The membrane lipid composition in an average mammalian cell

Large scale conformational transitions can occur in lipid bilayers. These include bud formation, formation of vesicles which split off from the membrane, and fusion of membrane vesicles, all of which are important biologically. To some extent, the tendency to engage in these larger shape transitions depends on the local curvature of the vesicle, which depends on the local phospholipid composition and fluidity of the membrane. We have previously seen that segregation of lipids into rafts (or domains) within a leaflet depends on the phospholipid content. Baumgart et al. have made giant unilamellar vesicles (GUV) containing a mixture of three lipids: sphingomyelin (SM), dioleylphosphatidylcholine (DOPC), and cholesterol (C) to study transitions in the bilayers. SM and C segregate laterally into a "liquid phase" domain characterized by significant order (Lo) while DOPC forms a "liquid phase" with more disorder (Ld). They could detect the different phases through fluorescence microscopy of GUV's labeled with fluorophores (molecules that absorb visible or UV light and emit photons of lower energy (higher wavelength). The first, N-lissamine rhodamine dipalmitoylphosphatidylethanolamine (red fluorescence), partitions almost exclusively into the Ld phase. The second, perylene (blue fluorescence), partitioned with great selectivity into the Lo phase. The results of their studies are shown in the figures below. This images provide stunning visualization of lipid domains (rafts) including those in the shapes of circles, stripes, and rings. Their works shows that the boundary between the domains is important in determining lipid structures. Structures are favored which reduce the perimeter at the boundaries of the phases. It appears that the Ld phase (and associated lipids) is found preferentially in area of high membrane curvature, while the Lo phase is found is regions of lower curvature. The two figures below are reprinted with permission of Nature and the authors: Baumgart, Hess, and Webb, W. Imaging coexisting fluid domains in biomembrane models coupling curvature and line tension. Nature. 425, pg 821 (2003)

Cell Membrane Lipids: More than Fat Chance | Evolution …

30/10/2017 · Unlike plasma membrane where separated “lipid raft” domains have ..

the plasma-membrane lipid bilayer ..

Rafts probably bind or exclude binding of other biological molecules like proteins. Some proteins are chemically modified with a glycosylphosphoinositol (GPI) group at the carboxy terminus. The PI group can insert into the membrane, anchoring the protein to the bilayer. Protein also appear to induce raft formation. Lipids rafts appear to be enriched in GPI-anchored proteins. Recent studies have shown that the Ebola virus interacts with lipid rafts in the process of entering and exiting the infected cell. Rafts are also involved in how cells sense and respond to their environment. Signaling molecules on the outside of the cell can bind receptor proteins in the membrane. As we will see later, conformational changes in the receptor protein signals the inside of the cells that the receptor is bound with a ligand. Once bound, the receptor can move in the membrane and often cluster in outer leaflet rafts that contain cholesterol and sphingolipids. Inner leaflet rafts have also been observed. The figure below shows two versions of an animated version of a lipid raft. The large shapes represent membrane proteins selectively found in the rafts (a topic which will be discussed in ). The most modern definition of a lipid raft is a .

Plasma Membrane and Intracellular Lipid Synthesis in …

Lipids in membranes are often distributed asymmetrically. The inner and outer leaflet of a biological membrane usually have different PL compositions. For example, in red blood cell membranes, the outer leaflet is composed mostly of sphingomylein (SM) and PC, while the inner leaflet is composed mostly of PE and phosphatidyl serine (PS). This phospholipid contains the amino acid linked through its side chain (-CH2OH) to phosphate in position 3 of diacylglyerol. With a negative charge on the phosphate and carboxylate and a positive charge on the amine of PS, this phospholipid is acidic with a net negative charge. All the PS is located in the inner leaftet! This observation will become important latter on, when we discuss programmed cell death. A dying cell will expose PS in the outer leaflet. This is in fact one of the markers of a dying cell.

These isoprenylated proteinshave key roles in membrane attachment leading to central functionality in cellbiology and pathology.

Bile Acid Synthesis, Metabolism and Biological Functions

Eukaryotic cell-free synthesis was used to incorporate the large and complex multispan plant membrane transporter Bot1 in a functional form into a tethered bilayer lipid membrane. The electrical properties of the protein-functionalized tethered bilayer were measured using electrochemical impedance spectroscopy and revealed a pH-dependent transport of borate ions through the protein. The efficacy of the protein synthesis has been evaluated using immunoblot analysis.

The end products of cholesterol utilization are the bile acids

Lipogenesis occurs both in liver and adipose tissuesresulting in the synthesis of fatty acids fromacetyl CoA synthesized by glycolysis (). Acetyl CoA is then carboxylatedby ACC forming malonyl CoA. Malonyl CoA and acetyl CoA are furtherprocessed by fatty acid synthase (FAS) in palmitic acid, which isthen transformed by Elvol6 into stearic acid (). SCD1 catalyzes the formation ofpalmitoleoyl-CoA and oleoyl-CoA from palmitoyl-CoA andstearoyl-CoA, respectively (),which are preferentially transformed in triglycerides for storagein adipose tissue or phospholipids for membrane formation ().

Lipid Products | 14:0 PC (DMPC) | 850345 - Avanti Polar Lipids

Those lipids with double labels (TNB and 32 P) must have flipped from the inner leaftlet to the outer leaftlet where they could be labeled with TNBS. The cells incubated for 3 minutes before the addition of TNBS have a much higher level of doubly labeled PL's. Quantitating these data as a function of differing time of incubation at elevated temperatures show that the rate of flip-flop diffusion is much higher in cells than liposomes, which suggests that the process is catalyzed, presumably by a protein transporter (flipase or Transbilayer amphipath transporter - TAT) in cells.