical gradient depends on the difference in the concentration of the ion between the two sides of the membrane, the charge of the ion, and the transmembrane potential (the difference in voltage between the two sides of the membrane).

FIGURE 22.3 Circuit representation of membrane current. The conductance can be nonlinear as indicated by the powers p and q on the state variables m and hi, respectively. These state variables are typically time-varying functions of the transmembrane potential difference, V. The battery, , represents the electrochemical gradient of the ionic species responsible for the current. 

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. 

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