Protein membrane composition

We have recently studied the effect of the protein membrane composition on partial coalescence in 20% milk-fat emulsions (16). Emulsions were produced consisting of 20% fat (from sweet butter, 80% fat), and 0.085, 0.17, 0.25, 0.50, 0.75, 1.0, 1.5, or 2.0% whey protein isolate (WPI, 88% protein Protose Separations, Teeswater, ON, Canada). The emulsions were heated to 70 or 90°C for 30 min. 

Majd, S. and Mayer, M. (2005) Hydrogel stamping of arrays of supported lipid bilayers with various lipid compositions for the screening of drug-membrane and protein-membrane interactions. Angew. Chem. Int. Ed., 44, 6697-6700. 

The impact of hemodialysis on drug therapy depends on drug characteristics (e.g., molecular weight, protein binding, and Vp), dialysis prescription (e.g., dialysis membrane composition, filter surface area, blood and dialysate flow rates, and reuse of the dialysis filter), and clinical indication for dialysis. 

Recent results indicate that not only topogenic signals and membrane composition contribute to the proper topology of a membrane protein. The antimicrobial peptide nisin, produced by Lactococcus lactis, kills Gram-positive bacteria via pore formation, thus leading to the permeabilisation of the membrane. Nisin depends on the cell-wall precursor Lipid II, which functions as a docking molecule to support a perpendicular stable transmembrane orientation [43]. 

One approach to understanding membrane function is to study membrane composition—to determine, for example, which components are common to all membranes and which are unique to membranes with specific functions. So before describing membrane structure and function we consider the molecular components of membranes proteins and polar lipids, which account for almost all the mass of biological membranes, and carbohydrates, present as part of glycoproteins and glycolipids. 

The ADSA Committee on Milk Protein Nomenclature (Eigel et al. 1984) presented a tentative nomenclature for the new enzyme membrane proteins. While the primary structures of these proteins have not been established, sufficient information exists to obtain an operational definition. The total protein complement of the membrane as observed is dependent upon the past history of the membrane from its formation to its analysis. Both the temperature and the time of storage before analysis can alter the membrane composition and physical state (Wooding 1971). In addition, plasmin has been shown to be associated with preparations of the membrane, and proteolytic products of the membrane protein have been observed in milk (Hoffman et al. 1979 Kanno and Yamauchi 1979). Therefore, one should use fresh warm raw milk for the study of the native MFGM protein. 

Mather, I. H., Weber, K. and Keenan, T. W. 1977. Membranes of mammary gland. XII. Loosely associated proteins and compositional heterogeneity of bovine milk fat globule membrane. J. Dairy Sci. 60, 394-402. 

Several reports have shown that the kinetics of P-gp transport activity can be sufficiently described by one-site Michaelis-Menten saturable kinetics (199-206). Where JP.g ) is the flux mediated by P-gp transport activity,, /max is the maximal flux mediated by P-gp transport activity, Km is the Michaelis-Menten constant, and Ct is the concentration of substrate present at the target (binding) site of P-gp. When donor concentration is used in place of Ct, apparent Km and Jmax values are obtained. Binding affinity of the substrate to P-gp and the catalytic (ATPase) activity of P-gp combine to determine Km, and, /max is determined by the catalytic (ATPase) activity of P-gp and the expression of P-gp in the system (concentration of P-gp protein). It has recently been noted that since substrates must first partition or cross the membrane to access the binding site, accurate assessing of P-gp kinetics can be difficult (207). Furthermore, the requirement of first partitioning into the membrane has been shown to produce asymmetric apparent kinetics in polarized cells where AP and BL membrane compositions may be sufficiently different (206). 

Artificial membranes are used to study the influence of drug structure and of membrane composition on drug-membrane interactions. Artificial membranes that simulate mammalian membranes can easily be prepared because of the readiness of phospholipids to form lipid bilayers spontaneously. They have a strong tendency to self-associate in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [74], and the mechanism of vesicle (synonym for liposome) formation has been reviewed [75]. While the individual components of membranes, proteins and lipids, are made up of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by hydrophobic effects. The hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and solid state. 

Membrane Composition and Functioning of Membrane-embedded Proteins... 

The isolation of lipids from cells or tissues is not as simple and straightforward as one might desire, but is essentially an important adjunct to characterization of membranes (composition, lipid-to-protein ratio, structure proof, definition, new lipids, etc.). While this is recognized by many investigators in the field, it is difficult for the novice in this area to become aware of some of the potential problems in extraction procedures and the reasons for particular approaches. Thus it seems fitting at this point in time to comment on some of the nuances of the approaches used in isolation, purification, and identification of lipids present in cell membranes. These topics are subdivided into areas which are considered to be of major import to a successful consideration of the extraction procedure. 

The primary constituents of membranes are lipid and protein carbohydrate residues are found in relatively small amounts attached to members of both major classes. Any organism has fairly constant membrane composition, but membranes vary widely from organism to organism and among various kinds of cells. For example, membranes vary from 18% protein, 79% lipid, and 3% other compounds in the myelin sheath, to 75% protein and 25% lipid in the cell membrane of gram-positive bacteria. A typical figure would be about 60% protein and 40% lipid. 

The mutations in rpoB and rpoC in DapR S. aureus strains may alter the patterns of transcription, thus influencing membrane composition or key target protein levels associated with the dual mechanism of action. 

After this first series of experiments we incubated six devices in six different lysozyme solutions for 30 min. The results shown in Fig. 7 show a clear dependence of ArPH7 on the lysozyme concentration in the sample solution. We stirred during the 30 min incubation process in order to reduce protein concentration gradients in the solution. A different membrane composition may further im-... 

These results show that without any optimization of membrane composition, measurement of lysozyme in concentrations ranging from 3E-7 to 3E-4 M is possible with the pH step titration method. In order to lower the detection limit and to improve the device-to-device reproducibility, other membrane materials may have to be investigated. Especially for selective protein detection, requiring specific receptor molecules deposited in the membrane, more hydrophilic membrane materials are probably necessary, since the hydrophobic polystyrene beads are known to adsorb easily proteins aspecifically. 

Hydrolysis with peracetic acid reveals a membrane composition of about 2/3 protein and 1/3 lipid, plus small amounts of carbohydrates (32). This protein incorporates nearly 13% proline, an amino acid that prevents normal helix formation (68, 75). Membranes also contain about 7 % of half-cystine units this composition may permit substantial disulfide bonding (68). The formation of cross linkages like those in insect cuticle is postulated (32). A common origin of membrane and KH proteins is suggested by their similarly high contents of proline and half-cystine residues (75). 

The HIV protein gp41 contains a relatively small 23-residue fusion peptide, FP23, which mediates the fusion of the virus membrane with the target cell [101,102], Different studies have shown that FP23 has a pronounced conformational plasticity and can change between a-helix and [3-sheet, depending upon membrane composition and/or peptide concentration. Previously an a-helical, T-state-like structure [103-105], but also a [3-shcctcd assembly... 

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