thestructure of$...
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The Structure of Biological Membranes
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Func7ons of Cellular Membranes 1. Plasma membrane acts as a selec7vely permeable barrier to the
environment
• Uptake of nutrients
• Waste disposal • Maintains intracellular ionic milieau
2. Plasma membrane facilitates communica7on • With the environment
• With other cells • Protein secre8on
3. Intracellular membranes allow compartmentaliza7on and separa7on of different chemical reac7on pathways
• Increased efficiency through proximity
• Prevent fu8le cycling through separa8on
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Composi7on of Animal Cell Membranes
• Hydrated, proteinaceous lipid bilayers • By weight: 20% water, 80% solids • Solids: Lipid Protein Carbohydrate (~10%) • Phospholipids responsible for basic membrane bilayer structure and physical proper8es
• Membranes are 2-‐dimensional fluids into which proteins are dissolved or embedded
~90%
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The Most Common Class of Phospholipid is Formed from a Gycerol-‐3-‐P Backbone
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Saturated FaJy Acid
• Palmitate and stearate most common • 14-‐26 carbons • Even # of carbons
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Unsaturated FaJy Acid
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Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008)
Structure of Phosphoglycerides
All Membrane Lipids are Amphipathic
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Phosphoglycerides are Classified by their Head Groups
Phospha8dylethanolamine
Phospha8dylcholine
Phospha8dylserine
Phospha8dylinositol Ether Bond at C1
PS and PI bear a net nega8ve charge at neutral pH
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Sphingolipids are the Second Major Class of Phospholipid in Animal Cells
Sphingosine
Ceramides contain sugar moi8es in ether linkage to sphingosine
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Figure 10-18 Molecular Biology of the Cell (© Garland Science 2008)
Glycolipids are Abundant in Brain Cells
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Membranes are formed by the tail to tail associa7on of two lipid leaflets
Cytoplasmic Leaflet
Exoplasmic Leaflet
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Figure 10-11a Molecular Biology of the Cell (© Garland Science 2008)
(Hydrophobic)
(Hydrophilic)
Bilayers are Thermodynamically Stable
Structures Formed from Amphathic Lipids
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Lipid Bilayer Forma7on is Driven by the Hydrophobic Effect
• HE causes hydrophobic surfaces such as faVy acyl chains to aggregate in water
• Water molecules squeeze hydrophobic molecules into as compact a surface area as possible in order to the minimize the free energy (ΔG) of the system by maximizing the entropy (ΔS) or degree of disorder of the water molecules
• ΔG = ΔΗ ‒ ΤΔS
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Figure 1-12 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)
Lipid Bilayer Forma7on is a Spontaneous Process
Gently mix PL and water
Vigorous mixing
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Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008)
The Forma7on of Cell-‐Like Spherical Water-‐Filled Bilayers is Energe7cally Favorable
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Figure 10-7 Molecular Biology of the Cell (© Garland Science 2008)
Phospholipids are Mesomorphic
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Figure 10-11b Molecular Biology of the Cell (© Garland Science 2008)
PhosphoLipid Movements within Bilayers (µM/sec)
(1012-‐1013/sec) (108-‐109/sec)
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Pure Phospholipid Bilayers Undergo Phase Transi7on
Below Lipid Phase Transi7on Temp Above Lipid Phase Transi7on Temp
Gauche Isomer Forma7on
All-‐Trans
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Figure 12-57 Molecular Biology of the Cell (© Garland Science 2008)
Phosphoglyceride Biosynthesis Occurs at the Cytoplasmic Face of the ER
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Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008)
A “Scramblase” Enzyme Catalyzes Symmetric
Growth of Both Leaflets in the ER
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Figure 10-16 Molecular Biology of the Cell (© Garland Science 2008)
The Two Plasma Membrane Leaflets Possess Different Lipid Composi7ons
Enriched in PC, SM, Glycolipids
Enriched in PE, PS, PI
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Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008)
A “Flippase” Enzyme promotes Lipid
Asymmetry in the Plasma Membrane
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Membrane Proteins May Selec7vely Interact with Specific Lipids (Lipid Annulus or Halo)
Lipid Asymmetry Also Exists in the Plane of the Bilayer
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Figure 10-17a Molecular Biology of the Cell (© Garland Science 2008)
Phospholipids are Involved in Signal Transduc7on
PI-‐4,5P PI-‐3,4,5P
1. Ac7va7on of Lipid Kinases
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2. Phospholipases Produce Signaling Molecules via the Degrada7on of
Phospholipids
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Figure 10-17b Molecular Biology of the Cell (© Garland Science 2008)
Phospholipase C Ac7va7on Produces Two Intracellular Signaling Molecules
PI-‐3,4,5P
I-‐3,4,5P
Diacylglycerol
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Hydrophilic End
Flexible Hydrocarbon Tail
Amphipathic Lipids Containing Rigid Planar Rings Are Important Components of Biological Membranes
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Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008)
C12
How Cholesterol Integrates into a Phospholipid Bilayer
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Cholesterol Biosynthesis Occurs in the Cytosol and at the ER Membrane Through Isoprenoid Intermediates
(Rate-‐Limi8ng Step in ER)
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Figure 10-14b Molecular Biology of the Cell (© Garland Science 2008)
Lipid Rads are Microdomains Enriched in Cholesterol and SM that may be Involved in Cell
Signaling Processes
Insoluble in Triton X-‐100
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Figure 10-14a Molecular Biology of the Cell (© Garland Science 2008)
Electron Force Microscopic Visualiza7on of Lipid Rads in Ar7fical Bilayers
Yellow spikes are GPI-‐anchored protein
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3 Ways in which Lipids May be Transferred Between Different Intracellular Compartments
Vesicle Fusion Direct Protein-‐Mediated
Transfer Soluble Lipid
Binding Proteins
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Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008)
The 3 Basic Categories of Membrane Protein
Integral Lipid-‐Anchored
Peripheral (can also interact via PL headgroups)
Single-‐Pass Mul7-‐pass
FaJy acyl anchor
GPI Anchor
Transmembrane Helix
Linker Domain
β-‐Strands
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Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008)
3 Types of Lipid Anchors
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Figure 3-5 Molecular Biology of the Cell (© Garland Science 2008)
Membrane Domains are “Inside-‐Out”
Right-‐Side Out Soluble Protein
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Figure 10-31 Molecular Biology of the Cell (© Garland Science 2008)
Func7onal Characteriza7on of Integral Membrane Proteins Requires Solubiliza7on and Subsequent Recons7tu7on into a Lipid Bilayer
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(Used to denature proteins)
(Used to purify Integral Membrane Proteins)
Detergents are Cri7cal for the Study of Integral Membrane Proteins
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Figure 10-29b Molecular Biology of the Cell (© Garland Science 2008)
Detergents Exist in Two Different States in Solu7on
(Cri8cal Micelle Concentra8on)
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Detergent Solubiliza7on of Membrane Proteins
Desirable for Purifica8on of Integral Membrane Proteins
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Beta Sheet Secondary Structure
An7-‐parallel Strands
Side chains alternately extend into opposite sides of the sheet
H-‐Bond
Polypep7de backbone is maximally extended (rise of 3.3 Å/residue)
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β-‐Barrel Structure of the OmpX Porin Protein in the Outer Membrane of E. coli
Aroma7c Side Chain anchors
Alterna7ng Alipha7c Side Chains
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Transmembrane α-‐Helices 1. Right-‐handed 2. Stability in bilayer results from
maximum hydrogen bonding of pep8de backbone
3. Usually > 20 residues in length (rise of 1.5 Å/residue)
4. Exhibit various degrees of 8lt with respect to the membrane and can bend due to helix breaking residues
5. Mostly hydrophobic side-‐chains in single-‐pass proteins
6. Mul8-‐pass proteins can possess hydrophobic, polar, or amphipathic transmembrane helices
H-‐Bond
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Figure 10-22b Molecular Biology of the Cell (© Garland Science 2008)
Transmembrane Domains Can Oden be Accurately Iden7fied by Hydrophobicity Analysis
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Figure 10-32 Molecular Biology of the Cell (© Garland Science 2008)
Bacteriorhodopsin of Halobacterium
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Figure 10-33 Molecular Biology of the Cell (© Garland Science 2008)
Structure of Bacteriorhodopsin
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Structure of the Glycophorin A Homodimer
Arg and Lys Side Chain Anchors
23 residue transmembrane helices
Coiled-‐Coil Domains in Van Der Waals Contact
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Figure 10-34 Molecular Biology of the Cell (© Garland Science 2008)
Structure of the Photosynthe7c Reac7on Center of Rhodopseudomonas Viridis
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Figure 10-39 Molecular Biology of the Cell (© Garland Science 2008)
4 Ways that Protein Mobility is Restricted in Biological Membranes
Intramembrane Protein-‐Protein Interac8ons
Interac8on with the cytoskeleton
Interac8on with the extracellular maxtrix
Intercellular Protein-‐Protein Interac8ons