Membranes, natural structure

Grobner G, Taylor A, Williamson PT, Choi G, Glaubitz C, Watts JA, de Grip WJ, Watts A (1997) Macroscopic orientation of natural and model membranes for structural studies. Anal Biochem 254 132-138... 

Sretcher, M.S., 1972a, Asymmetrical lipid bilayer structure for biological membranes. Nature New Biol, 236 11-12. 

The circulatory system of fish is also unique structurally and functionally. Structurally, the membranous nature of the vasculature makes for a friable high-capacitance system under low pressure. Low blood flows result in somewhat longer distributional phases for many drugs. Processes such as heart rate and stroke volume that influence drug distribution are themselves influenced by external factors such as temperature and stress. In addition, total plasma protein content differs in fish as compared to mammals. Total plasma protein in the trout and flounder is approximately one-half that of mammals such as dogs and cats. For many compounds protein binding is considerably lower in fish than their mammalian counterparts (19, 20). 

The physical and functional properties of natural and artificial membranes which have been discussed in brief in this section have been the subject of extensive investigations. Several books and reviews have been published on these topics [8, 126], The described features of membranes contribute to the physicochemical properties that make biological membranes highly structured fluids, in both space and time. They confer on the membranes particular structural, dynamic, and functional properties. 

In contrast to their limited importance in nature, the /1-barrel proteins are most prominent in the list of established membrane protein structures. Moreover, they show a high degree of internal chain-fold symmetry and therefore convey the impression of beautiful proteins (Fig. 1). One should not forget that the first protein structure, myoglobin, caused some disappointment among those who solved it as it showed no symmetry whatsoever even the a-helices were not whole-numbered but about 3.6 residues per turn. Accordingly, the symmetric transmembrane /1-barrels stand out from the bulk of asymmetric chain folds of water-soluble proteins. 

At the start of the twenty-first century, the pace of membrane protein structure determinations is clearly accelerating (Figure 1). With the exceptions of rhodopsin (Palczewski et al., 2000) and the calcium ATPase (Toyoshima et al., 2000), however, eukaryotic channels, transporters, and receptors are conspicuously absent from the list of known membrane protein structures. These two exceptions, as proteins of naturally high abundance, highlight the current reality that no structure has been determined for an overexpressed eukaryotic membrane protein. This situation reflects the present difficulties in the reliable overexpression of membrane proteins, particularly those of eukaryotic organisms. Just as the development 20 years ago of overexpression systems for water-soluble proteins revolutionized the structure determinations of this class of proteins, advances in membrane protein expression will be essential to successful realization of the goal of routine structural analysis of membrane proteins. 

Ostermeier, C., Iwata, S., Lubwig, B., and Michel, H., 1995, Fv fragment-mediated crystallization of the membrane protein bacterial cytochrome c oxidase, Nature Structural Biology, 2 842n846. 

Liposomes have aroused interest in a great variety of areas from biochemistry and molecular biology to cosmetics and food technology. One of the most salient applications of liposomes has been promoted by their high similarity to natural cell membranes, for which they are extensively used as substitutes in medical and pharmaceutical research. Since their inception, liposomes have often been used as models for studying the nature of cell membranes, the structure and functions of which they can mimic quite closely. One example is the determination of membrane distribution coefficients of drugs with a view to estimate their ability to penetrate cells. Interactions between analytes and phospholipid membranes depend on the characteristics of both the analytes and the membrane. 

Polymerized lipids do not occur in natural cell membranes. Nature tends to support fragile membrane structures with polymeric skeletons, i.e. protein cytoskeletons, polysaccharide cell walls etc. Analogous synthetic polymeric nets are simply constructed from polymerizable counterions. Negatively charged dihexadecyl phosphate vesicles can be neutralized with choline methacrylate polymerization of the latter produces a polycationic vesicle coat which is not inserted into the membrane (Figure 4.30). A cytoskeleton at the... 

Zhou, F. X., Cocco, M. J., Russ, W. P., et al. (2000) Interhelical hydrogen bonding drives strong interactions in membrane proteins. Nature Structural Biology, 7(2), 154—160. 

HJM Kramer, R van Grondelle, CN Hunter, WHJ Westerhuis and J Amesz (1984) Pigment organization of the B800-850 antenna complex of Rps. sphaeroides. Biochim Biophys Acta 765 156-165 TG Monger and WW Parson (1977) Singlet-triplet fusion in Rhodopsudomonas sphaeroides chromatophores. A probe of the organization of the photosynthetic apparatus. Biochim Biophys Acta 460 393-407 KR Miller (1979) Structure of a bacterial photosynthetic membrane. Proc Nat Acad Sci, USA 76 6415-6419. KR Miller (1982) Three-dimensional structure of a photosynthetic membrane. Nature 300 53-55... 

These observations are consistent with the mosaic model of the membrane that was derived from monolayer studies (2, 4, 5, 12, 13). Therein, the structural or bimodal (amphipathic) protein in the membrane (natural or artificial) interacts with the polar peripheries of the polymeric lipid structures alongside the protein. The EPR data of Jost et al. (29) support this concept, i.e., an appreciable portion of the lipid is in lateral hydrophilic bonding with the protein whereas the other lipid is free, probably within the lipid cluster, and preserves the lipid character. 

Membranes naturally tend to form closed structures, to avoid exposing the hydrophobic ends of lipid bilayers to the solvent. Synthetic closed structures, called liposomes, can be made with membrane fragments or synthetic phospholipids. Liposomes can be made to contain compounds buried in the membrane or totally enclosed and are a... 

Singer, S. Jonathan, and Nicolson, Garth L. (1972). The Fluid Mosaic Model of the Structure of Cell Membranes. Nature 175 720-731. 

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