Phosphatidylethanolamine structure

The visual pigment present in rods has been termed rhodopsin and consists of 11-m-retinal, a derivative of vitamin A1( and a lipoprotein called opsin. Recent evidence(43) suggests that in native rhodopsin the retinal chromo-phore is covalently bonded to a phosphatidylethanolamine residue of the lipid portion of opsin. The structure of 1 l-cis-retinal is as follows ... 

Figure 2 Structure of functionalized anchors and heterobifunctional reagents. DPPE is given as an example. Abbreviations DPPE, l,2-dipalmitoyl-sn-glycero-3-phos-phoethanolamine DPPE-AcBr, bromoacetyl dipalmitoyl phosphatidylethanolamine.

Lipid transfer peptides and proteins occur in eukaryotic and prokaryotic cells. In vitro they possess the ability to transfer phospholipids between lipid membranes. Plant lipid transfer peptides are unspecific in their substrate selectivity. They bind phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and glycolipids. Some of these peptides have shown antifungal activity in vitro The sequences of lipid transfer proteins and peptides contain 91-95 amino acids, are basic, and have eight cysteine residues forming four disulfide bonds. They do not contain tryptophan residues. About 40% of the sequence adopts a helical structure with helices linked via disulfide bonds. The tertiary structure comprises four a-helices. The three-dimensional structure of a lipid transfer peptide from H. vulgare in complex with palmitate has been solved by NMR. In this structure the fatty acid is caged in a hydrophobic cavity formed by the helices. 

The Guy open conformation model docked structure was minimized in vacuo followed by a 1-ns molecular dynamics simulation of the complex embedded in a phosphatidylethanolamine (POPE) lipid bilayer. Adjustments were made to the model, and simulations were repeated so that very little movement occurred during the hnal iterations. Similar methods were used to dock the two domains in transitional and resting states. However, these results are more tenuous as little experimental data is available. In particular, the position of the S4-S5 linker and its role in opening and closing the pore are uncertain. The supplemental movie accompanying reference 36 illustrates the open-to-close-to-open cycle resulting from the simulations. 

The illustration shows a model of a small section of a membrane. The phospholipids are the most important group of membrane lipids. They include phosphatidylcholine (lecithin), phosphatidylethanolamine, phos-phatidylserine, phosphatidylinositol, and sphingomyelin (for their structures, see p. 50). in addition, membranes in animal cells also contain cholesterol (with the exception of inner mitochondrial membranes). Clycoli-pids (a ganglioside is shown here) are mainly found on the outside of the plasma membrane. Together with the glycoproteins, they form the exterior coating of the cell (the gly-cocalyx). 

A certain type of lipid (or lipid-like) molecules are found that when dispersed in water tend to make self-assembly structures (Figure 4.13). Detergents were shown to aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-assembly structure, which is called liposome or vesicle. 

Phosphatidylethanolamine (PE) is a good coemulsifier for PC. PE like PC also has a zwitterionic structure however, it does not form a bilayer membrane in water (Table 8). 

Glycerophospholipids differ in the structure of their head group common gycerophospholipids are phosphatidylethanolamine and phosphatidylcholine. The polar heads of the gycerophospholipids carry electric charges at pH near 7. 

Hay and Morrison (1971) later presented additional data on the fatty acid composition and structure of milk phosphatidylethanolamine and -choline. Additionally, phytanic acid was found only in the 1-position of the two phospholipids. The steric hindrance presented by the four methyl branches apparently prevents acylation at the 2-position. The fairly even distribution of monoenoic acids between the two positions is altered when the trans isomers are considered, as a marked asymmetry appears with 18 1 between the 1- and 2-positions of phosphatidylethanolamine, but not of phosphatidylcholine. Biologically, the trans isomers are apparently handled the same as the equivalent saturates because the latter have almost the same distribution. There are no appreciable differences in distribution of cis or trans positional isomers between positions 1 and 2 in either phospholipid. Another structural asymmetry observed is where cis, cis nonconjugated 18 2s are located mostly in the 2-position in both phospholipids. It appears that one or more trans double bonds in the 18 2s hinders the acylation of these acids to the 2-position. 

Lipids are the major components of membranes they have complex structures comprising fatty acids esterified with alcohols to form glycerides, and other lipids based upon esters of phosphatidylethanolamine. Other important lipid components are based on sterols. Within this hydrophobic structure, proteins provide ports of entry and exit from the interior of the cell and distinguish the inside from the outside of the cell. Figure 5.5 illustrates the complexity of this structure. 

Figure 8.1. The structure of some common phospholipids ( ) phosphatidylcholine, (b) phosphatidylethanolamine and (c) phosphatidic acid.

Guan Z., Wang Y., Cairns N. J., Lantos P L., Dallner G., and Sindelar P. J. (1999). Decrease and structural modifications of phosphatidylethanolamine plasmalogen in the brain with Alzheimer disease. J. Neuropathol. Exp. Neurol. 58 740-747. 

Fig. 2 Chemical structures of the zwitterionic neutral lipids DOPC (1,2-dioleoyl-.s -glycero-3-phosphatidylcholine) and DOPE (l,2-dioleoyl-S7J-glycero-3-phosphatidylethanolamine) and the cationic lipids DOTAP (l,2-dioleoyl-3-trimethylammonium-propane, a UVL) and MVL5 (a custom-synthesized MVL)...

The plasma membrane of fungi has the typical lipid bilayer structure, with phosphatidylcholine, phosphatidylethanolamine, and ergosterol as major lipid con-... 

For measurements between crossed mica cylinders coated with phospholipid bilayers in water, see J. Marra andj. Israelachvili, "Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions," Biochemistry, 24, 4608-18 (1985). Interpretation in terms of expressions for layered structures and the connection to direct measurements between bilayers in water is given in V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayers in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). The bilayer-bilayer interactions are reported in E. A. Evans and M. Metcalfe, "Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by van der Waals attraction," Biophys. J., 46, 423-6 (1984). 

The methodology used here for structure proof of phosphatidylethanolamine (and its analogs) is very similar to that described in Chapter 4 for phosphatidylcholine (and its analogs). Consequently a more limited discussion of the procedures and protocols for phosphatidylethanolamine will be undertaken... 

Phospholipase A2 Action. As in the case of phosphatidylcholine, the above-mentioned phospholipases will attack only the sn-3 form of naturally occurring (as well as synthetic) phosphatidylethanolamine. The products are, of course, lysophosphatidylethanolamine (1 -6>-acyl-2-lyso-.rn-glycero-3-phosphoethanolamine) and the fatty acids (liberated from the sn-2 position). The latter can be analyzed for composition and structure, as the methyl esters, by gas-liquid chromatography coupled with mass spectrometry. Usually these acyl groups are largely the unsaturated types. 

A typical biological membrane is a complex structure composed primarily of lipids and proteins. The major structural components of the bilayer are various lipids. In eukaryotes, the most common type of lipids are phosphatidylcholines, whereas in prokaryotes (such as Escherichia coli), the main lipids are typically phosphatidylethanolamines (1). One example of a typical eukaryotic neutral (zwitterionic) phospholipid is palmitoyl-oleoy 1-phosphatidylcholine (POPC). The molecular structure of POPC is compared to those of dimyristoylphosphatidyl-choline (DMPC) and the negatively charged dimyristoylphosphatidylglycerol (DMPG), commonly used in membrane mimetics, in Fig. 1. 

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