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Lipids: Chapter 10

Lipids: Chapter 10. Major characteristic: hydrophobicity (water insolubility) But typically amphipathic Lipophilic (hydrophobic) chain Polar/charged (hydrophilic) headgroup. “Fatty acids”. “Fatty” = lipid component: chain of hydrocarbons, “acid” = carboxylic acid (hydrophilic headgroup)

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Lipids: Chapter 10

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  1. Lipids: Chapter 10 • Major characteristic: hydrophobicity (water insolubility) • But typically amphipathic • Lipophilic (hydrophobic) chain • Polar/charged (hydrophilic) headgroup

  2. “Fatty acids” • “Fatty” = lipid component: chain of hydrocarbons, “acid” = carboxylic acid (hydrophilic headgroup) • Lipids components interact via hydrophobic and van der Waals forces • Stronger forces = higher melting points, less fluidity • Variation in chains: determines physical properties of the lipid portion (mp, fluidity, water solubility) • Length: ~4-40 carbons in the backbone • Longer chains = stronger forces • “Saturation”: refers to # double bonds (fully saturated means saturated with hydrogens, no double bonds) • Unsaturation: lack of free rotation around double bond • Typically in ‘cis’ conformation: introduces a kink in the chain • Reduces intermolecular interactions

  3. Saturation

  4. Derivation of the carboxylic acid • Commonly ester linkage to glycerol • Three positions for acylation

  5. Storage lipids • Energy storage: triacylglycerol • Three fatty acid groups linked to glycerol • Efficient relative to sugar • Energy in C-C bonds is higher • Water insolubility aids storage • But sugars are better as ‘quick’ sources of energy • Waxes • Typically solid (vs. oils) • What types of lipids will form waxes? • Length? Saturation?

  6. Membrane Lipids • Lipid bilayer: lipid component cluster together, polar headgroup exposed to aqueous environments • Again, these lipids are amphipathic • Types of membrane lipids: • Glycerophosphates • Galactolipids/sulfolipids • Tetraether lipids • Sphingolipids • Sterols

  7. Glycerophospholipids • Glycerol: three –OH groups • Glycerophospholipids: • Phosphate plus polar/charged group • Fatty acid ester • Fatty acid ester

  8. Polar headgroup constituents of GPLs -3?

  9. Phosphatidylinositol can be phosphorylated enzymatically on multiple –OH groups“Combinatorial complexity” PI (net -1) PI(4)P 5-kinase PI 4-kinase adds phosphate to 4 position adds phosphate to 5 position PI (4,5) P2 PI (4) P (-3) (-2) PI(4,5)P2 3-kinase PI(3,4,5)P3 (-4)

  10. Galactolipids • Plant-specific • Similar to glycerophospholipids • Glycerol “backbone” with two fatty acid esters • Polar headgroup: no phosphate linkage, typically galactose (polar) or sulfonated galactose (charged)

  11. Sphingolipids • Common part of mammalian membranes • Sphingosine backbone • Similar idea as glycerol • Intrinsic long chain • Fatty acid attached in amide linkage • Polar headgroup

  12. Mammalian cell membraneLiver cell plasma membrane: Percent of total lipid by mass • Phosphatidylcholine: 24 • Sphingomyelin: 19 • Cholesterol: 17 • Phosphatidylethanolamine: 7 • Glycolipids: 7 • Phosphatidylserine: 4 • Others: 22 Alberts: The Molecular Biology of the Cell

  13. But, lipid composition is dynamiceg. phosphorylation of phosphatidylinositoleg. enzymatic addition/removal of lipid headgroupseg. removal/addition (typically through vesicles) of lipids: changing fatty acid compositionWhy multiple compositions?ie. what effect do lipids have on cell function?

  14. Effects of lipid composition of cell physiology • Membrane fluidity • Length/saturation of the fatty acid chain • Attraction/repulsion among headgroups • Activity of integral membrane proteins • Lipids act as the solvent • Composition of the membrane • can have drastic effects on the • proteins’ activities

  15. Effects of lipid composition of cell physiology 3. Binding sites for peripheral membrane proteins PH domain from DAPP1 binding to PI (3,4,5)P3

  16. Effects of lipid composition of cell physiology • Precursors to other molecules: • Membrane acts as a store of other important compounds: typically released enzymatically Hydrolysis by phospholipase enzymes Phospholipase A1/2: yield glycerophospholipid plus fatty acid Phospholipase C: yields diacylglycerol plus free phosphorylated headgroup Phospholipase D: yields phosphatidic acid (ie. phosphorylated DAG) plus free headgroup

  17. Role of cholesterol • Amphipathic compound: incorporates into lipid bilayer • Disrupts close packing of lipid chains: increases membrane fluidity • Precursor to steroid hormones (eg. estrogen, testosterone) • Excess (water-insoluble) cholesterol can clog the arteries during transport

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