Phospholipid | EPFL Graph Search

Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typically have omega-3 fatty acids EPA and DHA integrated as part of the phospholipid molecule. The phosphate group can be modified with simple organic molecules such as choline, ethanolamine or serine. Phospholipids are a key component of all cell membranes. They can form lipid bilayers because of their amphiphilic characteristic. In eukaryotes, cell membranes also contain another class of lipid, sterol, interspersed among the phospholipids. The combination provides fluidity in two dimensions combined with mechanical strength against rupture. Purified phospholipids are produced commercially and have found applications in nanotechnology and materials science. The first phospholipid identified in 1847 as such in biological tissues was lecithin, o

Lipid membranes for the fabrication of functional micro- and nano-structures

Gopakumar Gopalakrishnan

The central goal of this thesis work is to fabricate novel, functional fluorescent nanostructures in confined systems offered by phospholipid membranes, which are known to have highly ordered, thermotropic and lyotropic structures. In separate approaches, we have used three different lipid systems: multilamellar planar lipid membranes, unilamellar vesicular membranes as well as lipid monolayers for the development of functional fluorescent nano-, micro- and meso-scopic structures. Techniques like fluorescence microscopy, single particle imaging, electron microscopy, electron diffraction were used to achieve fundamental understanding of the resulting structures. Multilayer stacks of phospholipid membranes have been used as an effective template for the growth of high aspect ratio fluorescent nanowires. The room temperature synthesis was achieved in the confined nanometer-sized interlamellar water space of lipid multilayers where supersaturating CdCl2 concentrations were induced by acidification leading to controlled unidirectional growth of nanowires. The possibility to render the nanowires fluorescent by doping with CdS quantum dots (QDs) and the light waveguiding along hundreds of micrometers together with the possibility of lateral manipulation make these nanowires attractive candidates for future optoelectronic applications. Novel organic-inorganic functional nanocontainers have been designed and tested by making use of vesicle forming lipid bilayers in combination with semiconductor QDs. Hydrophobic QDs can be integrated into bilayers of lipid vesicles and such lipid/QD hybrid vesicles are capable to fuse with live cells, thereby stain the cell's plasma membrane selectively with fluorescent QDs and transfer the vesicle's cargo into the cell. Modification of the membrane of such hybrid vesicles on the other hand, made them capable to enter the cytoplasm of live cells. Additionally, these hybrid vesicles were found extremely useful for long-term model membrane imaging studies. The results described in this thesis imply that cell and lipid membranes can integrate any kind of hydrophobic nanoparticle whose size matches the membrane thickness, opening novel possibilities to manipulate them as individuals or in ensemble with wide-ranging applications for nanobiotechnology. In a further step, the ability of phospholipid molecules to exhibit lamellar to non-lamellar transition using external stimuli enabled a directed self-assembly of QDs into mesoscale fluorescent structures. An easy and versatile method for the surface modification of TOPO coated CdSe QDs using 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid (DPPA) have been achieved. DPPA predominantly form non-lamellar phases when dispersed in water, for example, upon addition of Ca2+ or at a pH below 6 are known to form hexagonal II phases. This particular property of DPPA has been exploited to form mesoscale self-assemblies of QD based structures both in solution and in confined systems. Potential applications include the detection or removal of Ca2+ ions in attoliter volumes, the construction of functional devices where QDs are reversibly organized in different forms as well as use of fluorescently labeled DPPA molecules for cellular studies using fluorescence microscopy.

EPFL2006

Critical roles of isoleucine-364 and adjacent residues in a hydrophobic gate control of phospholipid transport by the mammalian P4-ATPase ATP8A2

Matteo Dal Peraro, Thomas Lemmin, Anna Vestergaard

P4-ATPases (flippases) translocate specific phospholipids such as phosphatidylserine from the exoplasmic leaflet of the cell membrane to the cytosolic leaflet, upholding an essential membrane asymmetry. The mechanism of flipping this giant substrate has remained an enigma. We have investigated the importance of amino acid residues in transmembrane segment M4 of mammalian P4-ATPase ATP8A2 by mutagenesis. In the related ion pumps Na+, K+-ATPase and Ca2+-ATPase, M4 moves during the enzyme cycle, carrying along the ion bound to a glutamate. In ATP8A2, the corresponding residue is an isoleucine, which recently was found mutated in patients with cerebellar ataxia, mental retardation, and dysequilibrium syndrome. Our analyses of the lipid substrate concentration dependence of the overall and partial reactions of the enzyme cycle in mutants indicate that, during the transport across the membrane, the phosphatidylserine head group passes near isoleucine-364 (I364) and that I364 is critical to the release of the transported lipid into the cytosolic leaflet. Another M4 residue, N359, is involved in recognition of the lipid substrate on the exoplasmic side. Our functional studies are supported by structural homology modeling and molecular dynamics simulations, suggesting that I364 and adjacent hydrophobic residues function as a hydrophobic gate that separates the entry and exit sites of the lipid and directs sequential formation and annihilation of water-filled cavities, thereby enabling transport of the hydrophilic phospholipid head group in a groove outlined by the transmembrane segments M1, M2, M4, and M6, with the hydrocarbon chains following passively, still in the membrane lipid phase.

Natl Acad Sciences2014

Lipid membranes for the fabrication of functional micro- and nano-structures

Gopakumar Gopalakrishnan

EPFL2006