lipids: chapter 10 major characteristic: hydrophobicity (water insolubility) –but typically...
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Lipids: Chapter 10
• Major characteristic: hydrophobicity (water insolubility)– But typically amphipathic
• Lipophilic (hydrophobic) chain• Polar/charged (hydrophilic) headgroup
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“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
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Saturation
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Derivation of the carboxylic acid
• Commonly ester linkage to glycerol– Three positions for acylation
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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?
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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
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Glycerophospholipids
• Glycerol: three –OH groups
• Glycerophospholipids:1. Phosphate plus polar/charged group
2. Fatty acid ester
3. Fatty acid ester
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Polar headgroup constituents of GPLs
-3?
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Phosphatidylinositol can be phosphorylated enzymatically on multiple –OH groups
“Combinatorial complexity”
PI (net -1)
PI 4-kinase
PI (4) P
PI(4)P 5-kinase
adds phosphateto 4 position
adds phosphateto 5 position
PI (4,5) P2
PI(4,5)P2 3-kinase
PI(3,4,5)P3
(-2) (-3)
(-4)
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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)
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Sphingolipids
• Common part of mammalian membranes• Sphingosine backbone
– Similar idea as glycerol– Intrinsic long chain– Fatty acid attached in amide linkage– Polar headgroup
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Mammalian cell membrane
Liver 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
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But, lipid composition is dynamic
eg. phosphorylation of phosphatidylinositoleg. enzymatic addition/removal of lipid headgroupseg. removal/addition (typically through vesicles) of
lipids: changing fatty acid composition
Why multiple compositions?
ie. what effect do lipids have on cell function?
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Effects of lipid composition of cell physiology
1. Membrane fluidity• Length/saturation of the fatty acid chain• Attraction/repulsion among headgroups
2. Activity of integral membrane proteins
•Lipids act as the solvent•Composition of the membrane can have drastic effects on the proteins’ activities
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3. Binding sites for peripheral membrane proteins
Effects of lipid composition of cell physiology
PH domain from DAPP1 bindingto PI (3,4,5)P3
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Effects of lipid composition of cell physiology
4. Precursors to other molecules:• Membrane acts as a store of other important compounds:
typically released enzymatically
Hydrolysis by phospholipase enzymesPhospholipase 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
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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