The Structure of Membranes

Mutations that affect the structure of membrane proteins (teceptots, ttanspotters, ion channels, enzymes, and stmctutal proteins) may cause diseases examples include cystic fibrosis and familial hypetcholes-terolemia. 

If we had asked this question a few years ago, the answer would have been at best equivocal. However, as was pointed out in an earlier chapter, since the enormous progress in the determination of the structure of membrane proteins, we can, on the basis of the X-ray structures of an increasing number of ion transport proteins, begin to advance hypotheses that have more and more likelihood to be close to reality. In the case of Na+ channels we are still pretty much in the dark. However, the successful determination of the structure of a number of K+ channels of both bacterial and mammalian origins represents a great leap forward in our understanding of how these channels function. 

The generally low sensitivity of solid state NMR is significantly enhanced by using selective F-labels to analyze the structures of membrane-associated peptides and proteins [13,16-24]. This opens up a previously inaccessible range of experimental conditions, such as peptideiHpid ratios as low as 1 3000 [2 5 ]. This is particularly relevant for antibiotic peptides, as their concentration dependence constitutes an essential functional aspect [26]. A suitable side chain for labelling is 4F-phenylglycine (4F-Phg), as the F-reporter is fixed... 

Cholesterol The pathway for synthesis of cholesterol is described in Appendix 11.9. Cholesterol is important in the structure of membranes since it can occupy the space that is available between the polyunsaturated fatty acids in the phospholipid (Chapter 4). In this position, cholesterol restricts movement of the fatty acids that are components of the phosphoglycerides and hence reduces membrane fluidity. Cholesterol can be synthesised de novo in proliferating cells but it can also be derived from uptake of LDL by the cells, which will depend on the presence of receptors for the relevant apoUpoproteins on the membranes of these cells (Appendix 11.3). 

Although numerous models for the structure of membranes have been proposed, the structural features which are generally accepted at present are rather similar to the original Danielli-Davson model. There is convincing evidence that the structure is dominated by lipid bilayers. The state of order of the hydrocarbon chains is now being studied extensively by many groups (see below). Less is known about the proteins. Besides the proteins that are located on the outside according to the Danielli-Davson model, there are also proteins that are partly buried in the hydro-phobic interior of the lipid layer however, little is known about the lipid-protein interaction. 

Reiss-Husson, F., and Luzzati, V. Phase transitions in lipids in relation to the structure of membranes. Adv. in BioL Medical Physis//, 87-105 (1967). 

The plasma membrane is a major barrier to the diffusion of solutes into and out of plant cells, the organelle membranes play an analogous role for the various subcellular compartments, and the tonoplast performs this function for the central vacuole. For instance, although H20 and C02 readily penetrate the plasma membrane, ATP and metabolic intermediates usually do not diffuse across it easily. Before we mathematically describe the penetration of membranes by solutes, we will briefly review certain features of the structure of membranes. 

With this knowledge of the structure of membranes, we turn to a quantitative analysis of the interactions between membranes and diffusing solutes. In Chapter 3 (Section 3.3E,F,G), we will discuss active transport, which is important for moving specific solutes across membranes, thereby overcoming limitations posed by diffusion. 

Although membrane proteins are more difficult to purify and crystallize than are water-soluble proteins, researchers using x-ray crystallographic or electron microscopic methods have determined the three-dimensional structures of more than 20 such proteins at sufficiently high resolution to discern the molecular details. As noted in Chapter 3. the structures of membrane proteins differ from those of soluble proteins with regard to the distribution of hydrophobic and hydrophilic groups. We will consider the structures of three membrane proteins in some detail. 

On recommending membrane processes as modern and effective tools which may be successfully applied in waste management technologies, we must not neglect many serious limits pertaining to the transition of membrane methods from their well-established application range to the one proposed here. It is not easy to arbitrate which of the difficulties encountered in this connection rank first. To begin with, let us consider those associated with the essence and the structure of membrane. 

From these arguments, we may conclude that trehalose is the best compromise to maintain an artificial H-bond network that temporarily replaces that developed by H2O molecules and avoids both those catastrophic consequences, that is the formation upon freezing of crystalline ice in that part outside macromolecules that is in contact with liquid water, and the collapse of the structure of membranes or of secondary structures of proteins upon drying due to the escape of H2O molecules. This artificial H-bond network is much less flexible than that established by H2O molecules and considerably slows down the dynamics of H2O molecules (23, 28). It consequently does not allow normal activity. It does not allow life to proceed in the same way as in the H2O network of living conditions. It nevertheless avoids irreversible transformation of the structure of the macromolecule by hydric stress, thus allowing resumption of living activities by rehydration. The discussion that has appeared in the literature to decide which mechanism is the most important, the glass... 

This work demonstrated (a) that there are distortions in the CD of particulate systems (this had not previously been appreciated and the distorted spectra had been interpreted in terms of unique conformational features), (b) that the distorted spectra can be calculated and hence corrected, and (c) that there is a measurable differential scatter of left and right circularly polarized light by optically active particles. Thus, in addition to correcting spectra for suspensions of particulate systems that may be interpreted in terms of biomolecular conformation, the third point makes it possible to obtain an optical rotatory dispersion spectrum for the particle surface. In the case of membranes this will allow determination of relative amounts of surface area which are covered by ordered protein. This information coupled with the CD spectrum for the whole membrane will provide considerable information on the structure of membranes. 

FT-IR spectroscopy is particularly useful for probing the structure of membrane proteins. Until recently, a lack of adequate experimental techniques has been the reason for the poor understanchng of the secondary structure of most membrane proteins. X-ray diffraction requires high quality crystals and these are not available for many membrane proteins. Circular dichroism (CD) has been widely used for studying the conformation of water-soluble proteins, but problems arise in its use for membrane proteins. The light scattering effect may distort CD spectra and lead to substantial errors in their interpretation. In addition, the reference spectra used for the analysis of CD spectra are based on globular proteins in aqueous solution and may not be applicable to membrane proteins in the hydrophobic environment of lipid bilayers. 

Helices that form pores will be amphiphilic because it is more favorable to have situated in the inner side of the pore hydrophilic amino acid side chains, while the outer side of the pore represents a more favorable environment for hydrophobic amino acid side chains since these are in contact with lipids. Some authors point to the possibility that such a structure contains hydrogen bonds between amino acid residues and the main chain in order to compensate opposite charges and oppositely oriented dipoles. A comparison between the strength of different interactions in the structure of soluble and membrane proteins leads to the conclusion that because of the decreased strength of hydrophobic interactions and increased strength of electrostatic interactions (because of the reduced dielectric constant), the electrostatic interactions play the main role in stabilizing the structure of membrane proteins. ... 

It is generally believed that the structure of membrane proteins is simpler than that of soluble proteins because the lipid bilayer in which the membrane proteins are immersed diminishes the degrees of freedom. Thus, the prediction of the membrane protein structures is expected to be more accurate and the obtained models should be of considerable help when the activity of the membrane proteins is analyzed. 

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