Membrane lateral structure

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

The primary function of the TMDs is to provide a pathway through which allocrites can cross the membrane. It is not difficult to imagine how such a pathway could readily be adopted for channel function (e.g., CFTR), but for receptor-type ABC proteins (such as SURl), the evolutionary step is less obvious. However, for all these functions, the TMDs appear to be the main determinants of specificity [3, 39, 40[. TMDs are much more variable in their amino acid sequences than the NBDs [15,41[. Hydropathy predictions, which have so far been borne out by later structural studies, imply multiple transmembrane a-helices, typically six per domain (see Section 1.2), but with many exceptions [3[. Eukaryotic ABC proteins in the C class (ABCC subdivision) frequently contain a whole extra TMD with five predicted transmembrane spans [7[. 

Many electron spin resonance (ESR) studies of different systems have shown that phase separation in lipid layers may lead to a domain-like lateral structure. The area of domain formation can be extended over several hundred A. In this connection the charge-induced domain formation in biomembranes is of special interest for the medicinal chemist. Especially the addition of Ca to negatively charged lipids leads to domain formation. Each lipid component is expected to have a characteristic spontaneous curvature. The Ca - induced domains lead to protrusions in the membrane plane. The lateral variation in the concentration in the plane of the membrane would then lead to a parallel variation in... 

Structure below 873 K, exhibited quite similar behavior in terms of H2 permeability to that of the Pd-40 wt% Cu membrane. Later, Pd alloy—wall catalyzed WGS reactions were conducted in a Pd-20 wt% Cu membrane because of its excellent resistance to H2S at the entire temperature region (lyoha et al., 2007). For H2S-ffee case, the CO conversion in the Pd-20 wt% Cu membrane reactor was much lower than that in the pure Pd membrane reactor due to the lower H2 permeabihty of the alloy membrane. 

The layer structure is an oversimplified model, since the membranes also have a lateral structure. However the real membrane density can be separated into a pure layer structure and a lateral density Ap f ... 

Given a j rticular curvilinear system x, y, z we can deflne the subset of all membranes (of an infinite set), whose wavy surface is parallel to the sheets z =const. It seems reasonable to assume that the lateral structure does not depend on the particular surface form of the membranes. We therefore conclude that the layer structure... 

Lipid rafts are lateral structural components of the plasma membrane enriched with cholesterol (Choi) and sphingomyelin (SM), onto which specific proteins attach within the bilayers. Lipid mixtures consisting of Choi, phosphatidylcholine (PC), and SM have often been used to mimic the rafts in cells. It is well established that this mixture gives rise to coexisting liquid-ordered (Lo) domains (SM/Chol-enriched) and liquid-disordered (Ld) phase (PC-enriched) in the bilayers. " " ... 

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

FIGURE 9.6 The fluid mosaic model of membrane structure proposed by S. J. Singer and G. L. Nicolsou. In this model, the lipids and proteins are assumed to be mobile, so that they can move rapidly and laterally in the plane of the membrane. Transverse motion may also occur, but it is much slower. 

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

Biological membranes are asymmetric structures. There are several kinds of asymmetry to consider. Both the lipids and the proteins of membranes exhibit lateral and transverse asymmetries. Lateral asymmetry arises when lipids or proteins of particular types cluster in the plane of the membrane. 

Protein lateral motion is much slower than that of lipids because proteins are larger than lipids. Also, some membrane proteins can diffuse freely through the membrane, whereas others are bound or anchored to other protein structures in the membrane. The diffusion constant for the membrane protein fibronectin is approximately 0.7 X 10 cmVsec, whereas that for rhodopsin is about 3 X 10 cmVsec. 

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