1 / 41

Lipids II; Membranes

Lipids II; Membranes. Andy Howard Introductory Biochemistry 25 September 2008. Lipids Plasmalogens Glycosphingolipids Isoprenoids Steroids Other lipids Membranes Bilayers Fluid mosaic model Physical properties Lipid Rafts Membrane proteins. Membrane transport Passive & active

nerita
Télécharger la présentation

Lipids II; Membranes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lipids II; Membranes Andy HowardIntroductory Biochemistry25 September 2008 Biochemistry: Lipid2/Membranes

  2. Lipids Plasmalogens Glycosphingolipids Isoprenoids Steroids Other lipids Membranes Bilayers Fluid mosaic model Physical properties Lipid Rafts Membrane proteins Membrane transport Passive & active Thermodynamics Pores & Channels Protein-mediated transport What we’ll discuss Biochemistry: Lipid2/Membranes

  3. iClicker quiz question 1 • What is the most common fatty acid in soybean triglycerides? • (a) Hexadecanoate • (b) Octadecanoate • (c) cis,cis-9,12-octadecadienoate • (d) all cis-5,8,11,14-eicosatetraeneoate • (e) None of the above Biochemistry: Lipid2/Membranes

  4. iClicker quiz, question 2 • Which set of fatty acids would you expect to melt on your breakfast table? • (a) fatty acids derived from soybeans • (b) fatty acids derived from olives • (c) fatty acids derived from beef fat • (d) fatty acids derived from bacteria • (e) either (c) or (d) Biochemistry: Lipid2/Membranes

  5. iClicker quiz question 3 • Suppose we constructed an artificial lipid bilayer of dipalmitoyl phosphatidylcholine (DPPC) and another artificial lipid bilayer of dioleyl phosphatidylcholine (DOPC).Which bilayer would be thicker? • (a) the DPPC bilayer • (b) the DOPC bilayer • (c) neither; they would have the same thickness • (d) DOPC and DPPC will not produce stable bilayers Biochemistry: Lipid2/Membranes

  6. Plasmalogens • Another major class besides phosphatidates • C1 linked via cis-vinyl ether linkage. • n.b. The textbook figure 8.10 is one page later than the discussion of it • Ordinary fatty acyl esterification at C2 • Phosphatidylethanolamine at C3 Biochemistry: Lipid2/Membranes

  7. Specific plasmalogens Biochemistry: Lipid2/Membranes

  8. Roles of phospholipids • Most important is in membranes that surround and actively isolate cells and organelles • Other phospholipids are secreted and are found as extracellular surfactants (detergents) in places where they’re needed, e.g. the surface of the lung Biochemistry: Lipid2/Membranes

  9. Sphingolipids • Second-most abundant membrane lipids in eukaryotes • Absent in most bacteria • Backbone is sphingosine:unbranched C18 alcohol • More hydrophobic than phospholipids Biochemistry: Lipid2/Membranes

  10. Varieties of sphingolipids SphingomyelinImage on steve.gb.com • Ceramides • sphingosine at glycerol C3 • Fatty acid linked via amideat glycerol C2 • Sphingomyelins • C2 and C3 as in ceramides • C1 has phosphocholine Biochemistry: Lipid2/Membranes

  11. Cerebrosides • Ceramides with one saccharide unit attached by -glycosidic linkage at C1 of glycerol • Galactocerebrosides common in nervous tissue Biochemistry: Lipid2/Membranes

  12. Gangliosides • Anionic derivs of cerebrosides (NeuNAc) • Provide surface markers for cell recognition and cell-cell communication Biochemistry: Lipid2/Membranes

  13. Isoprenoids • Huge percentage of non-fatty-acid-based lipids are built up from isoprene units • Biosynthesis in 5 or 15 carbon building blocks reflects this • Steroids, vitamins, terpenes • Involved in membrane function, signaling, feedback mechanisms, structural roles Biochemistry: Lipid2/Membranes

  14. Steroids • Molecules built up from ~30-carbon four-ring isoprenoid starting structure • Generally highly hydrophobic (1-3 polar groups in a large hydrocarbon); but can be derivatized into emulsifying forms • Cholesterol is basis for many of the others, both conceptually and synthetically Cholesterol:Yes, you need to memorize this structure! Biochemistry: Lipid2/Membranes

  15. Other lipids Image courtesy cyberlipid.org • Waxes • nonpolar esters of long-chain fatty acids and long-chain monohydroxylic alcohols, e.g H3C(CH2)nCOO(CH2)mCH3 • Waterproof, high-melting-point lipids • Eicosanoids • oxygenated derivatives of C20 polyunsaturated fatty acids • Involved in signaling, response to stressors • Non-membrane isoprenoids:vitamins, hormones, terpenes Images Courtesy Oregon State Hort. & Crop Sci. Biochemistry: Lipid2/Membranes

  16. Example of a wax • Oleoyl alcohol esterified to stearate (G&G, fig. 8.15) Biochemistry: Lipid2/Membranes

  17. Isoprene units: how they’re employed in real molecules • Can be linked head-to-tail • … or tail-to-tail (fig. 8.16, G&G) Biochemistry: Lipid2/Membranes

  18. Membranes • Fundamental biological mechanism for separating cells and organelles from one another • Highly selective barriers • Based on phospholipid or sphingolipid bilayers • Contain many protein molecules too(50-75% by mass) • Often contain substantial cholesterol too:cf. modeling studies by H.L. Scott Biochemistry: Lipid2/Membranes

  19. Solvent Bilayers • Self-assembling roughly planar structures • Bilayer lipids are fully extended • Aqueous above and below, apolar within Solvent Biochemistry: Lipid2/Membranes

  20. Salmonella ABC transporter MsbAPDB 3B603.7Å2*64 kDa Fluid Mosaic Model • Membrane is dynamic • Protein and lipids diffuse laterally;proteins generally slower than lipids • Some components don’t move as much as the others • Flip-flops much slower than lateral diffusion • Membranes are asymmetric • Newly synthesized components added to inner leaflet • Slow transitions to upper leaflet(helped by flippases) Biochemistry: Lipid2/Membranes

  21. Fluid Mosaic Model depicted Courtesy C.Weaver, Menlo School Biochemistry: Lipid2/Membranes

  22. Physical properties of membranes • Strongly influenced by % saturated fatty acids: lower saturation means more fluidity at low temperatures • Cholesterol percentage matters too:disrupts ordered packing and increases fluidity (mostly) Biochemistry: Lipid2/Membranes

  23. Chemical compositions of membranes (fig. 9.10, G&G) Biochemistry: Lipid2/Membranes

  24. Lipid Rafts • Cholesterol tends to associate with sphingolipids because of their long saturated chains • Typical membrane has blob-like regions rich in cholesterol & sphingolipids surrounded by regions that are primarily phospholipids • The mobility of the cholesterol-rich regions leads to the term lipid raft Biochemistry: Lipid2/Membranes

  25. Significance of lipid rafts:still under discussion • May play a role as regulators • Sphingolipid-cholesterol clusters form in the ER or Golgi and eventually move to the outer leaflet of the plasma membrane • There they can govern protein-protein & protein-lipid interactions • Necessary but insufficient for trafficking • May be involved in anaesthetic functions:Morrow & Parton (2005), Traffic 6: 725 Biochemistry: Lipid2/Membranes

  26. Membrane Proteins • Many proteins associate with membranes • But they do it in several ways • Integral membrane proteins:considerable portion of protein is embedded in membrane • Peripheral membrane proteins:polar attachments to integral membrane proteins or polar groups of lipids • Lipid-anchored proteins:protein is covalently attached via a lipid anchor Biochemistry: Lipid2/Membranes

  27. Integral(Transmembrane) Proteins Drawings courtesy U.Texas • Span bilayer completely • May have 1 membrane-spanning segment or several • Often isolated with detergents • 7-transmembrane helical proteinsare very typical (e.g. bacteriorhodopsin) • Beta-barrels with pore down the center: porins Biochemistry: Lipid2/Membranes

  28. Peripheral Membrane proteins • Also called extrinsic proteins • Associate with 1 face of membrane • Associated via H-bonds, salt bridges to polar components of bilayer • Easier to disrupt membrane interaction:salt treatment or pH Chloroflexus auracyanin PDB 1QHQ1.55Å15.4 kDa Biochemistry: Lipid2/Membranes

  29. Lipid-anchored membrane proteins • Protein-lipid covalent bond • Often involves amide or ester bond to phospholipid • Others: cys—S—isoprenoid (prenyl) chain • Glycosyl phosphatidylinositol with glycans Biochemistry: Lipid2/Membranes

  30. N- Myristoylation & S-palmitoylation Biochemistry: Lipid2/Membranes

  31. Membrane Transport • What goes through and what doesn’t? • Nonpolar gases (CO2, O2) diffuse • Hydrophobic molecules and small uncharged molecules mostly pass freely • Charged molecules blocked Biochemistry: Lipid2/Membranes

  32. Transmembrane Traffic:Types of Transport (Table 9.3) Type Protein Saturable Movement Energy Carrier w/substr. Rel.to conc. Input? Diffusion No No Down No Channels Yes No Down No & pores Passive Yes Yes Down No transport Active Yes Yes Up Yes Biochemistry: Lipid2/Membranes

  33. Cartoons of transport types • From accessexcellence.org Biochemistry: Lipid2/Membranes

  34. Thermodynamics ofpassive and active transport • If you think of the transport as a chemical reaction Ain Aout or Aout Ain • It makes sense that the free energy equation would look like this: • Gtransport = RTln([Ain]/[Aout]) • More complex with charges;see eqns. 9.4 through 9.6. Biochemistry: Lipid2/Membranes

  35. Example • Suppose [Aout] = 145 mM, [Ain] = 10 mM,T = body temp = 310K • DGtransport = RT ln[Ain]/[Aout]= 8.325 J mol-1K-1 * 310 K * ln(10/145)= -6.9 kJ mol-1 • So the energies involved are moderate compared to ATP hydrolysis Biochemistry: Lipid2/Membranes

  36. Charged species • Charged species give rise to a factor that looks at charge difference as well as chemical potential (~concentration) difference • Most cells export cations so the inside of the cell is usually negatively charged relative to the outside Biochemistry: Lipid2/Membranes

  37. Quantitative treatment of charge differences • Membrane potential (in volts  J/coul):DY = Yin - Yout • Gibbs free energy associated with change in electrical potential isDGe = zFDYwhere z is the charge being transported and F is Faraday’s constant, 96485 JV-1mol-1 • Faraday’s constant is a fancy name for 1. Biochemistry: Lipid2/Membranes

  38. Faraday’s constant • Relating energy per moleto energy per coulomb: • Energy per mole of charges,e.g. 1 J mol-1, is1 J / (6.022*1023 charges) • Energy per coulomb, e.g, 1 V = 1 J coul-1, is1 J / (6.241*1018 charges) • 1 V / (J mol-1) =(1/(6.241*1018)) / (1/(6.022*1023) = 96485 • So F = 96485 J V-1mol-1 Biochemistry: Lipid2/Membranes

  39. Total free energy change • Typically we have both a chemical potential difference and an electrical potential difference so • DGtransport = RTln([Ain]/[Aout]) + zFDY • Sometimes these two effects are opposite in sign, but not always Biochemistry: Lipid2/Membranes

  40. Pores and channels • Transmembrane proteins with centralpassage for small molecules,possibly charged, to pass through • Bacterial: pore. Usually only weakly selective • Eukaryote: channel. Highly selective. • Usually the DGtransport is negative so they don’t require external energy sources • Gated channels: • Passage can be switched on • Highly selective, e.g. v(K+) >> v(Na+) Rod MacKinnon Biochemistry: Lipid2/Membranes

  41. Protein-facilitated passive transport • All involve negative DGtransport • Uniport: 1 solute across • Symport: 2 solutes, same direction • Antiport: 2 solutes, opposite directions • Proteins that facilitate this are like enzymes in that they speed up reactions that would take place slowly anyhow • These proteins can be inhibited, reversibly or irreversibly Diagram courtesy Saint-Boniface U. Biochemistry: Lipid2/Membranes

More Related