Transfer across membranes mechanisms

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

Such a mechanism appears to be adequate enough for the description of the carrier-mediated electron transfer across membranes. It allows, for example, one to describe quantitatively the influence of electrical polarization on the transmembrane electron transfer. 

Membrane transport of toxic heavy metals not only controls their access to intracellular target sites but also helps to determine their uptake, distribution, and excretion from the body. The critical role of membranes in the toxicology of metals has attracted the attention of many investigators, and extensive information has been collected on the mechanisms of metal transfer across membranes. Characteristics of metal transport in different cells (see also Part II, Chapter 4), or even on opposite sides of the same cell, or under different physiological conditions, are not identical, and no unitary hypothesis has been formulated until now to explain this process in all cells (Foulkes 2000). 

Two mechanisms of transmembrane electron transfer were elucidated (i) via the translocation of viologen radical across membrane and (ii) via... 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

One paradigm for membrane transport of iron is the binding of the receptor protein to an iron-free siderophore molecule, followed by exchange of iron from an external ferri-siderophore to the receptor bound iron-free siderophore, and subsequent transfer across the cellular membrane. This shuttle mechanism has been explored in the transport system of ferric pyoverdine in P. aeruginosa (215,216). It is unclear why the bacterial system behaves in this manner, but mutagenesis studies of the protein suggest that residues involved in the closure of the P-barrel will not interact in the same way with the iron-free siderophore as they do with the ferri-siderophore. A similar mechanism has been suggested for A hydrophila and E. coli (182). 

The precise mechanism(s) by which oligos enter cells is not fully understood. Most are charged molecules, sometimes displaying a molecular mass of up to 10-12 kDa. Receptor-mediated endocytosis appears to be the most common mechanism by which charged oligos, such as phosphorothioates, enter most cells. One putative phosphorothioate receptor appears to consist of an 80 kDa surface protein, associated with a smaller 34 kDa membrane protein. However, this in itself seems to be an inefficient process, with only a small proportion of the administered drug eventually being transferred across the plasma membrane. 

The mechanism of transport of GPG using SLM has been studied at the authors laboratory [56]. GPG could be permeated from alkaUne feed of carbonate buffer into an acidic stripping solution of acetate buffer across the membrane comprising Aliquat-336 in -butyl acetate immobiUzed in a polypropylene (Gelgard 2400) support. The transport mechanism is a case of counter transport exhibiting overall rate dependence on solute diffusion in the membrane phase as well as the mass transfer across the aqueous boundary films. 

The proposed mechanism of electron transfer across Chl-containing membranes of vesicles in A // Chi // D (i.e. for systems containing Chi as a photosensitizer in the membrane and donor, D, and acceptor, A, particles outside and inside the vesicle, respectively) and D // Chi //A systems was outlined in early papers [42,43,... 

Experimentally, electron transfer across vesicle membranes with an asymmetrically embedded photosensitizers was first observed in System 17 of Table 1. Katagi et al. [64, 65] succeeded in embedding a photosensitizer (ZnC18TMPyP3+) into the bilayer membrane both uniformly and selectively in its outer monolayer, i.e. asymmetrically. In the latter case no electron transfer across the membrane took place until the other photosensitizer (ZnTPP) was introduced into the membrane uniformly. The proposed mechanism of electron transfer involved two photochemical steps ... 

As it has been already indicated in Sect. 2, electron transport through the hydrocarbon core of the bilayer (see reactions (8), (14), (26)) is a key step of any transmembrane PET, and usually it controls the rate and efficiency of the PET process as a whole. Therefore, the data concerning the mechanism of this stage of PET and the factors which affect it are of crucial importance for the development of photochemical systems based on PET across the membranes. Unfortunately, for the majority of the systems listed in Table 1 the proposed mechanisms of electron transfer across the membrane seem to be rather tentative because of insufficient information about the localization of the redox-active components and their diffusion mobility inside the membranes. Only for few systems the studies were detailed enough to propose convincing mechanisms and to give a quantitative description of the kinetics of electron transfer across the membrane. 

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