Electrochemical lipid bilayers

Mechanosensitive ion channels can be looked at as membrane-embedded mechano-electrical switches. They play a critical role in transducing physical stresses at the cell membrane (e.g. lipid bilayer deformations) into an electrochemical response. Two types of stretch-activated channels have been reported the mechanosensitive channels of large conductance (MscL) and mechanosensitive channels of small conductance (MscS). 

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

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. 

Fig. 14.38. Idealized drawing of a lipid bilayer membrane showing three possible modes of binding for proteins. (Reprinted from H. F. Rusling, Electrochemical Enzyme Catalysis, Interface, 6(4) 26-31, Fig. 1, 1997. Reproduced with permission from the Electrochemical Society, Inc.)...

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

The storage and reactivity of electroactive molecules in polymerized diacetylene vesicles was the subject of studies reported by Stanish, Singh, and coworkers [109, 110], They entrapped ferricyanide in large unilamellar vesicles of photopolymerized PCg PC (1 - palmitoyl - 2 - (tricosa - 10,12-diynoyl)-OT-glycero-3-phosphocholine). Cyclic voltammetry was used to demonstrate that the ferricyanide was electrochem-ically isolated by the poly(lipid) bilayer [110]. At pH7 and 25°C, an anomalously long half-life of 2.4 weeks was calculated for Fe (CN)g- retention in polymerized vesicles. In a subsequent study [109], vesicles with entrapped ferricyanide were prepared from 2-bis(10,12-tricosadiynoyl)-OT-glycero-3-phosphatidylcholine (DCs.gPC) doped with a disulfide-capped lipid (Af-3-(pyridyl-2-dithio)propionyl-2-... 

Membranes containing the visual pigment rhodopsin, a G-protein-linked receptor, were chosen as a model system for this work. Rhodopsin was one of the first integral membrane proteins whose amino acid sequence was determined (16-18). More than 40 receptors have been reported to have structural and functional homologies with rhodopsin (19). This chapter describes the use of electrochemical impedance spectroscopy to evaluate lipid bilayer membranes containing rhodopsin formed on electrode surfaces. 

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