Hydrophobic proteins

Reconstitution experiments have shown that the interaction of OSCP and Fi with the membrane unit occurs independent of the presence of phospholipids, the latter only being required for the expression of enzymatic activity. Therefore, the association of the two extrinsic components with the membrane is probably mediated by protein-protein interactions. The association of the complex with the lipid bilayer, however, is dependent on the lipid-binding properties of the highly hydrophobic proteins of the membrane unit. There are other examples of membrane enzyme complexes containing both extrinsic and intrinsic protein subunits to suggest that this may be a general organizational plan of membrane enzymes. 

In the example cited above, the intrinsic proteins were subunits of a larger enzyme complex this, however, need not necessarily be true in all cases. For example, rhodopsin is an intrinsic protein that exists as a monomeric species in the disc membranes of the retinal outer rod segment. An analogous situation prevails in the plasma membrane of Halo-bacterium halobium, which contains patches of membrane consisting of phospholipid and a single rhodopsin-like protein. 

There is a substantial body of evidence suggesting that cellular membranes are assembled by a process of accretion of new material into a preexisting structure. Starting with this permise, the problem of membrane biogenesis can be conceptually simplified by posing the question of how newly synthesized lipids and proteins are integrated into membranes. 

Most membrane proteins are made on cytoplasmic ribosomes that also synthesize the bulk cytosolic proteins of the cell. The exceptions are a limited number of mitochondrial and chloroplast proteins that are translated by a special class of ribosomes present in their respective organelles. These ribosomes differ from their cytoplasmic counterparts in size and RNA and protein composition, but there is no reason to believe that the translational mechanism itself is fundamentally different (Chapters 4 and 7). 

Other mechanisms can also be visualized. For example, there may exist special adaptor proteins or lipids (lyso compounds) which could act as 

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Kovacs H, A E Mark and W F van Gunsteren 1997. Solvent Structure at a Hydrophobic Protein Surface. Proteins Structure, Function and Genetics 27 395-404. 

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

Figure 12.2 (a) Schematic drawing of membrane proteins in a typical membrane and their solubilization by detergents. The hydrophilic surfaces of the membrane proteins are indicated by red. (b) A membrane protein crystallized with detergents bound to its hydrophobic protein surface. The hydrophilic surfaces of the proteins and the symbols for detergents are as in (a). (Adapted from H. Michel, Trends Biochem. Sci. 8 56-59, 1983.)... 

Amphoteric hydrophobic Blue dextran, collagen, gelatin, hydrophobic proteins Hydrophobic peptides Buffer or salt solution with organic solvent (e,g, 20% CH3CN in 0.1 M NaNOi) 35-45% CH3CN in 0.1% TFA... 

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

Table 3 shows some physicochemical properties used as international GA quality parameters, for example moisture, total ash content, volatile matter and internal energy, with reference to gums taken from Acacia Senegal species in Sudan (FAO, 1990, Larson Bromley, 1991). The physicochemical properties of GA may vary depending on the origin and age of trees, the exudation time, the storage type, and climate. The moisture content facilitates the solubility of GA carbohydrate hydrophilic and hydrophobic proteins. The total ash content is used to determine the critical levels of foreign matter, insoluble matter in... 

Hemoglobin is another heme-containing protein, which has been shown to be active towards PAH, oxidation in presence of peroxide [420], This protein was also modified via PEG and methyl esterification to obtain a more hydrophobic protein with altered activity and substrate specificity. The modified protein had four times the catalytic efficiency than that of the unmodified protein for pyrene oxidation. Several PAHs were also oxidized including acenaphthene, anthracene, azulene, benzo(a)pyrene, fluoranthene, fluorene, and phenanthrene however, no reaction was observed with chrysene and biphenyl. Modification of hemoglobin with p-nitrophenol and p-aminophenol has also been reported [425], The modification was reported to enhance the substrate affinity up to 30 times. Additionally, the solvent concentration at which the enzyme showed maximum activity was also higher. Both the effects were attributed to the increase in hydrophobicity of the active site. 

Sackett, D. L. and Wolff, J. (1987). Nile red as a polarity-sensitive fluorescent-probe of hydrophobic protein surfaces. Anal. Biochem. 167, 228-234. 

Gretch, D.R., Suter, M., and Stinski, M.F. (1987) The use of biotinylated monoclonal antibodies and streptavidin affinity chromatography to isolate herpesvirus hydrophobic proteins or glycoproteins. Anal. Biochem. 163, 270-277. 

Ecroyd, H., Belghazi, M., Dacheux, J.L. and Gatti, J.L. (2005) The epididymal soluble prion protein forms a high-molecular-mass complex in association with hydrophobic proteins. Biochem. J. 392,211-219. 

B. Apparent Paradox of Hydrophobic Proteins and Water Spinning ... 21... 

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