Chapter 11: Biological Membranes and Transport Dr. Clower Chem 4202

Lipid Aggregates  Lipids are not “free”  Virtually insoluble in water  Associate to form separate phase –Reduces contact of nonpolar chain with H 2 O –Solvate polar head groups  Micelles  Bilayers –Structural basis for biological membranes

Micelles  Spherical  10 – 1000s of lipids  Free fatty acids  Detergents

Bilayer  Two monolayers (leaflets)  3 nm (30 Å) thick  Lipids are structurally similar –Glycerophospholipids –Sphingolipids

Liposome  Bilayer folded back on itself  Hollow sphere  Maximum stability in aqueous environment –Loss of hydrophobic edge of bilayer

Biological Membranes  Surround cells  Partition two aqueous environments of different concentrations  Formed from lipid bilayers –Inner and outer leaflet  Flexible –Change shape without compromising integrity  Lipid mobility –Transfer of lipid through bilayer  Transverse diffusion  Lateral diffusion

Transverse Diffusion  “Flip-flop”  From one bilayer leaflet to the other  Rare  Very slow without catalyst –Polar head pass through anhydrous core –Catalyst = flippase

Lateral Diffusion  Exchange of neighboring lipids in same bilayer leaflet  Measure with: –Fluorescence recovery after photobleaching (FRAP) –Single particle tracking

FRAP

Single Particle Tracking

Membrane Fluidity  Changes in conformation of chains keep interior in constant motion –Low viscosity in interior –Increases close to head (limited mobility)  Liquid-disordered state (fluid) vs. liquid-ordered state vs. paracrystalline state (gel)  Temperature dependent  Favored by unsaturated FAs, shorter FAs  Sterols –Reduce fluidity –Reduce freedom of movement/rotation

Membrane Fluidity  Lipids synthesized by cells to keep fluidity constant

Membrane Structure and Assembly  Contain lipids and proteins –Percent composition varies with function  Lipids –Can be the same or different –Most commonly:  Glycerophospholipids, sphingolipids, and sterols

Membrane Proteins  Composition varies –More widely than lipids  Catalyze chemical reactions  Relay information  Transport across membranes  3 classes A. A.Integral/intrinsic B. B.Lipid-linked C. C.Peripheral/extrinsic

A. Integral Proteins  Strongly associate to membranes –Hydrophobic interactions  Difficult to separate from membrane –Need detergent, denaturant  Amphiphilic –Nonpolar section in membrane –Polar section(s) on one or both sides of membrane  Example: cyclooxygenase

COX-1 with NSAID

Intergral Membrane Proteins  Types I - VI  Transmembrane proteins –Span membrane –3 domains –Preference for one face or the other –Sugar residues outside

Transmembrane Domain  Hydrophobic region  Domain structure –  -helix –  -barrel  Protein tertiary structure difficult to determine –10-20% are integral –1% structure determined  Predict presence when > 20 nonpolar AA residues  Use hydropathy index

Hydropathy Index  Free energy change accompanying movement of AA side chain from hydrophobic solvent into water –Charged or polar = exergonic –Aromatic, aliphatic = endergonic

Glycophorin A Single  -helix

Bacteriorhodopsin 7 helices connected by hydrophilic loops

Threonine and Tyrosine  Interact with both polar and nonpolar regions  Located on surface  Tyr = orange  Thr = red  Charged = blue

Rhodopseudomonas viridis  Photosynthetic reaction center  1200 residues  1 st protein determined by crystallography  4 non-identical subunits  Transmembrane section = 11  -helices  Red = prosthetic groups

 -barrel   -sheets not found in membrane interior   -barrels are  stranded anti-parallel sheet  Typically 7-9 residues to span  Alternate residues (at least) are hydrophobic –Interact with lipid  Ex: porins –Found in membranes of gram-negative bacteria –Trimers of identical subunits –Barrel forms channel  Allows entry of charged/polar molecules  R groups in channel can be polar

Membrane Proteins with  - Barrel Structure

B. Lipid-linked proteins  Covalently attached to lipids (anchor)  Not as strongly associated as integral; more strongly associated than peripheral  3 varieties 1. 1.Prenylated proteins 2. 2.Fatty acylated proteins 3. 3.GPI-linked proteins

1. Prenylated proteins  Lipid synthesized from isoprene  Linkage to Cys residue at C- terminus

2. Fatty Acylated Proteins  Myristic acid (14:0) –Links to amine N of Gly at N-terminus  Palmitic acid (16:0) –Thioester linkage to internal Cys

3. GPI-linked Proteins  Glycosyl- phosphatidylinositol  Exterior surface only  Glycerophospholipid linked to tetrasaccharide –(3 Man; 1 Glc) –linked to C-terminus through ethanolamine phosphate

C. Peripheral/Extrinsic Proteins  Easy to separate from membranes  Associate with membranes by binding at surface to lipids or integral proteins –H-bond or electrostatic  Do not bind lipids  Regulate membrane- bound enzymes or limit mobility of integral proteins (tether to intracellular structures)

Assembly of Membranes  Fluid-mosaic model  Proteins move in membranes due to lipid mobility  Leaflets not equivalent in composition or function

Transport across Membranes  Nonmediated –Diffusion of nonpolar molecule through membrane –From high concentration to low concentration  Mediated –Through action of specific proteins –Carrier proteins –Integral protein channels

Carrier Proteins  Shuttle amino acids, ions, sugars etc. into cells  Hydrophobic on outside  Specific for ligands/substrates

Integral Protein Channels  Means by which hydrophilic molecules/ions move through hydrophobic membrane  Typically selective for one molecule/ion  Channel = protein complex –Transverse cell membrane –Hollow, hydrophilic core –Hydrophobic outside interact with lipids

Transport Systems  Integral proteins with binding sites on either side of membrane  Reversible process  More than one type of molecule can be transported  Ex: lactose transporter of E. coli –Lactose and H +

Summary of Transport Types

Chapter 11 Problems  3-4, 6, 11-15, 18