chapter 9 - lipids and membranes lipids are essential components of all living organisms lipids are...
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Chapter 9 - Lipids and Membranes
• Lipids are essential components of all living organisms
• Lipids are water insoluble organic compounds
• They are hydrophobic (nonpolar) or amphipathic (containing both nonpolar and polar regions)
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Fig 9.1 Structural relationships of major lipid classes
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Fatty Acids
• Fatty acids differ from one another in: (1) Length of the hydrocarbon tails(2) Degree of unsaturation (double bond)(3) Position of the double bonds in the chain
• Fatty acids - R-COOH (R=hydrocarbon chain) are components of triacylglycerols, glycerophospholipids, sphingolipids
Nomenclature of fatty acids
• Most fatty acids have 12 to 20 carbons• Most chains have an even number of carbons• IUPAC nomenclature: carboxyl carbon is C-1• Common nomenclature: etc. from C-1 • Carbon farthest from carboxyl is
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Prentice Hall c2002 Chapter 9 4
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Prentice Hall c2002 Chapter 9 5
Structure and nomenclature of fatty acids
• Saturated - no C-C double bonds
• Unsaturated - at least one C-C double bond
• Monounsaturated - only one C-C double bond
• Polyunsaturated - two or more C-C double bonds
Double bonds in fatty acids• Double bonds are generally cis
• Position of double bonds indicated by n, where n indicates lower numbered carbon of each pair
• Shorthand notation example: 20:45,8,11,14 (total # carbons : # double bonds, double bond positions)
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(a) Stearate (octadecanoate)
(b) Oleate (cis-9-octadecenoate)
(c) Linolenate (all-cis-9,12,15-octadecatrienoate)
• The cis double bonds produce kinks in the tails of unsaturated fatty acids
Fig. 9.3 Structures of three C18 fatty acids
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Triacylglycerols
• Fatty acids are stored as neutral lipids, triaclyglycerols (TGs)Fats have 2-3 times the energy of proteins or carbohydrates
• TGs are 3 fatty acyl residues esterified to glycerol
• TGs are hydrophobic, stored in fat cells (adipocytes)
Fig 9.5 Structure of a triacylglycerol
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Fig 9.9 Phospholipases hydrolyze phospholipids
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Fig 9.10 Structure of an ethanolamine plasmalogen
• Plasmalogens - C-1 hydrocarbon substituent attached by a vinyl ether linkage (not ester linkage)
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Prentice Hall c2002 Chapter 9 13
Sphingolipids
• Sphingolipids - sphingosine is the backbone abundant in central nervous system tissues
• Ceramides - fatty acyl group linked to C-2 of sphingosine by an amide bond
• Sphingomyelins - phosphocholine attached to C-1 of ceramide
• Cerebrosides - glycosphingolipids with one monosaccharide residue attached via a glycosidic linkage to C-1 of ceramide
• Galactosylcerebrosides - a single -D-galactose as a polar head group
• Gangliosides - contain oligosaccharide chains with N-acetyl-neuraminic acid (NeuNAc) attached to a ceramide
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Fig 9.11
(a) Sphingosine (b) Ceramides(c) Sphingomyelin
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Fig 9.12
• Structure of a galactocerebroside
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Fig 9.13 Ganglioside GM2
(NeuNAc in blue)
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Prentice Hall c2002 Chapter 9 17
Steroids
• Classified as isoprenoids - related to 5-carbon isoprene (found in membranes of eukaryotes)
• Steroids contain four fused ring systems: 3-six carbon rings (A,B,C) and a 5-carbon D ring
• Ring system is nearly planar
• Substituents point either down () or up ()
Fig 9.14 Isoprene Fig 9.15
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Fig 9.15 Structures of
several steroids
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Fig 9.15 Structures of
several steroids
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Cholesterol
• Cholesterol modulates the fluidity of mammalian cell membranes
• It is also a precursor of the steroid hormones and bile salts
• It is a sterol (has hydroxyl group at C-3)
• The fused ring system makes cholesterol less flexible than most other lipids
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Cholesterol esters
• Cholesterol is converted to cholesteryl esters for cell storage or transport in blood
• Fatty acid is esterified to C-3 OH of cholesterol
• Cholesterol esters are very water insoluble and must be complexed with phospholipids or amphipathic proteins for transport
Fig 9.17 Cholesteryl ester
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Waxes• Waxes are nonpolar esters of long-chain fatty acids and
long chain monohydroxylic alcohols
• Waxes are very water insoluble and high melting
• They are widely distributed in nature as protective waterproof coatings on leaves, fruits, animal skin, fur, feathers and exoskeletons
Fig 9.18 Myricyl palmitate, a wax
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Eicosanoids
• Eicosanoids are oxygenated derivatives of C20 polyunsaturated fatty acids (e.g. arachidonic acid)
• Prostaglandin E2 - can cause constriction of blood vessels
• Thromboxane A2 - involved in blood clot formation
• Leukotriene D4 - mediator of smooth-muscle contraction and bronchial constriction seen in asthmatics
• Aspirin alleviates pain, fever, and inflammation by inhibiting cyclooxygenase (COX), an enzyme critical for the synthesis of prostaglandins. (NSAID family of compounds)
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Fig 9.19 Arachidonic acid and eicosanoid derivatives
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Lipid vitamins• Vitamins A,D,E,
and K are isoprenoid derivatives
Vitamin E
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Prentice Hall c2002 Chapter 9 26
Biological Membranes Are Composed of Lipid Bilayers and Proteins
• Biological membranes define the external boundaries of cells and separate cellular compartments
• A biological membrane consists of proteins embedded in or associated with a lipid bilayer
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Several important functions of membranes
• Some membranes contain protein pumps for ions or small molecules
• Some membranes generate proton gradients for ATP production
• Membrane receptors respond to extracellular signals and communicate them to the cell interior
Lipid Bilayers
• Lipid bilayers are the structural basis for all biological membranes
• Noncovalent interactions among lipid molecules make them flexible and self-sealing
• Polar head groups contact aqueous medium• Nonpolar tails point toward the interior
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Fig 9.21 Membrane lipid and bilayer
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Fluid Mosaic Model of Biological Membranes
• Fluid mosaic model - membrane proteins and lipids can rapidly diffuse laterally or rotate within the bilayer (proteins “float” in a lipid-bilayer sea)
• Membranes: ~25-50% lipid and 50-75% proteins
• Lipids include phospholipids, glycosphingolipids, cholesterol (in some eukaryotes)
• Compositions of biological membranes vary considerably among species and cell types
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Fig 9.22 Structure of a typical eukaryotic plasma membrane
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Lipid Bilayers and Membranes Are Dynamic Structures
Fig 9.23 (a) Lateral diffusion is very rapid (b) Transverse diffusion (flip-flop) is very slow
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Fig 9.25 Freeze-fracture electron microscopy, distribution of membrane proteins
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Fig 9.22 Structure of a typical eukaryotic plasma membrane
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Three Classes of Membrane Proteins
(1) Integral membrane proteins Contain hydrophobic regions embedded in the lipid bilayer• Usually span the bilayer completely
(2) Peripheral membrane proteins
• Associated with membrane through charge-charge or hydrogen bonding interactions to integral proteins or membrane lipids
• More readily dissociated from membranes than covalently bound proteins
• Change in pH or ionic strength often releases these proteins
(3) Lipid-anchored membrane proteins
• Tethered to membrane through a covalent bond to a lipid
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Fig 9.22 Structure of a typical eukaryotic plasma membrane
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Prentice Hall c2002 Chapter 9 36
Membrane Transport
• Three types of integral membrane protein transport:
(1) Channels and pores
(2) Passive transporters
(3) Active transporters
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Table 9.3 Characteristics of membrane transport
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Pores and Channels• Pores and channels are transmembrane proteins with a
central passage for ions and small molecules
• Solutes of appropriate size, charge, and molecular structure can diffuse down a concentration gradient
• Process requires no energy
• Central passage allows molecules and ions of certain size, charge and geometry to transverse the membrane.
• Figure 9.30
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Passive Transport• Passive transport (facilitated diffusion) does not require an
energy source
• Protein binds solutes and transports them down a concentration gradient
Types of passive transport systems
• Uniport - transporter carries only a single type of solute
• Some transporters carry out cotransport of two solutes, either in the same direction (symport) or in opposite directions (antiport)
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Fig 9.31
• Types of passive transport(a) Uniport(b) Symport(c) Antiport
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Fig 9.32 Kinetics of passive transport
• Initial rate of transport increases until a maximum is reached (site is saturated)
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Prentice Hall c2002 Chapter 9 42
Active Transport
• Transport requires energy to move a solute up its concentration gradient
• Transport of charged molecules or ions may result in a charge gradient across the membrane
Types of active transport
• Primary active transport is powered by a direct source of energy as ATP, light or electron transport
• Secondary active transport is driven by an ion concentration gradient
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Fig 9.34 Secondary active transport in E. coli
• Oxidation of Sred generates a transmembrane proton gradient
• Movement of H+ down its gradient drives lactose transport (lactose permease)
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Fig 9.35 Secondary active transport in animals: Na+-K+ ATPase
• Na+ gradient (Na+-K+ATPase) drives glucose transport
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Endocytosis and Exocytosis
• Cells import/export molecules too large to be transported via pores, channels or proteins by:
• Endocytosis - macromolecules are engulfed by plasma membrane and brought into the cell inside a lipid vesicle
• Exocytosis - materials to be excreted from the cell are enclosed in vesicles that fuse with the plasma membrane
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Transduction of Extracellular Signals
• Specific receptors in plasma membranes respond to external chemicals (ligands) that cannot cross the membrane: hormones, neurotransmitters, growth factors
• Signal is passed through membrane protein transducer to a membrane-bound effector enzyme
• Effector enzyme generates a second messenger which diffuses to intracellular target
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Fig 9.37 General mechanism of signal transduction across a membrane
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Fig 9.43
• Summary of the adenyl cyclase signaling pathway