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Chapter 10: Lipids, 10.3: Lipids as Signals, Cofactors, and Pigments, 10.2…

- - - - found in the cytoplasmic face of plasma membranes
      - serves as a reservoir of messenger molecules that are released in response to extracellular signals
        
        phospholipase C hydrolyzes PIP2 to IP3 and diacylglycerol (intracellular messengers)
    - - signaling proteins bind specifically to PIP3 in the plasma membrane, initiating formation of multienzyme complexes at the membrane's cytosolic surface
- - - - contain a five-carbon ring
      - functions:
        
        stimulate contraction of the smooth muscle of the uterus
        
        affect blood flow to specific organs, the wake-sleep cycle, and the responsiveness of certain tissues to hormones
        
        elevate body temperature and cause inflammation and pain
    - - have a six-membered ring containing an ether
      - produced by platelets
      - function: act in the formation of blood clots and reduction of blood flow to the site of a clot
    - - contain three conjugated double bonds
      - function: powerful biological signals-induces contraction of the smooth muscle lining the airways to the lung
    - - linear and contain several hydroxyl groups along the chain
      - function: potent anti-inflammatory agents
- - - - formed in the skin in a photochemical reaction driven by the UV component of sunlight
      - not biologically active
      - converted by enzymes in the liver and kidney to calcitriol
        
        calcitrol: hormone that regulates calcium uptake in the intestine and calcium levels in the kidney and bone
      - deficiency leads to defective bone formation and the disease rickets
      - aka cholecalciferol
    - - aka all-trans-retinol
      - acts in processes of development, cell growth and differentiation, and vision
      - can be stored for some time in the body
      - can be converted enzymatically to all-trans-retinoic-acid
        
        all-trans-retinoic-acid: retinoid hormone that acts through a family of nuclear receptor proteins to regulate gene expression
      - beta-carotene pigment is a source of vitamin A
        
        carotenoids: natural products with a characteristic extensive system of conjugated double bonds, which makes possible their strong absorption of visible light
      - deficiencies:
        
        in pregnant women, leads to congenital malformations and growth retardation in the infant
        
        in adults, leads to dryness of skin, eyes, and mucous membranes, and night blindness
      - golden rice: genetically engineered rice enriched in beta-carotene
    - - Vitamin E
        
        collective name for a group of lipids called tocopherols
        
        tocopherols: hydrophobic compounds that contain a substituted aromatic ring and a long isoprenoid side chain
        
        biological antioxidants
        
        associate with cell membranes, lipid deposits, and lipoproteins
        
        deficiencies:
        
        cause scaly skin, muscular weakness and wasting, and sterility
        
        very rare in humans
        
        principal symptom is fragile erythrocytes
      - Vitamin K
        
        contains an aromatic ring that undergoes a cycle of oxidation and reduction during the formation of active prothrombin
        
        prothrombin: a blood plasma protein essential in blood clotting
        
        warfarin: a synthetic compound that inhibits the formation of active prothrombin
        
        potent rodenticide
        
        invaluable anticoagulant drug for treating humans at risk for excessive blood clotting
        
        deficiencies:
        
        slow blood clotting
        
        extremely uncommon in humans
      - ubiquinone (coenzyme Q) and plastiquinone: isoprenoids that function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively
- - - - a C16 or C18 fatty acid at C-1
      - a C18 or C20 unsaturated fatty acid at C-2
  - - - releases from leukocytes called basophils
      - stimulates platelet aggregation and serotonin release
      - plays a role in inflammation and the allergic response
    - - one or two galactose residues are connected by a glycosidic linkage to C-3 of a 1,2-diacylglycerol
- - - - do not contain phosphate
      - occur largely in the outer face of plasma membranes
      - include...
        
        cerebrosides: have a single sugar linked to ceramide
        
        those with glucose are found in the plasma membranes of cells in nonneural tissues
        
        those with galactose are found in the plasma membranes of cells in neural tissue
        
        Globosides: glycosphingolipids with 2+ sugars, usually D-glucose, D-galactose, or N-acetyl-D-galactosamine
        
        sometimes called neutral glycolipids, as they have no charge at pH 7
        
        gangliosides: have oligosaccharides as their polar head groups and 1+ resideus of N-acetylneuraminic acid, a sialic acid, at the termini
        
        genetic defects in any hydrolytic enzymes leads to accumulation of gangliosides in the cell
- - - - amphipathic
      - polar head group
      - nonpolar hydrocarbon body
      - membrane constituents
      - similar to stigmasterol in plants and ergosterol in fungi
- - - - polyunsaturated FAs: contain more than one double bond in their backbone
        
        omega-3 fatty acids: double bond between C-3 and C-4 relative to the most distant (omega) carbon
        
        omega-6 fatty acids: double bond between C-6 and C-7 relative to the most distant (omega) carbon
    - - for unbranched:
        
        the chain length and number of double bonds, separated by a colon
        
        numbering begins at the carboxyl carbon
        
        positions of double bonds are indicated by delta and a superscript number
    - - usually have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons
      - in monounsaturated fatty acids, the double bond is usually between C-9 and C-10 (delta9)
      - in polyunsaturated fatty acids...
        
        the double bonds are usually delta12 and delta15
        
        double bonds are usually separated by a methylene group
      - double bonds are usually in the cis conformation
    - - have poor solubility in water due to the nonpolar hydrocarbon chain
        
        decreased double bond number decreases solubility
        
        carboxylic acid group is polar and ionized at neutral pH
        
        increased chain length decreases solubility
      - fatty acid melting points:
        
        at room temperature:
        
        saturated fatty acids have a waxy consistency
        
        unsaturated fatty acids are oily liquids
        
        extent of packing depends on degree of saturation
  - - - storage methods:
        
        plants store triacylglycerols in the seeds
        
        vertebrates store triacylglycerols as lipid droplets in adipocytes (fat cells)
      - lipases: enzymes that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuel; found in adipocytes and germinating seeds
      - advantages of using triacylglycerols as stored fuels
        
        because carbon atoms of fatty acids are more reduced than those of sugars, oxidation of fatty acids yields more energy
        
        because triacylglycerols are hydrophobic and unhydrated, the organism does not have to carry the extra weight of water hydration that is associated with stored polysaccharides
      - insulation:
        
        seals, walruses, and other warm-blooded polar animlas are amply padded with triacylglycerols that provide insulation against low termperatures
        
        hibernating animals accumulate huge fat reserves before hibernation, providing insulation and energy storage