Chapter 11: Biological Membranes and Transport

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

2. Lipid Aggregates Lipids are not �free� Virtually insoluble in water Associate to form separate phase Reduces contact of nonpolar chain with H2O Solvate polar head groups Micelles Bilayers Structural basis for biological membranes

3. Micelles Spherical 10 � 1000s of lipids Free fatty acids Detergents

4. Bilayer Two monolayers (leaflets) 3 nm (30 �) thick Lipids are structurally similar Glycerophospholipids Sphingolipids

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

6. 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

7. Transverse Diffusion �Flip-flop� From one bilayer leaflet to the other Rare Very slow without catalyst Polar head pass through anhydrous core Catalyst = flippase

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

9. FRAP

10. Single Particle Tracking

11. 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

12. Membrane Fluidity Lipids synthesized by cells to keep fluidity constant

13. 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

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

15. 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

16. COX-1 with NSAID

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

18. Transmembrane Domain Hydrophobic region Domain structure a-helix b-barrel Protein tertiary structure difficult to determine 10-20% are integral 1% structure determined Predict presence when > 20 nonpolar AA residues Use hydropathy index

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

20. Glycophorin A

21. Bacteriorhodopsin

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

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

24. b-barrel b-sheets not found in membrane interior b-barrels are 16-20 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

25. Membrane Proteins with b-Barrel Structure

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

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

28. 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

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

30. 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)

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

32. 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

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

34. 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

35. 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+

36. Summary of Transport Types

37. Chapter 11 Problems 3-4, 6, 11-15, 18