Lipids II

1. Lipids II Andy HowardIntroductory Biochemistry, Fall 2014 6 October 2014 Lipids, Membranes

2. Lipids and Membranes • Lipids are critical as energy storage molecules and as components of membranes • Membranes have characteristic structures and functional properties, and the way that substances pass through them is worthy of attention Lipids, Membranes

3. Lipids Periodic table Fatty acid properties Triacylglycerols Glycerophospholipids Plasmalogens Sphingolipids Isoprenoids Steroids Other lipids Membranes Fluid-mosaic model Components Lipid rafts Membrane proteins Plans for Today Lipids, Membranes

4. Triglyceride composition by source • Courtesy Charles Ophardt, Elmhurst College Lipids, Membranes

5. Nomenclature for fatty acids • IUPAC names: hexadecanoic acid, etc. • Trivial names from sources (Table 8.1) • Laurate (dodecanoate) • Myristate (tetradecanoate) • Palmitate (hexadecanoate) • Palmitoleate (cis-9-hexadecenoate) • Stearate (octadecanoate) • Oleate (cis-9-octadecenoate) • Linoleate (cis,cis-9,12-octadecadienoate) • Arachidonate(allcis-5,8,11,14-eicosatetraeneoate) Lipids, Membranes

6. Saturated Fatty Acids Contrast withmelting points of Unsaturated C18 FAs: 16ºC, -5ºC -11ºC;C20, 4 double bonds: -50ºC Lipids, Membranes

7. How fatty acids really appear • Almost always esterified or otherwise derivatized • Most common esterification is to glycerol • Note that glycerol is achiral but its derivatives are often chiral • Triacylglycerols; all three OHs on glycerol are esterified to fatty acids • Phospholipids: 3-OH esterified to phosphate or a phosphate derivative glycerol Lipids, Membranes

8. Triacyl-glycerols • Neutral lipids • R1,2,3 all aliphatic • Mixture of saturated & unsaturated; unsaturatedmore than half • Energy-storage molecules • Yield >2x energy/gram as proteins or carbohydrates, independent of the water-storage issue … • Lipids are stored anhydrously; carbohydrates & proteins aren’t Lipids, Membranes

9. Catabolism of triacylglycerol • Lipases break these molecules down by hydrolyzing the 3-O esters and 1-O esters • Occurs in presence of bile salts(amphipathic derivatives of cholesterol) • These are stored in fat droplets within cells, including specialized cells called adipocytes Cartoon courtesy Adipocyte Proteome Database, Max-Planck Inst. Lipids, Membranes

10. Glycerophospholipids (G&G §8.3) • Also called phosphoglycerides • Primary lipid constituents of membranes in most organisms • Simplest: phosphatides(3-phosphoesters) • Of greater significance: compounds in which phosphate is esterified both to glycerol and to something else with an —OH group on it Lipids, Membranes

11. Glycerophospholipid Types • Generally categorized first by the polar “head” group; secondarily by fatty acyl chains • Usually C-1 fatty acid is saturated • C-2 fatty acid is unsaturated • Think about structural consequences! Cartoon courtesy Nature Scitable Lipids, Membranes

12. Varieties of head groups • Variation on otherphosphoester position • Ethanolamine (R1-4 = H)(—O—(CH2)2—NH3+) • Serine (R4 = COO-)(—O—CH2-CH-(COO-)—NH3+) • Methyl, dimethylethanolamine(—O—(CH2)2—NHm+(CH3)2-m) • Choline (R4=H, R1-3=CH3) (—O—(CH2)2—N(CH3)3+) • Glucose, glycerol . . . Lipids, Membranes

13. Phospholipids aren’t interchangeable! • Phosphatidylcholine and phosphatidylethanolamine are the major components of eukaryotic membranes • Phosphatidylserine and P-inositol tend to be on the inner leaflet only, and are more prevalent in brain tissue than other tissues • Good reference: http://lipidlibrary.aocs.org/ Lipids, Membranes

14. Chirality in common lipids • Fatty acyl chains themselves are generally achiral • Glycerol C2 is often chiral (unless C1 and C3 fatty acyl chains are identical) • Phospholipid polar groups are achiral except for phosphatidylserine and a few others Lipids, Membranes

15. Charges in lipids • Triacylglycerols are neutral • Many phospholipids are zwitterionic (phosphatidylcholine, for example) • Some phospholipids are anionic(phosphatidylglycerol, phosphatidylserine, cardiolipin) • Many protein-lipid interactions in membranes are governed by charge-charge interactions Lipids, Membranes

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

17. iClicker quiz, question 2 2. Which set of fatty acids would you expect to melt most readily 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) Lipids, Membranes

18. iClicker quiz question 3 3. Suppose we constructed an artificial lipid bilayer of dipalmitoylphosphatidylcholine (DPPC) and another artificial lipid bilayer of dioleylphosphatidylcholine (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 Lipids, Membranes

19. Plasmalogens • Ether phospholipids have an ether link to C1 instead of an ester linking • Plasmalogens are ether phospholipids with C1 linked via cis-vinyl ether linkage. • They constitute the other major category of phospholipids besides esterified glycerophospholipids • Ordinary fatty acyl esterification at C2…platelet activating factor has R2 = CH3 • Usually PE or PC at C3 position Lipids, Membranes

20. Specific plasmalogens Lipids, Membranes

21. 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 Lipids, Membranes

22. Sphingolipids (G&G §8.4) • Second-most abundant membrane lipids in eukaryotes • Absent in most bacteria • Backbone is sphingosine:unbranched C18 alcohol • More hydrophobic than phospholipids sphingosine Lipids, Membranes

23. 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 Lipids, Membranes

24. Cerebrosides • Ceramides with one saccharide unit attached by -glycosidic linkage at C1 of glycerol • Galactocerebrosides common in nervous tissue Lipids, Membranes

25. Gangliosides • Anionic derivs of cerebrosides (NeuNAc) • Provide surface markers for cell recognition and cell-cell communication Lipids, Membranes

26. Isoprenoids(G&G §8.6) • 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 Lipids, Membranes

27. Isoprene units: how they’re employed in real molecules • Can be linked head-to-tail • … or tail-to-tail (fig. 8.12, G&G) Lipids, Membranes

28. Steroids (G&G §8.7) • 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! Lipids, Membranes

29. Other lipids Image courtesy cyberlipid.org • Waxes (G&G §8.5) • 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 Image Courtesy Oregon State Hort. & Crop Sci. Lipids, Membranes

30. Example of a wax • Oleoyl alcohol esterified to stearate (G&G, fig. 8.15) Lipids, Membranes

31. 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 Lipids, Membranes

32. Solvent Bilayers • Self-assembling roughly planar structures • Bilayer lipids are fully extended • Aqueous above and below, apolar within Solvent Lipids, Membranes

33. 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) Lipids, Membranes

34. Fluid Mosaic Model depicted Courtesy C.Weaver, Menlo School Lipids, Membranes

35. Physical properties of membranes (G&G §9.1) • 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) Lipids, Membranes

36. Chemical compositions of membranes (fig. 9.10, old G&G) Lipids, Membranes

37. 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 Lipids, Membranes

38. 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), Traffic6: 725 Lipids, Membranes

39. Membrane Proteins (G&G §9.2) • 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 Lipids, Membranes

40. 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 Lipids, Membranes

41. 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 15.4 kDa PDB 1QHQ1.55Å Lipids, Membranes

42. 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 Lipids, Membranes

43. N- Myristoylation & S-palmitoylation Lipids, Membranes

44. 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 Lipids, Membranes

45. 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 Lipids, Membranes

46. Cartoons of transport types • From accessexcellence.org Lipids, Membranes