Membrane bilayer lipid translocation across

Biosensors based on metal-supported bilayer lipid membranes. BLMs, especially s-BLMs, have been used in the last three years as lipid bilayer-based biosensors [10,11,74-76,82], Hianik etal [75] have carried out a detailed physical study on the elasticity modulus of s-BLMs. They found that the dynamic viscosity of s-BLMs is one order of magnitude less than that of conventional BLMs [75]. It should be mentioned that in the s-BLM system, albeit attractive for certain purposes such as biosensors and molecular devices, the metallic substrate precludes ion translocation across the lipid bilayer. Therefore, the pursuit of a simple method for obtaining long-lived, planar BLMs separating two aqueous media has been an elusive one until now [81]. As reported, this much improved... 

Non-bilayer-forming lipids are also required for protein translocation across the membrane of E. coli. The only non-bilayer-forming lipid in E. coli mutants lacking PE is CL. Protein translocation into inverted membrane vesicles prepared from PE-lacking cells (now enriched in CL) is reduced with divalent cation-depletion but can be enhanced by inclusion of Mg or Ca [ 1 ]. Protein translocation in the absence of divalent cations was restored by incorporation of non-bilayer PE (18 1 acyl chains) but not by bilayer-prone PE (14 0 acyl chains). These results indicate that lipids with a tendency to form non-bilayer stmctures provide a necessary environment for translocation of proteins across the membrane. 

Additional work with closed vesicles derived from B. megaterium membranes demonstrates that NBD analogs of PE, PG, and PC can translocate across the membrane with a /i/2 of 30 s at 37°C (S. Hraffnsdottir, 1997). Similar types of experiments conducted with closed vesicles isolated from E. coli inner membrane reveal that NBD phospholipids traverse the bilayer with a of 7 min at 37°C (R. Huijbregts, 1996). This latter process is insensitive to protease and A-ethylmaleimide treatments and does not require ATP. Collectively, the data indicate that transbilayer lipid movement is rapid and does not require metabolic energy in bacterial membranes that harbor the biosynthetic enzymes for phospholipids. The basic characteristics of lipid translocation in the intact cell appear to be retained in isolated membranes. 

Charge generation, separation, and translocation are the main concern of electrochemistry. To relate these phenomena to membranes, the principal focus of research deals with the mechanisms of reactions through the membrane proper and at, as well as across, the solution-membrane interface. The electrical double layer at the interface plays a crucial role. The reconstituted planar bilayer lipid membrane (BLM) that separates two aqueous solutions was first reported in 1961. The BLM has proven to be an excellent model for biomembranes it permits charge separation and translo-... 

The previous sections on single-channel conductance in Upid bilayers show that pores of molecular dimensions and possessing selectivity can be demonstrated, but can single-channel conductance be demonstrated in biological membranes, or rather, are ions translocated across biological membranes in ways similar to those in lipid bilayers treated with channel-forming substances ... 

Cell-penetrating peptides (CPPs) are a class of short, often cationic peptides that have the capability to translocate across cellular membranes, and although the translocation most likely involves several pathways, they interact directly with membranes, as well as with model bilayers. A review focuses on solution NMR as a tool for investigating CPP-lipid interactions. Structural propensities and cell-penetrating capabilities can be derived from a combination of CPP solution structures and studies of the effect that the peptides have on bilayers and the localization in a bilayer. 

Figure 7. Mechanism of the proton-translocating ubiquinol cytochrome c reductase (complex III) Q cycle. There is a potential difference of up to 150 mV across the hydrophobic core of this complex (potential barrier represented by the vertical broken line). Cytochromes hour and b N are heme groups on the same peptide subunits of complex III which can transfer electrons across the hydrophobic core. The movement of two electrons provides the driving force to transfer two protons from the matrix to the cytosol. Diffusion of UQ and UQHj, which are uncharged, in the hydrophobic core, and lipid bilayer of the inner membrane is not influenced by the membrane potential (see Nicholls and Ferguson, 1992).

Models of lipid bilayers have been employed widely to investigate diffusion properties across membranes through assisted and non-assisted mechanisms. Simple monovalent ions, e.g., Na+, K+, and Cl, have been shown to play a crucial role in intercellular communication. In order to enter the cell, the ion must preliminarily permeate the membrane that acts as an impervious wall towards the cytoplasm. Passive transport of Na+ and Cl ions across membranes has been investigated using a model lipid bilayer that undergoes severe deformations upon translocation of the ions across the aqueous interface [126]. This process is accompanied by thinning defects in the membrane and the formation of water fingers that ensure appropriate hydration of the ion as it permeates the hydrophobic environment. 

Metalloprotein protein that binds a specific metal ion and requires that metal ion for proper function Metal transporter transmembrane protein responsible for the translocation of metal ions across a lipid bilayer MTMl mitochondrial inner membrane transporter needed for activating SOD2 with manganese SCO Copper carrying molecule, possibly the copper chaperone or copper insertion factor for cytochrome oxidase SMF2 intracellular metal transporter essential for manganese trafficking... 

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