Hydrophobic membrane interior

Karlin-Neumann et al. [31] have presented a model for the main apoprotein of LHC II which has 3 a-helices (Chapter 11), consistent with the direct determination of the a-helical content of LHC II [32]. There is a large domain of surface-exposed protein (= 48%) this is also consistent with the electron microscopic pictures of reconstituted LHC II [18,19], While most of the Chi molecules are thought to reside in the hydrophobic membrane interior, there are insufficient histidine residues present for the co-ordination of all Chi a molecules, and the ligand for Chi b has not been recognized yet. The location of the carotenoids, so often ignored, but always a constituent of all Chl-proteins, is not established [30],... 

Matters become more complicated when the molecule is highly polar For example, sodium ions are present at 143 mM outside a typical cell at 14 mM inside the cell, yet sodium does not freely enter the cell, bee the charged ion cannot pass through the hydrophobic membrane interior... 

For simplicity, the hydrophobic membrane interior is not indicated in this diagram. 

Although the contribution is rather small, the partial discharging of the anesthetics in membranes can be important in the mechanism of the anesthetic action. The most plausible mechanism can be summarized as only a small portion of the cationic species are neutralized (deprotonated) at the bilayer surface and the neutral species are deeply penetrated and widely distributed in the hydrophobic bilayer interior, while the cationic species still remain at the hydrophilic bilayer surface where the hydration is significant. 

Considering only the lipid phase as the transport pathway for the peptide, as the solute enters and diffuses across the membrane it will encounter a number of different microenvironments. The first is the aqueous membrane interface (Fig. 23). In this region, the hydrated polar headgroups of the membrane phospholipids separate the aqueous phase from the apolar membrane interior. It has been shown that this region is capable of satisfying up to 70% of the hydrophobic effect... 

Ionophores are necessary since the lipid components of biological membranes tend to be orientated such that their polar groups face the membrane surfaces while the non-polar hydrocarbon portions occupy the membrane interior. The hydrophobic nature of the centre of the membrane thus acts as a barrier to the passage of ions such as sodium or potassium. 

There is not a unique binding site for all sorts of xenobiotics, but the compounds are intercalated in such a way into the membrane that they interact most favourably with the membrane components and with least perturbation. Some compounds, such as hydrophobic and neutral molecules, are actually dissolved in the membrane interior, whereas others exhibit more specific interactions in the polar region of the membrane. In general, interaction of the xenobiotics with the head groups leads to a stronger perturbation of the bilayer than intercalation in the membrane core [170]. 

Herbette et al. compared the sorption site of the structurally similar tertiary amines propranolol and timolol [164]. Propranolol has a naphthalene substituent on the aliphatic chain, which is deeply incorporated into the hydrophobic core of the membrane. In contrast, timolol carries a partially charged morphine ring at the same place. This substituent, due to its polarity and partial charge, does not interact favourably with the membrane interior. Consequently, the AnW at pH 7.5 is 20 times higher for propranolol than for timolol, and timolol has less influence on the phase transition. 

The major route for bioaccumulation of hydrophobic organic compounds in aquatic animals is passive diffusion over cell membranes. In fish, the gill epithelia are the predominant port of entry, with less than 40% of uptake across the skin [181]. Since permeability of the membrane is a direct function of the membrane-water partition coefficient and the diffusion coefficient across the membrane interior [182], the bioconcentration factor (logBCF) can be directly correlated with log K0Vl. or log Km%v for compounds with intermediate hydro-phobicity [183,184],... 

For the membrane interior, independent sources of informahon exclude the existence of vapor and of hydrophobic pores ... 

The activity of 2,3-oxidosqualene cyclases is associated with microsomes, indicating their membrane-bound nature. However, the predicted amino acid sequences of these enzymes generally lack signal sequences and obvious transmembrane domains. Addition of hydrophobic membrane-localising regions to OSCs during evolution may have removed selection pressures that maintained alternate mechanisms for membrane localisation [33]. Consistent with this, there is a non-polar plateau on the surface of the A. acidocaldarius SC enzyme which is believed to be immersed in the centre of the membrane. The squalene substrate for SC is likely to diffuse from the membrane interior into the central cavity of the enzyme via this contact region [55,56]. 

COX-1 and COX-2 have virtually identical tertiary and quaternary structures, but they differ subtly in a long, thin hydrophobic channel extending from the membrane interior to the lumenal surface. The channel includes both catalytic sites and is presumed to be the binding site for the hydrophobic substrate, arachidonate. Both COX-1 and COX-2 have been crystallized in the presence of several different bound NSAID compounds, defining the NSAID-binding site (Fig. 1). The bound drugs block the hydrophobic channel and prevent arachidonate entry The subtle differences between the channels of COX-1 and COX-2 have guided... 

The influence of size and shape on the diffusion of hydrophobic solutes was estimated by simulations involving artificial Lennard-Jones particles those intermolecu-lar interaction parameters were based on those for ammonia or oxygen, respectively. The results on the size dependence of diffusion confirmed that the membrane interior differs strongly from a bulk hydrocarbon. In the center of the bilayer, the excess free energy for hydrophobic Lennard-Jones particles remained low irrespective of the size of the particles. This can be explained by the large fraction of accessible volume in that region. 

The accessibility to paramagnetic reagents is measured by the experimental accessibility parameter n, a quantity directly proportional to the collision frequency of the nitroxide with the reagent. As is intuitively reasonable, n is directly proportional to the exposure of R1 on the surface of the protein. The commonly used paramagnetic reagents include O2 and the metal ion complex NiEDDA. Because oxygen has sufficient solubility in both membrane interiors and water, 11(02) is useful to explore the exposure of R1 on both hydrophobic and hydrophilic surfaces of a membrane protein. 

Polymer films have been obtained by plasma polymerization of hexafluorobenzene, N-vinylpyrrolidine, and chloracrylonitrile (Munro). Higuchi et al. have shown that irradiation of an azobenzene-modified poly(Y-methyl-L-glutamate-CO-L-glutamic acid) in bilayer membrane vesicles of distearyldimethylammonium chloride leads to trans-cis isomerization of the polymer this leads to transfer of the polypeptide from the hydrophobic bilayer membrane interior to the hydrophilic surface. As a result, there was a decrease in the ion permeability through the bilayer membrane and the formation of intervesicular adhesion. Eisner and Ritter have prepared photosensitive membranes from an aromatic polyamide and a cinnamate that incorporates a liquid crystalline component. 

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