Membrane lipids hydrophobicity

At the N-terminal end of the f loop, near the membrane lipid interface, there is an autoinhibitory domain, rich in both basic and hydrophobic residues and consisting of a 20-aminoacid sequence (219-238). This aminoacid sequence, named exchange inhibitory peptide (XDP), is involved in NCX activity regulation. 

Figure 41-3. Diagrammatic representation of a phospholipid or other membrane lipid. The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic. The fatty acids in the tails are saturated (S) or unsaturated (U) the former are usually attached to carbon 1 of glycerol and the latter to carbon 2. Note the kink in the tail of the unsaturated fatty acid (U), which is important in conferring increased membrane fluidity.

Interaction with a lipid bilayer driven by a potential difference and by polar and/or hydrophobic forces between the amino acid side chains of the pardaxin tetramers and the polar membrane lipid head group triggers insertion from a "raft" like structure. 

Because the carotenoids favour hydrophobic domains they are generally localised in the membranes and lipoproteins of animal cells. In this location they can influence the oxidation of membrane lipids and prevent the passage of free radicals from one cellular compartment to another. Thus, DNA in the nucleus is protected from intracellularly generated ROS by (at least) the nuclear membrane and from extracellular ROS by a number of membranes. Should ROS reach the nucleus, base oxidation can occur. The base most susceptible to oxidation is guanine, although all other bases can also be affected. The cell has the ability to detect damaged bases, excise them. 

Figure 3 A hydrophobic permeant must negotiate through a complex series of diffu-sional and thermodynamic barriers as it penetrates into a cell. The lipid and protein compositions and charge distribution of the inner and outer leaflets of the membrane lipid bilayer can play limiting roles, particularly at the tight junction. Depending upon the permeant s characteristics, it may remain within the plasma membrane or enter the cytoplasm, possibly in association with cytosolic proteins, and partition into cytoplasmic membranes.

Membrane lipid composition -hydrophobic matching and interfacial charges-... 

In the following section, the role of the various types of complexes mentioned above will be discussed with regard to various mechanisms of interactions at biological interphases. It is clear that metal ions and hydrophilic complexes cannot distribute into the membrane lipid bilayer or cross it. The role of hydrophilic ligands has thus to be discussed in relation to binding of metals by biological ligands. In contrast, hydrophobic complexes may partition into the lipid bilayer of membranes (see below, Section 6). 

A clue as to why the cationic N-terminal region and the hydrophobic C-terminal portion of SP are required for full histamine-releasing activity comes from studies of the electrical conductivity of black lipid membranes in the presence of peptides. Using SP, these authors [176] concluded that SP probably binds by its N-terminal region to negatively charged sites on membrane lipids, while the C-terminal portion of the molecule penetrates the hydrophobic core of the lipid bilayer, which could induce an increase in membrane permeability or a slight alteration in membrane conformation. 

A conceptualized cross section through a portion of the cell wall (rectangles), periplasmic space, and cell membrane (lipid bilayer with polar head groups in contact with cytoplasm and external medium, and hydrophobic hydrocarbon chains) of an aquatic microbe. Reactive functional groups (-SH, -COOH, -OH, -NH2) present on the wall consitutents and extracellular enzymes (depicted as shaded objects) attached by various means promote and catalyze chemical reactions extracellularly. 

Fullerene showed antibacterial activity, which can be attributed to different interactions of C60 with biomolecules (Da Ros et al., 1996). In fact, there is a possibility to induce cell membrane disruption. The fullerene sphere seems not really adaptable to planar cellular surface, but for sure the hydrophobic surface can easily interact with membrane lipids and intercalate into them. However, it has been demonstrated that fullerene derivatives can inhibit bacterial growth by unpairing the respiratory chain. There is, first, a decrease of oxygen uptake at low fullerene derivative concentration, and then an increase of oxygen uptake, which is followed by an enhancement of hydrogen peroxide production. The higher concentration of C60 seems to produce an electron leak from the bacterial respiratory chain (Mashino et al., 2003). 

Membrane lipids are strongly amphipathic molecules with a polar hydrophilic head group and an apolar hydrophobic tail. in membranes, they are primarily held together by the hydrophobic effect (see p. 28) and weak Van der Waals forces, and are therefore mobile relative to each other. This gives membranes a more or less fluid quality. 

During the past decades, liposomes have been used for drug delivery on account of their unique solubilization characteristics for water-insoluble organic substances. A liposome encapsulates a region of aqueous solution inside a hydrophobic membrane dissolved hydrophilic solutes cannot readily pass through the lipids. Hydrophobic chemicals can be dissolved into the membrane, and this way, the liposome can carry... 

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