Phospholipids protein association

Figure 2.1 Structure of the plasma membrane. The plasma membrane is composed of a bilayer of phospholipid molecules. Associated with this bilayer are intrinsic proteins embedded within and spanning the membrane as well as intrinsic proteins found on the external or internal surface of the membrane. Molecules of cholesterol are found in the inner, nonpolar region of the membrane.

The composition of the membrane is about half phospholipid and half protein. In an aqueous medium, phospholipid molecules associate in such a way that their hydrophobic tails exclude water molecules, whereas the hydrophilic heads orient towards the water molecules (Figure 5.1). The result is two layers of phospholipid molecules the outer... 

We observed PKC-like activity in mouse forestomach extract (29). The activity was activated by TPA plus phosphatidylserine in the absence of calcium. Protein associated with this activity waspartially-puriEedby DEAE-cellulose chromatogrs hy. In mice pretreated with CLA 24 hours prior to sacrifice, the PKC-like activity was refractory to activation by TPA and phosphatidylserine. This observation indicates that cis-9, trans-11 incorporated into phospholipid might directly affect the interaction of TPA with PKC. Further, since PKC controls superoxide generation 30 CLA might in this way serve as an indirect antioxidant. 

As a second possibility, lipid-protein interaction must be considered. The red shift might be explained in terms of hydrophobic interaction of the hydrocarbon chains of phospholipids with the protein in such a way that the amide chromophores are transferred to a less polar environment (89). Again, the hypothesis can be tested by removal of lipid. The existence of the red shift in lipid-depleted mitochondria and in lipid-free mitochondrial structural protein shows that lipid-protein interaction is not necessary to produce the ORD spectra characteristic of membranes. It is possible that if some molecular rearrangement occurs during the extraction process, a red shift caused by hydrophobic lipid-protein association could be replaced with a red shift arising from hydrophobic protein-protein association. Such an explanation is unlikely, especially in view of the retention of the unit membrane structure in electron micrographs taken of extracted vesicles (30). On the basis of ORD, then, the most reasonable conclusion is that the red shift need not be assigned to lipid-protein association. 

According to the results of our experiments in vitro, the highest adsorptive capacity with reference to proteins and protein-associated phospholipids and triglycerides of blood serum was characteristic of the highly disperse silica modified with aluminium oxide. The mechanism of the therapeutic effect of HDS on treatment for intestinal infections seems to involve the following major aspects. 

The concentration of phospholipids in the milk fat ranges from 0.5 to 1.0% of the total (Patton and Jensen, 1976 Table 1.1). About 60 to 65% of these phospholipids are associated with the intact milk fat globule membrane (MFGM). The remaining 35 to 40% are found in the aqueous phase associated with protein/membrane fragment material in solution, rather than still attached to the MFGM (Huang and Kuksis, 1967 Patton and Keenan, 1971). 

In vivo, the folding process may be supported by a periplasmic chaperone called Skp. Skp is a 17 kDa protein associated with the plasma membrane that, together with peptidyl prolyl isomerases and disulfideexchanging enzymes, helps folding freshly synthesized proteins in the periplasm (Schafer et al., 1999). Skp binds to partially unfolded polypeptides. Depending on the presence of phospholipids, lipopolysaccharides, and bivalent cations, Skp exists in two conformations, one of which is protease-sensitive (DeCock et al., 1999). Moreover, it was shown that Skp binds to unfolded periplasmic proteins and inserts into phospholipid monolayers, corroborating its putative role as helper in folding and membrane insertion. 

Interactions of proteins with membranes are quite unspecific. Accordingly, typical PH-domain proteins, such as pleckstrin and spectrin, bind with no apparent stereospecificity and with quite low affinity to phosphotidylinositol bisphosphate (PtdInsP2) and IP3 in lipid vesicles (K > 30 (J.M and 40 (xM, respectively). Moreover, whether all PH-domain proteins associate with negatively charged membrane phospholipids is not certain. However, there is the case of PLC-81, where the PH domain is instrumental in bringing the soluble, cytosolic enzyme to its membrane-bound substrate, PtdInsP2. This function is supported by deletion mutants of PLC, lacking the PH domain, which caimot find the membrane-bound substrate. Therefore, while the role of the PH-domain motif in protein-protein interactions and interconnectivity is questionable, its role in membrane attachment of soluble, cytosolic proteins is quite persuasive. 

Similarly, supersaturation of saliva with respect to calcium phosphate salts is the driving force of calculus (i.e., mineralized dental plaque) and sialolith (i.e., salivary duct stones ) formation. In these cases, negatively charged phospholipids play a crucial role Ca + ions bind to the negative charges of such lipids, and inorganic phosphate associates with the bound calcium that forms a Ca-phosphate-phospholipid complex, which is an excellent nucleus of calcium-phosphate deposition. Salivary proteins may also play a role in this process because such complex formation occurs predominantly on lipids that are protein associated. The increase of pH facilitates these processes (13). 

Some peripheral proteins associate with the membrane by interactions with Integral proteins. Llpld-blndlng motifs In other peripheral proteins Interact with the polar head groups of membrane phospholipids (see Table 5-3). 

Explain the following statement The structure of all biomembranes depends on the chemical properties of phospholipids, whereas the function of each specific blomem-brane depends on the specific proteins associated with that membrane. 

Soybean tissue is composed of many cells containing oil, protein and metabolites, which supply energy, nitrogen storage reserves, and other important compounds, respectively, to support the germination of new plants. The triglycerides are stored in discrete bodies called oil bodies or spherozomes. The preponderance of the protein is storage protein, which is concentrated in other discrete bodies known as protein bodies. Most of the phospholipids are associated with membranes around the protein bod-... 

Leeder and Rippon [85] have analyzed the lipid composition of wool fibers after removing surface grease. Continued extraction with solvent removed the beta layers evidenced by electron microscopy however, the extract contained free cholesterol and free fatty acid and triglycerides but negligible quantities of phospholipid normally associated with biological membrane lipids. Koch [86], in his work with internal lipid of human hair, did not report significant quantities of phospholipid. These lipid-protein layers of hair are most likely related structurally to those of the epicuticle. 

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