Membranes structural complexity

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

Deisenhofer, J., et al. X-ray structure analysis of a membrane protein complex. Electron density map at 3 A resolution and a model of the chromophores of the photosynthetic reaction center from Rhodopseudomonas viridis. f. Mol. Biol. 180 385-398, 1984. 

Despite its weakness, the anisotropy of the g tensor of iron-sulfur centers can be used to determine the orientation of these centers or that of the accommodating polypeptide in relation to a more complex system such as a membrane-bound complex. For this purpose, the EPR study has to be carried out on either partially or fully oriented systems (oriented membranes or monocrystals, respectively). Lastly, the sensitivity of the EPR spectra of iron-sulfur centers to structural changes can be utilized to monitor the conformational changes induced in the protein by different factors, such as the pH and the ionic strength of the solvent or the binding of substrates and inhibitors. We return to the latter point in Section IV. 

MEMBRANES ARE COMPLEX STRUCTURES COMPOSED OF LIPIDS, PROTEINS, CARBOHYDRATES... 

It is now well established that the MN blood-group antigens are situated99-101 on the major sialoglycoprotein (glycophorin A) of the erythrocyte membrane. The complex, antigenic structure resides within the first five amino acid residues from the N-terminal portion of the molecule three of these residues are glycosylated. 

Proteins either strengthen the membrane structure (building proteins) or fulfil various transport or catalytic functions (functional proteins). They are often only electrostatically bound to the membrane surface (extrinsic proteins) or are covalently bound to the lipoprotein complexes (intrinsic or integral proteins). They are usually present in the form of an or-helix or random coil. Some integral proteins penetrate through the membrane (see Section 6.4.2). 

The photosynthetic apparatus in green plants and algae is located in the chloroplast, which is a flattened, double-membraned structure about 150-200 A thick/4,5 The two flat membranes lie one above the other and are united at their peripheries. These double-membraned structures have been termed thylakoids (from the Greek sacklike )/ Each membrane of the thylakoid consists of a water-insoluble lipoprotein complex which contains the light-absorbing chlorophyll and other pigments utilized in photosynthesis. 

Enveloped viruses Many viruses have complex membranous structures surrounding the nucleocapsid. Enveloped viruses are common in the animal world (for example, influenza virus), but some enveloped bacterial viruses are also known. The virus envelope consists of a lipid bilayer with proteins, usually glycoproteins, embedded in it. Although the glycoproteins of the virus membrane are encoded by the virus, the lipids are derived from the membranes of the host cell. The symmetry of enveloped viruses is expressed not in terms of the virion as a whole but in terms of the nucleocapsid present inside the virus membrane. 

The biochemical properties of these structures are known. Desmosomes display protease sensitivity, divalent cation dependency and osmotic insensitivity and their membranes are mainly of the smooth type. In direct contrast to desmosomes, the tight junctions as well as gap junctions and synapses display no protease sensitivity, divalent cation dependency or osmotic sensitivity, while their membranes are complex. These facts have been used in the development of techniques to isolate purified preparations of junctional complexes. 

In non-neuronal cells, electron microscopy studies reveal very complex endosomal compartments composed of a highly dynamic array of heterogeneous tubulovesicular-membrane structure extending from close vicinity to the plasma membrane to the cell interior, reaching the boundaries of the Golgi apparatus. Presynaptic terminals have similar endosomal systems, albeit less extensive [73, 74]. 

Babiychuk EB, Draeger A 2000 Annexins in cell membrane dynamics. Ca2+-regulated association of lipid microdomains. J Cell Biol 150 1113-1124 Ber DM 2001 Excitation-contraction coupling and cardiac contractile force, 2nd edn. Kluwer Academic Publishers, Dordrecht/Boston/London Blaustein MP, Golovina VA 2001 Structural complexity and functional diversity of endoplasmic reticulum Ca2+ stores. Trends Neurosci 24 602—608 Flynn ER, Bradley KN, Muir TC, McCarron JG 2001 Functionally separate intracellular Ca2+ stores in smooth muscle. J Biol Chem 276 36411-36418 Fry CH, WuCl 997 Initiation of contraction in detrusor smooth muscle. Scand J Urol Nephrol Suppl 184 7-14... 

Thus, lipoproteins could be injected over the surface of a lipid covered SPR sensor in a detergent free buffer solution and showed spontaneous insertion into the artificial membrane.171 Again two hydro-phobic modifications are necessary for stable insertion into the lipid layer, whereas lipoproteins with a farnesyl group only dissociate significantly faster out of the membrane. Therefore the isoprenylation of a protein is sufficient to allow interaction with membraneous structures, while trapping of the molecule at a particular location requires a second hydrophobic anchor. Interaction between the Ras protein and its effector Raf-kinase depends on complex formation of Ras with GTP (instead of the Ras GDP complex, present in the resting cell). If a synthetically modified Ras protein with a palmi-... 

The mercuric ion, Hg2 +, which is obtained after oxidation in the red blood cells and other tissues, is able to form many stable complexes with biologically important molecules or moieties such as sulphydryl groups. The affinity of mercury for sulphydryl groups is a major factor in the understanding of the biochemical properties of mercuric compounds, resulting in interference with membrane structure and function and with enzyme activity. 

Biological membranes fluidity order parameters lipid-protein interactions translational diffusion site accessibility structural changes membrane potentials complexes and binding energy-linked and light-induced changes effects of additives location of proteins lateral organization and dynamics... 

Mesoscale calculations, discussed for the membrane in Section 6.5.3, provide insights into segregation behavior, structural correlations, and d5mamical behavior of different phases in CLs. They contribute to furnishing relations among structure, transport properties, and reactivity. Compared to hydrated ionomer membranes (Section 6.5), structural complexity is more pronounced in CLs. 

The late factors C5 to C9 are responsible for the development of the membrane attack complex (bottom). They create an ion-permeable pore in the bacterial membrane, which leads to lysis of the pathogen. This reaction is triggered by C5 convertase [2]. Depending on the type of complement activation, this enzyme has the structure C4b2o3b or C3bBb3b, and it cleaves C5 into C5a and C5b. The complex of C5b and C6 allows deposition of C7 in the bacterial membrane. C8 and numerous C9 molecules—which form the actual pore—then bind to this core. 

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