Membrane hydrophobic core

The thickness of the membrane hydrophobic core determined in the author s laboratory on the basis of the difilfactometric measurements at 25 °C. For details see Gruszecki and Sielewiesiuk (1990, 1991) and Gruszecki et al. (1994). 

Our results confirmed the tendency of Lys-flanked peptides to compensate the positive mismatch between peptide and membrane hydrophobic core by tilting. Some of the peptides, however, prodnce superhelical donble-twisted structure. This only occurs in the membrane in the gel phase, where only a small hydrophobic mismatch exists. The peptide also alters certain properties of the surrounding hpids snch as membrane ordering, the amount of dihedral angles in tram conformation and the nnmber of transitions between tram and gauche conformation. It is likely that these effects shonld provide some preferable stmctnral state of the peptides in a membrane. The lipid stmctural state around the peptide is probably between gel and hqnid-ciystalline state. This effect depends on peptide amino acid composition. Amino acids with large side chains branched at (He, Val) produce hehx, which has more side chains finctuates than that of a poly-Len helix. This holds also for small side chains (Ala). 

Simple considerations show that the membrane potential cannot be treated with computer simulations, and continuum electrostatic methods may constimte the only practical approach to address such questions. The capacitance of a typical lipid membrane is on the order of 1 j.F/cm-, which corresponds to a thickness of approximately 25 A and a dielectric constant of 2 for the hydrophobic core of a bilayer. In the presence of a membrane potential the bulk solution remains electrically neutral and a small charge imbalance is distributed in the neighborhood of the interfaces. The membrane potential arises from... 

Why should the cores of most globular and membrane proteins consist almost entirely of a-helices and /3-sheets The reason is that the highly polar N—H and C=0 moieties of the peptide backbone must be neutralized in the hydrophobic core of the protein. The extensively H-bonded nature of a-helices and /3-sheets is ideal for this purpose, and these structures effectively stabilize the polar groups of the peptide backbone in the protein core. 

The final step of the reaction involves the transfer of two electrons from iron-sulfur clusters to coenzyme Q. Coenzyme Q is a mobile electron carrier. Its isoprenoid tail makes it highly hydrophobic, and it diffuses freely in the hydrophobic core of the inner mitochondrial membrane. As a result, it shuttles electrons from Complexes I and II to Complex III. The redox cycle of UQ is shown in Figure 21.5, and the overall scheme is shown schematically in Figure 21.6. 

Figure 7. Mechanism of the proton-translocating ubiquinol cytochrome c reductase (complex III) Q cycle. There is a potential difference of up to 150 mV across the hydrophobic core of this complex (potential barrier represented by the vertical broken line). Cytochromes hour and b N are heme groups on the same peptide subunits of complex III which can transfer electrons across the hydrophobic core. The movement of two electrons provides the driving force to transfer two protons from the matrix to the cytosol. Diffusion of UQ and UQHj, which are uncharged, in the hydrophobic core, and lipid bilayer of the inner membrane is not influenced by the membrane potential (see Nicholls and Ferguson, 1992).

As described above, some solutes such as gases can enter the cell by diffusing down an electrochemical gradient across the membrane and do not require metabolic energy. The simple passive diffusion of a solute across the membrane is limited by the thermal agitation of that specific molecule, by the concentration gradient across the membrane, and by the solubility of that solute (the permeability coefficient. Figure 41—6) in the hydrophobic core of the membrane bilayer. Solubility is... 

Meier, E. M. Schummer,D. Sandhoff, K., Evidence for the presence of water within the hydrophobic core of membranes, Chem. Phys. Lipids 55, 103-113 (1990). 

Owing to their chemical structure, carotenes as polyterpenoids are hydrophobic in nature (Britton et al., 2004). Therefore, as it might be expected, the carotenes are bound within the hydrophobic core of the lipid membranes. Polar carotenoids, with the molecules terminated on one or two sides with the oxygen-bearing substitutes, also bind to the lipid bilayer in such a way that the chromophore, constituted by the polyene backbone is embedded in the hydrophobic core of the membrane. There are several lines of evidence for such a localization of carotenoids with respect to the lipid bilayers. 

FIGURE 2.1 Energy of the 0-0 vibrational transition in the principal electronic absorption spectrum of violaxanthin (l Ag-—>1 BU+), recorded in different organic solvents, versus the polarizability term, dependent on the refraction index of the solvent (n). The dashed line corresponds to the position of the absorption band for violaxanthin embedded into the liposomes formed with DMPC (Gruszecki and Sielewiesiuk, 1990) and the arrow corresponds to the polarizability term of the hydrophobic core of the membrane (n = 1.44). 

In a lipid environment, van der Waals interactions become less specific. Electrostatic effects are enhanced up to 40-fold (according to Coulomb s law) due to the low dielectric constant, e s 2, of the hydrophobic core of membranes. As a result, weak electrostatic interactions, e.g., between the 71-electrons of an aromatic ring and a cation [51], may come into play. In addition, H-bond interactions, which can be considered as dipole-dipole interactions can also become relevant. 

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. 

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. 

However, there is another class of outer membrane proteins (such as Toc34) that do not have a cleavable transit peptide. These proteins appear to be relatively small. A study suggested that their signal resides on the N-terminal 30 residues (Li and Chen, 1996). The signal consists of a positively charged N-terminal portion followed by a hydrophobic core, although it is not certain whether this feature is general. 

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