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Biological Membranes
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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
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Types of Membrane Lipids
• Glycerophospholipids
• Sphingolipids
• Cholesterol
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Membrane Glycerophospholipids
O
CH
H2C O
C R1H2C
O
P
O
O–
O R3 Alcohol
Fatty Acids
OCR2
O
Glycerol
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Sphingolipids(Sphingomyelin)
CH3(CH2)12 CH CH CH
OH
CH
CH2
NH C R1
O
O P
O
O–
O CH2 CH2 N(CH3)3
Choline
+
Fatty Acid
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Cholesterol
HO
CH3
CH3CH
CH3
CH2 CH2 CH2 CH
CH3
CH3
Flexible Hydrophobic TailHydrophilic(Polar Head)
Rigid Fused Ring
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Amphiphilicity
Alcohol
Nonpolar Tail(Hydrophobic)
Polar Head(Hydrophilic)
Glycerol
P
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Properties of Lipid Aggregates
Micelles, Liposomes, and Bilayers
Driving Force = Hydrophobic Effect
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Figure 9-13a
Van der Waals Envelope(Fatty Acids)
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Micelle(single-tailed lipids)
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Cylindrical Lipids
Alcohol
Nonpolar Tail(Hydrophobic)
Polar Head(Hydrophilic)
Glycerol
P
Individual lipids are cylindrical-cross-section of head = tail
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Liposomes
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Figure 9-15
Electron Micrograph of Liposome
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Properties/Uses of Liposomes
Single Bilayer(inner and outer leaflets)
Delivery of Therapeutic Agents•Stable — purification•Manipulate internal content•Delivery — fusion with plasma membrane
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Bilayer Formation by Phospholipids
60Å
Outer Leaf let
I nner Leaf let
HydrophobicTails
HydrophilicHeads
Aqueous Phase
Aqueous Phase
60Å
Outer Leaf let
I nner Leaf let
HydrophobicTails
HydrophilicHeads
Aqueous Phase
Aqueous Phase
Aqueous phase
Aqueous phase
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Membrane composition
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Figure 9-18
Phase Transition in a Lipid Bilayer
(Transition Temperature)
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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
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Membrane composition
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Which membrane composition is more rigid?
A B
Average Chain length 16.0 17.0
RatioUnsaturated:Saturated
Fatty acids2.0 0.5
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Asymmetry within Membranes
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Lipid Diffusion in Membranes
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Figure 9-16a
Transverse Diffusion
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Flippase/Floppase/Scramblase
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Figure 9-16b
Lateral Diffusion
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Permeability of Lipid Bilayer
Semi-permeable
Hydrophilic molecules
Non-permeable
Facilitated diffusion
Active transport
Hydrophobic molecules
Permeable
Simple diffusion
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Membrane Carbohydrates
• Mostly oligosaccharides
• Variety of sugars
• Glycolipids• Glycoproteins
Glycoprotein
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Membrane Proteins
Peripheral or Extrinsic Proteins
Integral or Intrinsic Proteins
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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
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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
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Integral Membrane
proteins
Single transmembrane domain
Multple transmembrane domains
Lipid Linked
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Lipid Linked Proteins
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Page 268
Prenylated Proteins
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Page 268
Prenylated Proteins
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Glycosylphosphatidylinositol (GPI) Linked Proteins
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Figure 9-24
Core Structure of the GPI Anchors of Proteins
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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
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Asymmetric Orientation
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Detecting Asymmetric Orientation of Membrane
Proteins
Surface Labeling
Proteases
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Transmembrane Proteins
May contain -Helices(and -Sheets)
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Figure 9-20
Human Erythrocyte Glycophorin A
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Figure 9-21
Identification of Glycophorin A’s Transmembrane Domain
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Figure 9-22
Structure of Bacteriorhodopsin
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Figure 9-23a
X-Ray Structure of E. coli OmpF Porin
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Figure 9-23b
X-Ray Structure of E. coli OmpF Porin Trimer
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Functions of Membrane Proteins
• Catalysis of chemical reactions
• Transport of nutrients and waste products
• Signaling
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Hydrophillic compounds need help
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Glucose transporter
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Figure 9-25
Plasma Membrane StructureFluid Mosaic Model
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Evidence for Mobility of Membrane Proteins
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Figure 9-26 part 1
Fusion of Mouse and Human Cells
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Figure 9-26 part 2
Mixing of Human and Mouse Membrane Proteins
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Figure 9-27a
Fluoresence Recovery after Photobleaching (FRAP)
Technique
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Figure 9-27b
Fluoresence Recovery after Photobleaching (FRAP)
Results
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Distribution of Membrane Phospholipids
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Figure 9-32
Distribution of Membrane Phospholipids in Human Erythrocyte Membrane
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Figure 9-33
Reaction of TNBS with Membrane Surface
Phosphatidylethanolamine
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Figure 9-34
Location of Lipid Synthesis in a Bacterial Membrane
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Redistribution of Membrane Lipids
• Flipases
• Phospholipid Translocases(ATP-dependent active transport)
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Figure 9-32
Distribution of Membrane Phospholipids in Human Erythrocyte Membrane
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Exposure of Phosphatidylserine
Blood clotting (tissue damage)
Removal from circulation (erythrocytes)
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Membrane Subdomains
• Basolateral Cells– Two sided cells
• Microdomains– Concentration of specific lipids with specific
proteins• Cardiolipin and the electron transport chain
• Lipid Rafts
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Basolateral CellsAsymmetric cell
Glucose Glucose Glucose
I ntestinal Lumen CapillariesBrushBorder
Cell
Na+–glucose symport
Na+–K+–ATPase
Glucose uniport
Na+Na+
K+ K+
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Lipid RaftsSpecific Microdomain
– Glycosphingolipids– Cholesterol– GPI-linked proteins– Transmembrane signaling proteins– Caveolae — e.g. internalization of receptor-
bound ligands
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Lipid rafts