Membrane transport proteins structure

Igarashi, P., Cragoe, E.J., Jr. and Aronson, P.S. (1987) Xth ISN Congress Satellite Symposium on Structure, Function and Regulation of Membrane Transport Proteins, Furigen, p. 86. 

This chapter is divided into three sections. The first section covers renal tubule transport mechanisms. The nephron is divided structurally and functionally into several segments (Figure 15-1, Table 15-1). Many diuretics exert their effects on specific membrane transport proteins in renal tubular epithelial cells. Other diuretics exert osmotic effects that prevent water reabsorption (mannitol), inhibit enzymes (acetazolamide), or interfere with hormone receptors in renal epithelial cells (aldosterone receptor blockers). The physiology of each segment is closely linked to the basic pharmacology of the drugs acting there, which is discussed in the second section. Finally, the clinical applications of diuretics are discussed in the third section. 

The Si transporting proteins (structures of which are deduced from the cloned DNA sequences) are closely related in structure to other, well-characterized ion transporters14. They contain 12 a-helical transmembrane domains that fold to form a cylindrical channel— assembled like staves of a barrel — through the lipid bilayer membrane of the cell. However, the evidence suggesting an ion-type transporter needs to be resolved with results of recent physiological analyses of the pH-dependence of silicon uptake, suggesting that many diatom species most efficiently take up the unionized, neutral silicic acid16. 

X-ray structures of wild-type N. gonorrhoeae Fbp and mutant forms of H. influenzae Fbp have been described in which mnltinuclear oxo-metal clnsters are bomd to the protein. These surprising results are informative about aspects of the Fbp structure and formation of oxo-metal clusters on a protein surface, but it is not clear whether they have any physiological relevance. Sadler and coworkers present a case for this, noting that the onter membrane transport protein FecA passes diferric dicitrate into the periplasm and so at least one dinnclear iron complex is available to Fbp. 

Uptake capacity is dependent on the number and eiEciency of membrane transport proteins. It is not really known how the numbers of transporters (e.g. ammonium or nitrate) per cell varies as a function of physiological state, or how the kinetics of transport may be a function of the protein structure of the transporters themselves. Some studies indicate that the number of transporters per cell increase with nitrogen limitation, but kinetic data suggests otherwise and that transporter numbers are more or less constant as a function of growth rate under N limitation... 

Relatively few membrane transport proteins have been structurally characterized. Some of the best understood examples to date are the lactose permease and glycerol-3-phosphate transporter and the Ca + P-type ATPase (which is a primary ion pump). Other structurally well-characterized transport proteins include the bacterial porins and siderophore receptor proteins. In addition structures have been determined for several ion channels and additional bacterial transporters that are either directly relevant to or models for proteins important in drug transport. The following web sites maintained by Hartmut Michel and Stephen White respectively, contain exceptionally useful listings of these and other solved membrane protein structures and are frequently updated ... 

Analysis of the solution stmcture of a protein can be carried out by using redox-active metal centers (25). For example, [Cu(phen)2] was exploited in the structural analysis of Escherichia coli lactose permease, a paradigm for membrane transport proteins (73). By the reaction of Y with a functional lactose permease mutant, phen was covalently linked to a single cysteine residue situated within one of the 12 membrane-spanning a-helices of the protein. After Cu(n) ion was bound to the phen moiety of the modified protein and ascorbate was added in the presence of O2, efficient, localized scission (up to 30%) of the protein backbone was observed in adjacent helices, providing information on the structure of the protein. 

In all cells, the plasma membrane acts as a permeability barrier that prevents the entry of unwanted materials from the extracellular milieu and the exit of needed metabolites. Specific membrane transport proteins In the plasma mem brane permit the passage of nutrients Into the cell and metabolic wastes out of It others function to maintain the proper Ionic composition and pH ( 7.2) of the c3d osol. The structure and function of proteins that make the plasma membrane selectively permeable to different molecules are discussed In Chapter 7. 

The plasma membrane is a selectively permeable barrier between the cell and the extracellular environment. Its permeability properties ensure that essential molecules such as ions, glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell. In short, the selective permeability of the plasma membrane allows the cell to maintain a constant Internal environment. In Chapter 5, we learned about the components and structural organization of cell membranes. Movement of virtually all molecules and ions across cellular membranes is mediated by selective membrane transport proteins em bedded in the phospholipid bllayer. Because different cell types require different mixtures of low-molecular-weight compounds, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions and molecules to cross. Similarly, organelles within the cell often have a different internal environment from that of the surrounding cytosol, and organelle membranes contain specific transport proteins that maintain this difference. 

The coupling of these two reactions requires that charge is capable of being transferred across the membrane, most probably through the membrane-bound protein structures. The experimental and theoretical studies of charge transport in protein structures has been extensively reviewed elsewhere.The potential difference AV across the membrane would then have the value... 

In the rat, the observed effects of oral LD50 reflect both the intrinsic toxicity at the ultimate biophase site of action and the factors influencing distribution, membrane transport, protein binding, metabolism and excretion. The manifestation of acute mammalian toxicity is hence a much more complex response than can be described with the use of log alone, although individual processes such as bioavailability and adsorption depend on lipophilic-ity. The fit to a common QSAR model requires that each of these processes has similar structural dependences, qualitatively and quantitatively, within a given class of compounds. If a different process becomes predominant (i.e. rate limiting), the structure-toxicity relationship must alter thus the compound will be an outlier even when the principal mechanism of intrinsic toxicity remains the same. 

F. 3a and b. Fluid mosaic structure of the cell membrane, a Overall representation 1 — talayer of hpid molecules 2 — membrane proteins, including receptors, membrane transport proteins, proteins mediating signal transfer etc. 3 — polysaccharide layer — glycocalyx b molecular structure of the lipid bilayer a — cholesterol b — lateral chains of fatty acids c — glycCTol residue d — phosphate moiety of the phospholipid molecule e — positively charged (amino) part of the pho holipid molecule (From Refs. 

An additional feature in Fig. 2 worth noting is the amino-terminal 160 amino acids of mercuric reductase that lacked a fixed position in the crystal and therefore were not part of the solved structure. These contain the sequence that is homologous to MerP and postulated to be a mercurybinding domain. This region is drawn in Fig. 2 as an extension from the protein perhaps it functions like a baseball mitt that catches Hg from the membrane transport proteins and delivers Hg " to the carboxyl-terminal catalytic binding site, so that, as in the bucket brigade model above, Hg " is never found free within the cell. Mutant strains with the transport system but lacking the MerA detoxification enzyme are hypersensitive to mercury salts, as they accumulate Hg " but cannot get rid of it. After reduction by NADH (via FAD and the active site cysteine pair), metallic Hg is released... 

---