CHAPTER 11 Membranes

Transcription

1 CHAPTER 11 Membranes Key topics The function of biological membranes The structure and composition of membranes Dynamics of membranes Structure and function of membrane proteins Transport across biological membranes 1

2 Lipids aggregate into structures in water Structures formed depend on type of lipid, concentration Micelles Liposomes Bilayers 2

3 Micelle Forms in the solution of amphipathic molecules that have larger head than tail (Fatty acids, Sodium dodecyl sulfate) Each micelle has from a few dozen to a few thousand lipid molecules Aggregation occurs when the concentration of molecules is higher than a certain threshold 3

4 Vesicle (Liposome) Small bilayers will spontaneously seal into spherical vesicles Vesicle membranes can contain artificially inserted proteins The central aqueous cavity can enclose dissolved molecules They are useful artificial carriers of molecules (e.g., drugs) Vesicles fuse readily with cell membranes or with each other 4

5 Membrane Bilayer consists of two leaflets of lipid monolayers Hydrophilic head groups interact with water Hydrophobic fatty acid tails are packed inside One leaflet faces the cytoplasm Another leaflet faces the extracellular space or the inside of membrane-enclosed organelle 5

6 What are membranes? Complex lipid-based structures that form pliable sheets Composed of a variety of lipids and proteins Some membrane lipids and proteins are glycosylated All cells have a cell membrane, which separates the cell from its surrounding Eukaryotic cells have various internal membranes that divide the internal space into compartments

7 Electron Micrograph of Biological Membranes 7

8 Functions of Membranes Define the boundaries of the cell Allow import and export Selective import of nutrients (e.g., lactose) Selective export of waste and toxins (e.g., antibiotics) Retain metabolites and ions within the cell Sense external signals and transmit information into the cell Provide compartmentalization within the cell separate energy-producing reactions from energy-consuming ones keep proteolytic enzymes away from important cellular proteins Produce and transmit nerve signals Store energy as a proton gradient Support synthesis of ATP 8

9 Common Features of Membranes Sheet-like flexible structure, Å (3 10 nm) thick Main structure is composed of two leaflets of lipids (bilayer) With the exception of archaebacteria: monolayer of bifunctional lipids Form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect Protein molecules span the lipid bilayer Asymmetric Some lipids are found preferably inside Some lipids are found preferably outside Carbohydrate moieties are always outside the cell Electrically polarized (inside negative ~ 60mV) Fluid structures: two-dimensional solution of oriented lipids 9

10 Fluid Mosaic Model of Membranes Proposed in 1972 by Singer and Nicholson (UCSD) Lipids form a viscous, two-dimensional solvent into which proteins are inserted and integrated more or less deeply Integral proteins are firmly associated with the membrane, often spanning the bilayer Peripheral proteins are weakly associated and can be removed easily Some are noncovalently attached Some are linked to membrane lipids 10

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12 The Composition of Membranes Lipid composition of membranes is different in different organisms, tissues, and organelles Ratio of lipid to protein varies Type of phospholipid varies Abundance and type of sterols varies: prokaryotes lack sterols Cholesterol predominant in the plasma membrane, virtually absent in mitochondria Galactolipids abundant in plant chloroplasts but almost absent in animals 12

13 Membrane composition is highly variable in different organisms 13

14 Membrane Structure in Archaea Unique glycerol chirality in phospholipids L-glycerol in archaea D-glycerol in bacteria Unique fatty acids Branched isoprene chains in archaea Unbranched fatty acid chains in bacteria Unique linkages Ether linkages in archaea Ester linkages in bacteria Membrane topology Monolayer in some archaea Bilayer in all bacteria 14

15 Lipid Monolayer in Archaea Sulfolobus solfataricus and relatives Volcanic hot springs Temperatures C Acidic environment: ph 2 3 Better membrane stability by Isoprenoid tetraethers with unique alcohols 15

16 Membrane composition is highly variable in different organelles 16

17 Membrane bilayers are asymmetric Two leaflets have different lipid compositions Outer leaflet is often more positively charged Phosphatidylserine outside has a special meaning Platelets: Activates blood clotting Other cells: Marks the cell for destruction 17

18 Membrane composition and asymmetry 18

19 Functions of Proteins in Membranes Receptors: detecting signals from outside Light (opsin) Hormones (insulin receptor) Neurotransmitters (acetylcholine receptor) Pheromones (taste and smell receptors) Channels, gates, pumps Nutrients (maltoporin) Ions (K-channel) Neurotransmitters (serotonin reuptake protein) Enzymes Lipid biosynthesis (some acyltransferases) ATP synthesis (F 0 F 1 ATPase/ATP synthase) 19

20 Three Types of Membrane Proteins 20

21 Peripheral Membrane Proteins Associate with the polar head groups of membranes Relatively loosely associated with membrane through ionic interactions with the lipids or aqueous domains of integral membrane proteins Removed by disrupting ionic interactions either with high salt or change in ph Purified peripheral membrane proteins are no longer associated with any lipids 21

22 Integral Membrane Proteins Span the entire membrane Have asymmetry like the membrane Different domains in different compartments Tightly associated with membrane Hydrophobic stretches in the protein interact with the hydrophobic regions of the membrane Removed by detergents that disrupt the membrane Purified integral membrane proteins still have phospholipids associated with them 22

23 Six Types of Integral Membrane Proteins 23

24 Hydropathy plots can predict transmembrane domains 24

25 Amino acids in membrane proteins cluster in distinct regions Transmembrane segments are predominantly hydrophobic Tyr and Trp cluster at nonpolar/polar interface Charged amino acids are only found in aqueous domains 25

26 Membrane proteins also contain β-sheets 26

27 Lipid Anchors Some membrane proteins are lipoproteins They contain a covalently linked lipid molecule Long-chain fatty acids Isoprenoids Sterols Glycosylated phosphatidylinositol (GPI) The lipid part can become part of the membrane The protein is now anchored to the membrane reversible process allows targeting of proteins Proteins with GPI anchors are found only on the outer face of plasma membrane 27

28 Lipid-linked Membrane Proteins 28

29 Farnesylation of Proteins Proteins can be targeted to the inner leaflet of the plasma membrane by farnesylation Primary sequence of the protein contains a signature for farnesylation: CaaX C is a conserved Cys a is usually an aliphatic amino acid X is Met, Ser, Glu or Ala This reaction is catalyzed by farnesyl transferase Nonfarnesylated proteins do not go to the membrane and are inactive Promising cancer therapy (onco-ras) 29

30 Cell membranes are asymmetric Every component of the membrane exhibits asymmetry Lipids Outer and inner leaflets have different lipid compositions Proteins Individual peripheral membrane proteins are only associated with one side of the membrane Integral membrane proteins have different domains on different sides of the membrane. Specific lipid modification of proteins targets the protein to a specific leaflet Carbohydrates Only on the outside of cells 30

31 Physical Properties of Membranes Dynamic and flexible structures Can exist in various phases and undergo phase transitions Not permeable to large polar solutes and ions Permeable to small polar solutes and nonpolar compounds Permeability can be artificially increased by chemical treatment When we want to get DNA into the cell 31

32 Membrane Phases Depending on their composition and the temperature, lipid bilayer can be in gel or fluid phase Gel phase: individual molecules do not move around Fluid phase: individual molecules can move around Heating causes phase transition from the gel to fluid Under physiological conditions, membranes are more fluid-like than gel-like Must be fluid for proper function 32

33 Organisms can adjust the membrane composition Membrane fluidity is determined mainly by the fa composition More fluid membranes require shorter and more unsaturated fa * Remember differences in T M of fa At higher temperatures cells need more saturated fa: To maintain integrity At lower temperatures cells need more unsaturated fa: To maintain fluidity 33

34 Sterols and hopanols increase membrane rigidity and permeability HO cholesterol Cell membranes of many eukaryotes contain sterols Cholesterol in animals Phytosterols in plants Ergosterol in fungi Cell membranes of aerobic prokaryotes contain hopanols HO ergosterol OH OH O NH 2 OH OH HO OH OH one particular hopanol 34

35 Membrane Dynamics: Lateral Diffusion Individual lipids undergo fast lateral diffusion within the leaflet 35

36 Membrane Dynamics: Transverse Diffusion Spontaneous flips from one leaflet to another are rare because the charged head group must transverse the hydrophobic tail region of the membrane 36

37 Membrane Diffusion: Flippase Flippases catalyze transverse diffusion Flippases use energy of ATP to move lipids against the concentration gradient 37

38 Study of Membrane Dynamics Fluorescence Recovery After Photobleaching (FRAP) allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescence return From the rate of return of lipids, the diffusion coefficient of a lipid in the leaflet can be determined Rates of lateral diffusion are high (up to 1 µm/sec) a lipid can circumnavigate E.coli cell in one second

39 Movement of a Single Lipid in 56 ms 39

40 Membrane Rafts Lipid distribution in a single leaflet is not random or uniform Lipid rafts contain clusters of glycosphingolipids with longer-than-usual tails are more ordered contain specific doubly or triply acylated proteins allow segregation of proteins in the membrane 40

41 Caveolin forces membrane curvature Caveola are small invaginations in the plasma membrane 41

42 Other Modes of Membrane Curvature 42

43 Membrane Fusion Membranes can fuse with each other without losing continuity Fusion can be spontaneous or protein-mediated Examples of protein-mediated fusion are Entry of influenza virus into the host cell Release of neurotransmitters at nerve synapses

44 Neurotransmitter Release 44

45 Transport Across Membranes Cell membranes are permeable to small nonpolar molecules that passively diffuse through the membrane Passive diffusion of polar molecules involves desolvation and thus has a high activation barrier Transport across the membrane can be facilitated by proteins that provide an alternative diffusion path Such proteins are called transporters or permeases 45

46 Types of Transport 46

47 Polar solutes need alternative paths to cross cell membranes 47

48 Three Classes of Transport Systems 48

49 Glucose Transporter in the Membrane 49

50 Model for Glucose Transport 50

51 There are multiple glucose transporters A Na + -glucose symporter and a glucose uniporter operate on opposite sides of epithelial cells Cells can also have asymmetry, with distinct proteins confined to one side 51

52 Bicarbonate transporter is an antiporter Maintains the electrochemical potential across the membrane 52

53 Two Types of Active Transport 53

54 ABC transporters use ATP hydrolysis to drive transport of substrates (a) The multidrug transporter of animal cells (MDR1, also called P glycoprotein; PDB ID 3G60), responsible for pumping a variety of antitumor drugs out of human cells, (b) The vitamin B 12 importer BtuCD (c) Mechanisms proposed for the E. coli vitamin B 12 ABC transporter 54

55 Proton Transport and Chemical Energy of ATP Energy of ATP hydrolysis can be used to drive protons through the membrane ph control in the cell by F-type ATPase Energy of the proton gradient can be used to synthesize ATP in chloroplast and mitochondrial membranes by ATP synthase 55

56 Proton driven ATPases can function in both directions The V o V 1 H + ATPase uses ATP to pump protons into vacuoles and lysosomes, creating their low internal ph. Fo F1 ATPase/ATP synthase of mitochondria has an integral domain, Fo (orange) and a peripheral domain, F1

57 Aquaporins allow rapid water passage through membranes 57

58 Ion channels maintain gradients for active transport 58

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60 Cystic fibrosis is caused by a mutation to an ion channel in epithelial cells 60

61 In this chapter, we learned membranes are composed of various lipids and proteins phospholipids form a selectively permeable bilayer properties of the bilayer depend on the lipid composition, which varies strongly from organism to organism tissue to tissue organelle to organelle membrane proteins play a variety of structural and functional roles, especially in the transport of solutes across the membrane Active transport of solutes across membranes requires ATP but can be accomplished in many different ways 61