Protein membranes

In the currently accepted theories on membrane protein crystallization, discussed above, the only role of the detergent is to stabilize the membrane protein in aqueous solution. However, the effect of detergent structure on the crystallization of bovine heart cytochrome c oxidase. 

Orthorhombic crystals obtained from the preparations stabilized with the detergent decyl maltoside diffracted X-rays up to a maximum resolution of 2.3 resolution from non-frozen crystals (Yoshikawa et ah, 1998). No commercially available detergent other than decyl maltoside has been found that will produce orthorhombic crystals. Even dodecyl maltoside, only two alkyl chain units longer than decyl maltoside, did not produce crystals even though the detergent stabilizes the enzyme effectively. It should 

Given time, a cell membrane can be directly penetrated by quite large molecules. In a more efficient system, membrane proteins have evolved to simplify and control the transport through the protein itself. Membrane proteins form pores that act as a filter for transportation across the membrane. Several membrane proteins form channels for the penetration of potassium ions, K+, but do not allow the passage of smaller ions, such as Na+, Li+, and Mg +. Since the latter ions are small, the water molecules 

Of particular interest to us are the membrane proteins, numbered I-V. Four of them are part of the electron transport chain in accordance with the chemiosmotic model. FT in the chains is driven by the energy of food or by photosynthesis. Protons are pumped across the membrane to a more acid location. This is done in Complex I, Complex III, and Complex IV. Complex II is used in reduction of ubiquinone to ubiquinol. Another molecule of this type is Complex V, an ATP synthase where ATP is synthesized from ADP and P, as just mentioned. This complex does not have any electron transport chain. 

In the mitochondrial electron transport chain, electrons move from an electron donor (NADH, FADH, or QHj) to a terminal electron acceptor (O2) via a series of redox reactions. Complex III is called the cytochrome bcj complex. Complex IV is the end station where oxygen molecules are accepting electrons and protons and are reduced to water. Complex IV is also called cytochrome c oxidase (CcO). 

The mitochondrial membrane in photosynthetic organisms also contains proteins for harvesting energy from sunlight. The oxidation in the citric acid cycle provides energy for ATP synthase. 

An electron carrier is a subsystem that has accepted an electron in the lowest unoccupied molecular orbital (LUMO) or donated one from the highest occupied molecular orbital (HOMO). Usually, the carrier is a metal complex or a x-system with low reorganization energy. We will now discuss molecules that are involved in biological ET. PT reactions have been discussed already in Chapters 9 and 10, and not much needs to be added here. Proteins, also discussed in Chapter 9, can be electron carriers in the side groups of some of the peptides, for example, tyrosine. Next we will discuss some important molecules and molecular groups. 

Archer et al. constructed the BR-based homology model of the peptidergic cholecystokinin receptor CCKl [54] and compared its structure with the template. 

Challenges of Homology Model-Based Virtual Screening 

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All the long-range forces discussed in this chapter play a role in biological processes. Interactions between membranes, proteins, ligands, antibodies... 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Membrane proteins comprise another important class of protein crystallized in 2D. These proteins perform important functions as membrane channels and recognition sites for cells. Unlike the streptavidin crystals, membrane proteins... 

Walz T and Grigorieff N 1998 Eleotron orystallography of two-dimensional orystals of membrane proteins J. Struct. Biol. 121 142-61... 

Vos M H, Rappaport F, Lambry J-C, Breton J and Martin J-L 1993 Visualization of the coherent nuclear motion in a membrane protein by femtosecond spectroscopy Nature 363 320-5... 

The spatial arrangement of atoms in two-dimensional protein arrays can be detennined using high-resolution transmission electron microscopy [20]. The measurements have to be carried out in high vacuum, but since tire metliod is used above all for investigating membrane proteins, it may be supposed tliat tire presence of tire lipid bilayer ensures tliat tire protein remains essentially in its native configuration. 

Deisenhofer J, Epp O, Miki K, Huber R and Michei H 1984 X-ray structure anaiysis of a membrane-protein compiex eiectron density map at 3 A resoiution and a modei of the chromophores of the photosynthetic reaction center from Rhode pseudomonas viridis J. Mol. Biol. 180 385-98... 

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

Protein Computers. The membrane protein bacteriorhodopsin holds great promise as a memory component in future computers. This protein has the property of adopting different states in response to varying optical wavelengths. Its transition rates are very rapid. Bacteriorhodopsin could be used both in the processor and storage, making a computer smaller, faster, and more economical than semiconductor devices (34). 

Rotavirus. Rotavims causes infant diarrhea, a disease which has major socio-economic impact. In developing countries it is the major cause of death in infants worldwide, causing up to 870,000 deaths per year. In the United States, diarrhea is stiU a primary cause of physician visits and hospitalization, although the mortaUty rate is relatively low. Studies have estimated a substantial cost benefit for a vaccination program in the United States (67—69). Two membrane proteins (VP4 and VP7) of the vims have been identified as protective epitopes and most vaccine development programs are based on these two proteins as antigens. Both Hve attenuated vaccines and subunit vaccines are being developed (68). 

Many small molecules can penetrate the outer ced membrane by diffusion through channels created by outer-membrane proteins caded porins. 

The mechanism of inhibition has not been characterized, but it is probably related to the ionophoretic properties of these antibiotics. Monensin has been shown to inhibit the intracellular transport of viral membrane proteins of cells infected with Semliki Forest vims (169). The formation of syncytia, normally observed when T-lymphoblastoid cell line (CEM) cells are cocultivated with human immunodeficiency vims (HlV-l)-infected T-ceU leukemia cell line (MOLT-3) cells, was significantly inhibited in the presence of monensin (170). This observation suggests that the viral glycoproteins in the treated cells were not transported to the cell surface from the Golgi membrane. 

Fig. 4. Comparison of the three types of tetracycline resistance where T represents the tetracycline molecule O, a tetracycline transporter and aaa/, the ribosome A shows the effect of tetracycline exposure on a sensitive cell B, the efflux of resistance where a cytoplasmic membrane protein ( D) pumps tetracycline out of the cell as fast as the tetracycline transporter takes it up C, the ribosomal protection type of resistance where the ribosome is modified by ( ) to block productive binding and D, the tetracycline modification type of resistance where t is an inactive form of tetracycline. Reproduced with...

Aqueous-detergent solutions of appropriate concentration and temperature can phase separate to form two phases, one rich in detergents, possibly in the form of micelles, and the other depleted of the detergent (Piyde and Phillips, op. cit.). Proteins distribute between the two phases, hydrophobic (e.g., membrane) proteins reporting to the detergent-rich phase and hydrophilic proteins to the detergent-free phase. Indications are that the size-exclusion properties of these systems can also be exploited for viral separations. These systems would be handled in the same way as the aqueous two-phase systems. 

H. Michel (Ed.), Crystallisation of Membrane Proteins, CRC Press, Boca Raton, 1991. ISBN 0849348161. 

B30 611 1976 gave m 69-70°). Hydrolysis using an equivalent of base in methanol gave the desired glueoside. This is a non-ionie detergent for reeonstituting membrane proteins and has a critieal micelle concentration of 30 mM. [Shimamoto et al. J Biochem (Tokyo) 97 1807 I985 Saito and Tsuchiya Chem Pharm Bull Jpn 33... 

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