Biological Membranes

Biological Membranes Organized assemblies of lipids, proteins and small amounts of carbohydrates Regulate composition of intracellular medium by controlling flow of nutrients, waste products, ions, etc. in and out of cell Scaffolding Oxidative phosphorylation Photosynthesis Nerve impulses Hormone receptors Organelle Membranes

Membranes are incredibly complicated structures, with many different components.

Types of Membrane Lipids Glycerophospholipids Sphingolipids Cholesterol

Membrane Glycerophospholipids

Sphingolipids (Sphingomyelin)

Cholesterol

Amphiphilicity

Properties of Lipid Aggregates Micelles, Liposomes, and Bilayers Driving Force = Hydrophobic Effect

Van der Waals Envelope (Fatty Acids) Figure 9-13a

Micelle (single-tailed lipids)

Cylindrical Lipids Individual lipids are cylindrical -cross-section of head = tail

Liposomes

Electron Micrograph of Liposome Figure 9-15

Properties/Uses of Liposomes Single Bilayer (inner and outer leaflets) Delivery of Therapeutic Agents Stable — purification Manipulate internal content Delivery — fusion with plasma membrane

Bilayer Formation by Phospholipids Aqueous phase Aqueous phase

Membrane composition

Phase Transition in a Lipid Bilayer (Transition Temperature) Figure 9-18

Transition Temperature =more Rigid; =more fluid Increases with chain length Tm = more rigid Increases with degree of saturation More saturated = more rigid Cholesterol decreases membrane fluidity

Membrane composition Which of these might be the most rigid? Which one the most fluid?

Which membrane composition is more rigid? Average Chain length 16.0 17.0 Ratio Unsaturated:Saturated Fatty acids 2.0 0.5 B [Adaptation by fish and animals] and bacteria

Asymmetry within Membranes

Lipid Diffusion in Membranes

Transverse Diffusion Figure 9-16a

Flippase/Floppase/Scramblase

Lateral Diffusion Which we will get to more in a few minutes. Figure 9-16b

Permeability of Lipid Bilayer Semi-permeable Hydrophilic molecules Non-permeable Facilitated diffusion Active transport Hydrophobic molecules Permeable Simple diffusion

Membrane Carbohydrates Mostly oligosaccharides Variety of sugars Glycolipids Glycoproteins Glycoprotein

Peripheral or Extrinsic Proteins Integral or Intrinsic Proteins Membrane Proteins Peripheral or Extrinsic Proteins Integral or Intrinsic Proteins

Peripheral or Extrinsic Proteins Easily dissociated High ionic strength pH changes Free of attached lipid Water-soluble (e.g. cytochrome c) Normal amino acid composition

Integral or Intrinsic Proteins Not easily dissociated or solubilized Detergents Chaotropic agents — disrupt water structure Retain associated lipid >average hydrophobic amino acds Significant number hydrophilic amino acds Asymmetrically oriented amphiphiles Trans-membrane proteins

Integral Membrane proteins Single transmembrane domain Multple transmembrane domains Lipid Linked

Lipid Linked Proteins Lipid linked are sometimes grouped into each category, all the protein is outside the bilayer, but they are strongly attached to th

Prenylated Proteins Page 268

Prenylated Proteins Page 268

Glycosylphosphatidylinositol (GPI) Linked Proteins

Core Structure of the GPI Anchors of Proteins Figure 9-24

Composition of Biological Membranes (protein-lipid ratios) Myelin ~0.23 Eukaryotic plasma membrane ~1.0 (50% protein and 50% lipid) Mitochondrial inner membrane ~3.2

Asymmetric Orientation

Detecting Asymmetric Orientation of Membrane Proteins Surface Labeling Proteases

Transmembrane Proteins May contain -Helices (and -Sheets)

Human Erythrocyte Glycophorin A Figure 9-20

Identification of Glycophorin A’s Transmembrane Domain Figure 9-21

Structure of Bacteriorhodopsin Figure 9-22

X-Ray Structure of E. coli OmpF Porin Figure 9-23a

X-Ray Structure of E. coli OmpF Porin Trimer Figure 9-23b

Functions of Membrane Proteins Catalysis of chemical reactions Transport of nutrients and waste products Signaling

Hydrophillic compounds need help

Glucose transporter

Plasma Membrane Structure Fluid Mosaic Model Figure 9-25

Evidence for Mobility of Membrane Proteins

Fusion of Mouse and Human Cells Figure 9-26 part 1

Mixing of Human and Mouse Membrane Proteins Figure 9-26 part 2

Fluoresence Recovery after Photobleaching (FRAP) Technique Figure 9-27a

Fluoresence Recovery after Photobleaching (FRAP) Results Figure 9-27b

Distribution of Membrane Phospholipids

Distribution of Membrane Phospholipids in Human Erythrocyte Membrane Figure 9-32

Reaction of TNBS with Membrane Surface Phosphatidylethanolamine Figure 9-33

Location of Lipid Synthesis in a Bacterial Membrane Figure 9-34

Redistribution of Membrane Lipids Flipases Phospholipid Translocases (ATP-dependent active transport)

Distribution of Membrane Phospholipids in Human Erythrocyte Membrane Figure 9-32

Exposure of Phosphatidylserine Blood clotting (tissue damage) Removal from circulation (erythrocytes)

Membrane Subdomains Basolateral Cells Microdomains Lipid Rafts Two sided cells Microdomains Concentration of specific lipids with specific proteins Cardiolipin and the electron transport chain Lipid Rafts

Basolateral Cells Asymmetric cell Active transport of glucose (against a concentration gradient) from intestinal lumen to cytosol Mediated uniport of glucose (down a concentration gradient) from cytosol to capillaries Asymmetric Cells (sidedness): protein targeting

Lipid Rafts Specific Microdomain Glycosphingolipids Cholesterol GPI-linked proteins Transmembrane signaling proteins Caveolae — e.g. internalization of receptor-bound ligands

Lipid rafts Glycosphingolipids Cholesterol GPI-linked proteins Transmembrane signaling proteins Caveolae — e.g. internalization of receptor-bound ligands