Hydrophilic region

The L and the M subunits are firmly anchored in the membrane, each by five hydrophobic transmembrane a helices (yellow and red, respectively, in Figure 12.14). The structures of the L and M subunits are quite similar as expected from their sequence similarity they differ only in some of the loop regions. These loops, which connect the membrane-spanning helices, form rather flat hydrophilic regions on either side of the membrane to provide interaction areas with the H subunit (green in Figure 12.14) on the cytoplasmic side and with the cytochrome (blue in Figure 12.14) on the periplasmic side. The H subunit, in addition, has one transmembrane a helix at the car-boxy terminus of its polypeptide chain. The carboxy end of this chain is therefore on the same side of the membrane as the cytochrome. In total, eleven transmembrane a helices attach the L, M, and H subunits to the membrane. 

Thyrotropin (TSH) regulates the production and secretion of thyroid hormones as well as thyroid epithelial cell growth via the TSH receptor. The TSH receptor belongs to the family of G protein-coupled receptors. It is composed of 764 amino acids. The receptor contains a long hydrophilic region orientated towards the exterior of the cell (ectodomain), 7 hydrophobic transmembrane domains and a short cytoplasmic region. 

A similar argument can be made for electronic features such as electron density, polarization and polarizability. These are critically dependent on the ionization state of the molecule, but the conformahonal state is also highly influential. One highly approximate yet useful reflection of electron density is afforded by the polar surface area (PSA), a measure of the extent of polar (hydrophilic) regions on a molecular surface (see Chapter 5). 

Fig. 14.3 LogP distribution on diazepam. Green denotes lipophilic regions and red hydrophilic regions.

FIG. 7 Structures of various liquid-crystalline phases of membrane lipids. (A) Normal hexagonal phase (Hi) (B) lamellar phase (C) inverted hexagonal phase (Hu). Cubic phases consisting of (D) spherical, (E) rod-shaped, and (F) lamellar units. The hydrocarbon regions are shaded and the hydrophilic regions are white. (Reprinted by permission from Ref. 11, copyright 1984, Kluwer Academic Publishers.)... 

By swelling with aqueous electrolyte, cations (and, to lesser extent, also anions) penetrate together with water into the hydrophilic regions and form spherical electrolyte clusters with micellar morphology. The inner surface of clusters and channels is composed of a double layer of the immobilized —SO3 groups and the equivalent number of counterions, M+. Anions in the interior of the clusters are shielded from the —SOJ groups by hydrated cations and water molecules. On the other hand, anions are thus... 

Figure 6.4 Central hydrophilic region within a subunit of (a) HuHF, and (b) HoLF. From Chas-teen and Harrison, 1999. Reproduced by permission of Academic Press.

Tyrosine contains a phenolic side chain with a pKa of about 9.7-10.1. Due to its aromatic character, tyrosine is second only to tryptophan in contributing to a protein s overall absorptivity at 275-280nm. Although the amino acid is only sparingly soluble in water, the ionizable nature of the phenolic group makes it often appear in hydrophilic regions of a protein—usually... 

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