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The Structure of Biological Membranes

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The Structure of Biological Membranes The Structure of Biological Membranes Func7ons of Cellular Membranes 1. Plasma membrane acts as a selecvely permeable barrier to the environment • Uptake of nutrients • Waste disposal • Maintains intracellular ionic milieau 2. Plasma membrane facilitates communicaon • With the environment • With other cells • Protein secreon 3. Intracellular membranes allow compartmentalizaon and separaon of different chemical reac7on pathways • Increased efficiency through proximity • Prevent fu8le cycling through separaon Composi7on of Animal Cell Membranes • Hydrated, proteinaceous lipid bilayers • By weight: 20% water, 80% solids • Solids: Lipid Protein ~90% Carbohydrate (~10%) • Phospholipids responsible for basic membrane bilayer structure and physical proper8es • Membranes are 2-dimensional fluids into which proteins are dissolved or embedded The Most Common Class of PhospholipiD is FormeD from a Gycerol-3-P Backbone SaturateD FaJy AciD •Palmitate and stearate most common •14-26 carbons •Even # of carbons UnsaturateD FaJy AciD Structure of PhosphoglyceriDes All Membrane LipiDs are Amphipathic Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008) PhosphoglyceriDes are Classified by their Head Groups Phosphadylethanolamine Phosphadylcholine Phosphadylserine Phosphadylinositol Ether Bond at C1 PS and PI bear a net negave charge at neutral pH Sphingolipids are the SeconD Major Class of PhospholipiD in Animal Cells Sphingosine Ceramides contain sugar moies in ether linkage to sphingosine GlycolipiDs are AbunDant in Brain Cells Figure 10-18 Molecular Biology of the Cell (© Garland Science 2008) Membranes are formeD by the tail to tail associaon of two lipid leaflets Exoplasmic Leaflet Cytoplasmic Leaflet Bilayers are ThermoDynamically Stable Structures Formed from Amphathic LipiDs (Hydrophobic) (Hydrophilic) Figure 10-11a Molecular Biology of the Cell (© Garland Science 2008) Lipid Bilayer Formaon is Driven by the Hydrophobic Effect • HE causes hydrophobic surfaces such as fay 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 Lipid Bilayer Formaon is a Spontaneous Process Gently mix PL and water Vigorous mixing Figure 1-12 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008) The Formaon of Cell-Like Spherical Water-FilleD Bilayers is Energecally Favorable Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008) PhospholipiDs are Mesomorphic Figure 10-7 Molecular Biology of the Cell (© Garland Science 2008) PhosphoLipiD Movements within Bilayers (µM/sec) (1012-1013/sec) (108-109/sec) Figure 10-11b Molecular Biology of the Cell (© Garland Science 2008) Pure PhospholipiD Bilayers UnDergo Phase Transion Gauche Isomer Formaon All-Trans Below Lipid Phase Transion Temp Above LipiD Phase Transi7on Temp PhosphoglyceriDe Biosynthesis Occurs at the Cytoplasmic Face of the ER Figure 12-57 Molecular Biology of the Cell (© Garland Science 2008) A “Scramblase” Enzyme Catalyzes Symmetric Growth of Both Leaflets in the ER Figure 12-58 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 Figure 10-16 Molecular Biology of the Cell (© Garland Science 2008) A “Flippase” Enzyme promotes Lipid Asymmetry in the Plasma Membrane Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008) Membrane Proteins May Selec7vely Interact with Specific LipiDs (LipiD Annulus or Halo) LipiD Asymmetry Also Exists in the Plane of the Bilayer PhospholipiDs are Involved in Signal Transducon 1. Ac7va7on of LipiD Kinases PI-4,5P PI-3,4,5P Figure 10-17a Molecular Biology of the Cell (© Garland Science 2008) 2. Phospholipases ProDuce Signaling Molecules via the DegraDa7on of PhospholipiDs Phospholipase C Ac7va7on ProDuces Two Intracellular Signaling Molecules Diacylglycerol PI-3,4,5P I-3,4,5P Figure 10-17b Molecular Biology of the Cell (© Garland Science 2008) Amphipathic LipiDs Containing RigiD Planar Rings Are Important Components of Biological Membranes Flexible HyDrocarbon Tail Hydrophilic End How Cholesterol Integrates into a PhospholipiD Bilayer C12 Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008) Cholesterol Biosynthesis Occurs in the Cytosol anD at the ER Membrane Through Isoprenoid Intermediates (Rate-Limi8ng Step in ER) LipiD Rads are MicroDomains Enriched in Cholesterol anD SM that may be InvolveD in Cell Signaling Processes Insoluble in Triton X-100 Figure 10-14b Molecular Biology of the Cell (© Garland Science 2008) Electron Force Microscopic Visualiza7on of LipiD Rads in Ar7fical Bilayers Yellow spikes are GPI-anchoreD protein Figure 10-14a Molecular Biology of the Cell (© Garland Science 2008) 3 Ways in which LipiDs May be TransferreD Between Different Intracellular Compartments Direct Protein-Mediated Soluble Lipid Vesicle Fusion Transfer Binding Proteins The 3 Basic Categories of Membrane Protein GPI Anchor Single-Pass Mul7-pass β-StranDs Transmembrane Helix Linker Domain FaJy acyl anchor Integral Lipid- Peripheral Anchored (can also interact via PL headgroups) Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008) 3 Types of LipiD Anchors Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008) Membrane Domains are “InsiDe-Out” Right-SiDe Out Soluble Protein Figure 3-5 Molecular Biology of the Cell (© Garland Science 2008) Func7onal Characteriza7on of Integral Membrane Proteins Requires Solubilizaon anD Subsequent Reconstuon into a Lipid Bilayer Figure 10-31 Molecular Biology of the Cell (© Garland Science 2008) Detergents are Cri7cal for the StuDy of Integral Membrane Proteins (Used to denature proteins) (Used to purify Integral Membrane Proteins) Detergents Exist in Two Different States in Soluon (Cri8cal Micelle Concentraon) Figure 10-29b Molecular Biology of the Cell (© Garland Science 2008) Detergent Solubilizaon of Membrane Proteins Desirable for Purificaon of Integral Membrane Proteins Beta Sheet Secondary Structure Polypep7De backbone is maximally extenDed (rise of 3.3 Å/resiDue) H-Bond An7-parallel StranDs Side chains alternately extenD into opposite sides of the sheet β-Barrel Structure of the OmpX Porin Protein in the Outer Membrane of E. coli Aroma7c SiDe Chain anchors Alterna7ng Alipha7c Side Chains Transmembrane α-Helices 1. Right-handed 2. Stability in bilayer results from maximum hydrogen bonding of pepde backbone 3. Usually > 20 residues in length (rise of 1.5 Å/residue) H-Bond 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 Transmembrane Domains Can Oden be Accurately Idenfied by Hydrophobicity Analysis Figure 10-22b Molecular Biology of the Cell (© Garland Science 2008) Bacteriorhodopsin of Halobacterium Figure 10-32 Molecular Biology of the Cell (© Garland Science 2008) Structure of Bacteriorhodopsin Figure 10-33 Molecular Biology of the Cell (© Garland Science 2008) Structure of the Glycophorin A Homodimer Coiled-Coil Domains in Van Der Waals Contact Arg anD Lys SiDe Chain Anchors 23 residue transmembrane helices Structure of the Photosynthe7c Reac7on Center of Rhodopseudomonas Viridis Figure 10-34 Molecular Biology of the Cell (© Garland Science 2008) 4 Ways that Protein Mobility is RestricteD in Biological Membranes Intramembrane Protein- Interac8on with the Protein Interac8ons cytoskeleton Interac8on with the Intercellular Protein- extracellular maxtrix Protein Interac8ons Figure 10-39 Molecular Biology of the Cell (© Garland Science 2008) .
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