Chapter 12. Part II. Biological Membrane

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1 Chapter 12 Part II. Biological Membrane

2 Single-tailed lipids tend to form micelles Critical micelle concentration (cmc): minimum concentration that forms micelles e.g.) cmc for SDS 1mM; cmc for phospholipids < 1μM

3 Lipids with two hydrocarbon tails tend to form bilayers

4 Liposome Closed, self-sealing vesicles that are bounded by a single bilayer Several hundred Å Stable and purified by dialysis, gel filtration or centrifugation Models for biological membranes Drug delivery

5 Lipid bilayers are two-dimensional fluids

6 The fluorescence photobleaching recovery

7 Snapshot of a lipid bilayer 15 Å 30 Å 15 Å High viscosity in the head groups Hydrocarbon tails bend and interdigitate Water penetrates to the edge between head groups and hydrophobic cores

Phase transition in a lipid bilayer Thicker Highly mobile fluid (liquid crystal) Stiffening Highly ordered gel-like solid Longer the chain length and higher the degree of saturation Higher transition

8 Phase transition in a lipid bilayer Thicker Highly mobile fluid (liquid crystal) Stiffening Highly ordered gel-like solid Longer the chain length and higher the degree of saturation Higher transition temperature Cholesterol decreases membrane fluidity, facilitates the mobility of the fatty acid tails near their methyl ends and inhibits the crystallization of fatty acids, thereby broadening the temperature range of the phase transition

9 Composition of biological membranes

10 Membrane assembly

Membrane proteins Integral or intrinsic proteins Tightly bound by hydrophobic interaction Only isolable by treatment with agents that disrupt membranes (organic solvents, detegents or chaotropic

11 Membrane proteins Integral or intrinsic proteins Tightly bound by hydrophobic interaction Only isolable by treatment with agents that disrupt membranes (organic solvents, detegents or chaotropic agents) Tend to aggregate and precipitate in aqueous solution Peripheral or extrinsic proteins Bound to membrane head groups and/or integral proteins by electrostatic and hydrogen bonding interactions Isolable by mild procedures without membrane disruption (high ionic strength, metal chelators or ph change) Stable in aqueous solution (e.g. cytochrome c)

12 Integral proteins Asymmetrically oriented amphiphiles Monotopic proteins Transmembrane proteins Cytochrome b5 Human erythrocyte glycophorin A

13 Transmembrane domains contains α helix or β barrels Bacteriorhodopsin Photosynthetic reaction center E. coli OmpF porin

14 Lipid-linked proteins Covalent linkage between proteins and lipids Anchoring proteins to membranes Mediating protein-protein interactions Prenylation Farnesylation (C15) Geranylgeranylation (C20) Fatty acylation Myristic acid (C14) Palmitic acid (C16) Glycosylphosphatidylinositol (GPI)

15 Prenylation of proteins

16 GPI-linked proteins

17 Membranes are fluid Fluid mosaic model: proteins float freely in a sea of lipids By and large, this model is true, but not all parts of a membrane are accessible to all proteins

18 Fluid mosaic model has been verified Edidin (1970)

19 Freeze-fracture and freeze-etch techniques

20 The inner and outer leaflet of a membrane differ in lipid composition Human erythrocyte membrane

21 Lateral organization of lipids and proteins Non-uniform distribution of lipids and proteins, forming distinct domains in the plasma membranes Integral protein patches (aggregates) Specific interaction of integral proteins with particular lipids Phase separation by metal ions (e.g. Ca 2+ ) Glycosphigolipid-cholesterol rafts and caveolae

22 Erythrocyte membrane Easy preparation of erythrocyte membranes Erythrocyte is devoid of organelles ( membranous bag of hemoglobin) Osmotic lysis of erythrocyte and resealing the resultant membranous particles lead to the colorless erythrocyte ghost Erythrocyte membranes contain a small number different proteins 6-7 peripheral proteins 7-8 integral proteins

23 Biconcave disklike shape of erythrocytes is determined by a cytoskeleton of proteins

24 Gate and Fence Model of protein mobility Immobile Cytoskeleton Free rotation within a fence Diffusion by traveling through gates

25 Gap junctions Intercellular communication channels Heart contraction synchronization Embryonic development Nutrient supply of bone and lens cells Connexon (Hexagonal rings of connexins)

26 Channel-forming proteins Water soluble monomers oligomerize to form a transmembrane pore (e.g. bacterial channel forming toxins) α-hemolysin (Human pathogen from Staphylococcus aureus)

27 Chemical labeling of lipids

28 The location of lipid synthesis Lipid is synthesized in cytoplasmic face of the membrane Flipases facilitate transverse diffusions (flip-flops) Phospholipid translocases transport specific phospholipids across a bilayer with ATP hydrolysis

29 RER-associated protein synthesis Amino-terminus contains a signal sequence Signal sequence binds to the signal recognition particle (SRP) Signal sequence-srp binds to a receptor on the ER Protein is translocated to the ER lumen as translation proceeds Translocation stops after a hydrophobic sequence followed by basic amino acids

30 Synthesis of secreted and membrane proteins on RER

31 Signal peptides

32 Membrane, secretory and lysosomal proteins are transported in coated vesicles Clathrin

33 Clathrin cage Clathrin Schematic structure of a triskeleon

34 Fusion of vesicles with the plasma membrane

35 Vesicle fusion requires proteins SNARE-mediated membrane fusion

36 Model for SNARE-mediated membrane fusion

37 Targeting proteins to mitochondria TOM (Translocase of the outer membrane) TIM (Translocase of the inner membrane)

38 Lipoproteins Non-covalent assemblies of lipids and proteins in the blood plasma Chylomicrons Transport exogenous triacylglycerols and cholesterol from the intestines to the tissues Very low density lipoproteins (VLDL) Intermediate density lipoproteins (IDL) Low density lipoproteins (LDL) Transport endogenous triacylglycerols and cholesterol from the liver to the tissues High density lipoproteins (HDL) Transport endogenous cholesterol from the tissues to the liver

39 Lipoprotein structure LDL α Helix of apolipoprotein A-I Side view Structure of apo Δ(1-43)A-1 Top view

40 Triacylglycerol and cholesterol transport

41 Receptor-mediate endocytosis of LDL

Medical implications of lipoproteins High LDL (bad) cholesterol Build up in the inner walls of arteries (atherosclerosis) High HDL (good) cholesterol Removes excess cholesterol from arterial plaque (

42 Medical implications of lipoproteins High LDL (bad) cholesterol Build up in the inner walls of arteries (atherosclerosis) High HDL (good) cholesterol Removes excess cholesterol from arterial plaque ( exercise, weight loss, alcohol and estrogens) HDL/LDL ratio High Lp(a) High risk of atherosclerosis Arg158 Familial hypercholesterolemia Development of atherosclerosis due to the deficient LDL receptors One genetic variant of apoe (apoe4, R112, R158, 7% occurrence) is associated with an increased (16- Cys112 fold) incidence of Alzheimer s disease stronger molecular association with Aβ peptides than apoe3 (C112, R158, 78% occurrence) Another genetic variant of apoe (apoe2, C112, C158,15% occurrence) causes familial type III hyperlipoproteinemia low affinity for LDL receptor (0.1% of the affinity of apoe3 and apoe4) Receptor-binding domain of apoe3

I. Fluid Mosaic Model A. Biological membranes are lipid bilayers with associated proteins

Lecture 6: Membranes and Cell Transport Biological Membranes I. Fluid Mosaic Model A. Biological membranes are lipid bilayers with associated proteins 1. Characteristics a. Phospholipids form bilayers