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

Types of Membrane Lipids

• Glycerophospholipids

• Sphingolipids

• Cholesterol

Membrane Glycerophospholipids

O

CH

H2C O

C R1H2C

O

P

O

O–

O R3 Alcohol

Fatty Acids

OCR2

O

Glycerol

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

Cholesterol

HO

CH3

CH3CH

CH3

CH2 CH2 CH2 CH

CH3

CH3

Flexible Hydrophobic TailHydrophilic(Polar Head)

Rigid Fused Ring

Amphiphilicity

Alcohol

Nonpolar Tail(Hydrophobic)

Polar Head(Hydrophilic)

Glycerol

P

Properties of Lipid Aggregates

Micelles, Liposomes, and Bilayers

Driving Force = Hydrophobic Effect

Figure 9-13a

Van der Waals Envelope(Fatty Acids)

Micelle(single-tailed lipids)

Cylindrical Lipids

Alcohol

Nonpolar Tail(Hydrophobic)

Polar Head(Hydrophilic)

Glycerol

P

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

Liposomes

Figure 9-15

Electron Micrograph of Liposome

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

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

Membrane composition

Figure 9-18

Phase Transition in a Lipid Bilayer

(Transition Temperature)

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 membrane composition is more rigid?

A B

Average Chain length 16.0 17.0

RatioUnsaturated:Saturated

Fatty acids2.0 0.5

Asymmetry within Membranes

Lipid Diffusion in Membranes

Figure 9-16a

Transverse Diffusion

Flippase/Floppase/Scramblase

Figure 9-16b

Lateral Diffusion

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

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

Page 268

Prenylated Proteins

Page 268

Prenylated Proteins

Glycosylphosphatidylinositol (GPI) Linked Proteins

Figure 9-24

Core Structure of the GPI Anchors of Proteins

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)

Figure 9-20

Human Erythrocyte Glycophorin A

Figure 9-21

Identification of Glycophorin A’s Transmembrane Domain

Figure 9-22

Structure of Bacteriorhodopsin

Figure 9-23a

X-Ray Structure of E. coli OmpF Porin

Figure 9-23b

X-Ray Structure of E. coli OmpF Porin Trimer

Functions of Membrane Proteins

• Catalysis of chemical reactions

• Transport of nutrients and waste products

• Signaling

Hydrophillic compounds need help

Glucose transporter

Figure 9-25

Plasma Membrane StructureFluid Mosaic Model

Evidence for Mobility of Membrane Proteins

Figure 9-26 part 1

Fusion of Mouse and Human Cells

Figure 9-26 part 2

Mixing of Human and Mouse Membrane Proteins

Figure 9-27a

Fluoresence Recovery after Photobleaching (FRAP)

Technique

Figure 9-27b

Fluoresence Recovery after Photobleaching (FRAP)

Results

Distribution of Membrane Phospholipids

Figure 9-32

Distribution of Membrane Phospholipids in Human Erythrocyte Membrane

Figure 9-33

Reaction of TNBS with Membrane Surface

Phosphatidylethanolamine

Figure 9-34

Location of Lipid Synthesis in a Bacterial Membrane

Redistribution of Membrane Lipids

• Flipases

• Phospholipid Translocases(ATP-dependent active transport)

Figure 9-32

Distribution of Membrane Phospholipids in Human Erythrocyte Membrane

Exposure of Phosphatidylserine

Blood clotting (tissue damage)

Removal from circulation (erythrocytes)

Membrane Subdomains

• Basolateral Cells– Two sided cells

• Microdomains– Concentration of specific lipids with specific

proteins• Cardiolipin and the electron transport chain

• Lipid Rafts

Basolateral CellsAsymmetric cell

Glucose Glucose Glucose

I ntestinal Lumen CapillariesBrushBorder

Cell

Na+–glucose symport

Na+–K+–ATPase

Glucose uniport

Na+Na+

K+ K+

Lipid RaftsSpecific Microdomain

– Glycosphingolipids– Cholesterol– GPI-linked proteins– Transmembrane signaling proteins– Caveolae — e.g. internalization of receptor-

bound ligands

Lipid rafts