Biological Membranes & Transport

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1 Biological Membranes & Transport Life without membranes? Life without cells? 1. What are membranes made of? 2. Membranes, static or dynamic? 3. Transport across membranes? View The Inner Life of the Cell video 8 minute version: partial script (w/thumbnails): 1

2 I. Membrane Composition & Architecture A. Each membrane type: characteristic lipids, proteins: 1. Protein-lipid ratios, Table 11-1, p Different membranes-different lipids, Fig Different membranes-different proteins. a. rhodopsin (in our eyes & some bacteria) b. acetylcholine esterase (Where? ) But some tissue specificity 2

3 B. All biological membranes share fundamental properties 1. Fluid mosaic model (lipid is in liquid state) 2. Phospholipids form a bilayer (solvent of membranes) that is quantitatively asymmetric. 3. Membrane proteins are embedded in or stuck to the surface of the lipid bilayer. 4. Orientation of membrane proteins is qualitatively asymmetric. Summary in Fig (Wavy?) View animation? 3

4 C. Lipid bilayer is basic structural element of membranes & its formation is spontaneous (once you have the parts). Fig Types of aggregates: 1. Micelles (Note curvature) 2. Bilayers 3. Vesicles (liposomes) (Note curvature) 4. Back to asymmetric distribution of lipids, Fig Comment re micelles, vesicles, and bilayers. The dominant form that is present is determined by the structure of the lipid. Solubility vs. critical micelle concentration (cmc). 4

5 Erythrocyte plasma membranes (the archetypal membrane system): quantitatively asymmetric for inner/outer lipid mono-layer composition: D. 3 types of membrane protein differ in their association w/ the membrane. 1. See Fig re. peripheral, integral, amphitropic 2. Interactions responsible for holding in/on membrane: a) integral proteins b) peripheral proteins c) amphitropic Amphitropic distribution (membrane vs. cytosol) changes through time (usually in a regulated manner). 5

6 Fig E. Many membrane proteins span the lipid bilayer. 1. What thermodynamic problem is associated with proteins spanning (crossing?) the lipid bilayer? 2. Glycophorin (Fig. 11-8): well characterized example Fig Glycophorin Lot s of interesting details here. 6

7 Aside: I only semi-understand the book s comment re....residues 64 to 74 has some hydrophobic residues and probably penetrates the outer face of the lipid bilayer as shown. F. Integral proteins are held in the membrane by hydrophobic interactions with lipids. (Fig ) 1. Sequence was the initial clue for this. Topologies: Figure 11-9 (Error: labels Type IV & Type V are reversed?) 2. 3-D structure examined by X-ray methods: a) Bacteriorhodopsin: 7 spans! Fig , p. 391 b) Lipids coat the hydrophobic regions of aquaporin and F 0 Na + -ATPase. Fig

8 G. Topology of integral membrane proteins can be predicted from their sequence. 1. Most common ways to cross membrane: a) α-helix b) β-sheet (Fig ) 2. How do you find likely membrane spanning regions? Hydropathy plots for glycophorin and bacteriorhodopsin. Fig Positive inside rule: Arg-Arg-Leu-Ile-Lys-Lys glycophorin Remember hydropathy values from Chapter 3? (See next page.) How are hydropathy values determined? 8

9 Values come from Table 3-1 on p. 77. amino hydro pathy acid value Gly 0.4 Ala 1.8 Pro 1.6 Val 4.2 Leu 3.8 Ile 4.5 Met 1.9 Phe 2.8 Tyr 1.3 Trp 0.9 Ser 0.8 Thr 0.7 Cys 2.5 Asn 3.5 Gln 3.5 Lys 3.9 His 3.2 Arg 4.5 Asp 3.5 Glu 3.5 9

10 Fig Comment on windows? Back to Table 3-1? 3. Visual representation of composition in Fig Think about extractions you performed in organic chemistry lab. Spanning the membrane with β-sheets (barrels?) See also pdb 2omf H. Covalently linked lipids can anchor membrane proteins. 10

11 1. There s more than one way to stick to a membrane. 2. Summary: Fig a) Different residue linkages to proteins b) Different lipid components as anchors c) Different surface distributions of anchors and proteins 11

12 II. Membrane Dynamics The liquid state of membrane lipids implies that the molecules associated with the membrane will be quite mobile (dynamic) unless something ties them down. A. Acyl groups in the bilayer interior are ordered to varying degrees. States: 1. Gel (paracrystalline), highly low temp 2. Liquid ordered, intermediate middle temps 3. Fluid, liquid disordered, lots of high temp See Fig , p. 395 (density change?) View animation? 12

13 4. Some cells can alter their membrane lipid composition to maintain constant fluidity: Note Ratio row. B. Transbilayer lipid movement requires catalysis. 1. If a process is slow (uncatalyzed, this one is) what does that mean energetically (thermodynamically)? Contrast with diffusion within a monolayer: 2. Flippases, floppases, etc. increase the rate of transit between the inner and outer layers of a bilayer: 13

14 Extracellular monolayer PS signals...? C. Lipids & proteins diffuse laterally in the bilayer. Evidence: 1. Fluorescence recovery after photobleaching (FRAP). Fig An elegant experiment. View animation? 2. Single particle tracking: restrictions on movement. Fig : 2,250 frames. Time resolution = 25 μs 14

15 3. Protein based lattice work (ankyrin and spectrin, Fig 11-20) may represent some of the fencing in erythrocytes. Inner Life of the Cell video? D. Sphingolipids & cholesterol cluster together in membrane rafts. Solubility, functional significance? 1. Cholesterol/sphingolipid microdomains are thicker & more ordered than neighboring microdomains; rafts 2. These rafts are enriched in 2 classes of integral membrane proteins. a) anchored w/ 2 fatty acids covalently linked to Cys b) GPI-anchored proteins (see Fig to review both) 15

16 3. Raft details: a) diameter = 50 nm b) This size translates to a few thousand lipids and perhaps 10 to 50 membrane proteins (only a few protein types?). Functional significance? c) Up to 50% of a plasma membrane surface can be rafts. (A crowded ocean?) d) Lipids/proteins constantly move in & out of rafts. 4. Some rafts contain caveolin (see membrane attachment pattern in Fig ). Caveolin dimers force inward curvature of the membrane (Fig ). Caveolin rafts usually involve both monolayers. This is uncommon for rafts. Note violation of the can t go half-way in and come back out rule. 16

17 E. Membrane curvature & Fusion are central to many biological processes. When did you 1 st become you? List of processes, Fig Increased curvature due to: 1. Charge density issues Fig Individual protein action 3. Protein scaffolding interactions (BAR domains). 4. Some proteins function to bring membranes into close contact so fusion can occur Fig v- & t-snare zipper mechanism in synaptic vesicle fusion. 17

18 F. Integral membrane proteins are involved in surface adhesion, signaling, and other cellular processes. Note Ca 2+ binding domains. 1. What holds our cells together? a) integrin (αβ dimers) β subunit genetic disease. b) cadherin c) N-CAM (Ca 2+ independent interactions) d) selectin 2. Many of these perform other critical functions (blood clotting, catalysis, transport, etc.) Comment: anthrax toxin function/endosome ph changes. 18

19 III. Solute Transport across Membranes Some general comments (A, B & E), then many examples. Transport can be broadly characterized as passive ( with an electrochemical gradient) or active ( against said gradient). See 1 st summary figure, next page. 19

20 A. Passive transport: membrane proteins facilitate 1. Q: Why are specialized transport systems selectively advantageous? 2. The term transport implies change (in location). Recall from your previous chemistry that change requires a driving force. 3. Passive transport describes transport driven by an existing electrochemical gradient. Electrochemical? (see next pages) 20

21 a) Chemical gradient: [S] outside [S] inside (Really, activity.) Boltzmann program? b) Electrical gradient: charge outside charge inside Spend a few moments on Fig How to generate the left hand conditions in (a) & (b)? 4. Four types of passive transport: Back to Fig (6- noon). a) All four are biologically important. i) Simple diffusion ii) Ionophores iii) Ion channel iv) Fascilitated diffusion (down electro &/or chemical gradient) 21

22 b) What sorts of compounds can do simple diffusion (at reasonable rates)? c) What sorts of compounds can t do simple diffusion? 5. Thermodynamic picture of transport, Fig Look familiar? What is ΔGE, as shown? B. Transporters & Ion Channels are different. 1. Outcome of genomic analysis: lots of sequence info. 22

23 a) This allows us to establish links between genes. (Biochemical/genetic basis? Gene duplication!) b) Can you provide examples from earlier chapters? c) Do proteins with similar sequences always have similar (identical?) functions? d) Do proteins with different sequences ever have similar functions? 2. Many human transporter genes ($1000) exist. We therefore need to organize the way we view them. There are (many?) different ways to categorize, but 2 categories: Channels & Transporters. Fig

24 a) Channels generally: i) bind S with less stereospecificity than carriers ii) transport S at rates toward diffusion limits iii) are usually not saturable with respect to S c) Transporters (pumps) generally: i) bind substrates (S) with high stereospecificity ii) transport S at rates well below diffusion limits iii) are saturable with respect to S (like enzymes) Fig Note: This is a cartoon. 3. Channels & transporters represent two of the broadest ways to categorize transporter systems. 24

25 4. Structurally, some transporters are clustered α- helices (GLUT1) & some are β-barrels (porins). 5. Some carriers function through an existing electrochemical gradient; some use active transport. The remainder of Chapter 11 looks in more detail at a number (~8) different transport systems (or system types), active transport (generally and through ion gradients), & a specific method to measure ion channel function. 25

26 C. The glucose transporter of erythrocytes (GLUT1) mediates passive transport. 1. GLUT1 does facilitated diffusion (Fig 11-26, noon) 2. Structurally (Fig a): a) A type III integral membrane protein (Fig. 11-9) b) M r. 45,000 c) 12 (long enough to span membrane) hydrophobic runs d) Postulated arrangement in Fig b) & c) How do the helices fit together? Part of one helix shown in 11-3-b 4 helices shown 11-30c 26

27 3. Kinetics: Glucose by GLUT1 transport is 50,000 x faster than by simple diffusion through membrane. a) See Fig Look familiar? K t = K transport b) Binding of glucose is (stereo)specific & saturable. c) [glucose] blood. 5 mm (fasting?) i) D-glucose K t = 1.5 mm ii) D-mannose K t = 20 mm, D-galactose K t = 1.5 mm d) Postulated mechanism in Fig , below. Why (in vivo) is transport essentially unidirectional? 5. Other glucose transporters (12 known in us, Table 11-3) a) GLUT2 ships glucose out of liver (K t = 66 mm) b) GLUT4 and diabetes mellitus, Box

28 Box 11-1 D. The Cl! -HCO 3! exchanger catalyzes electroneutral cotransport of anions across the plasma membrane (example of antiport). 1. a.k.a. Anion exchange (AE) protein 2. Ultimate function: increase rate of HCO 3! transport by blood. Rate enhancement Structural pattern: similar to GLUT1? 28

29 4. Action: Cl! & HCO 3! co-transport must occur a) simultaneously (cotransport) Fig & b) in opposite directions (antiport see Fig ) Again, many different ways to categorize transport systems. 5. There are three different Cl! -HCO 3! exchanger genes a) Red blood cells express AE1 b) AE2 gene product present in large amounts in liver c) AE3 in plasma membranes of brain, heart, & retina E. Active transport results in solute movement against a concentration or electrochemical gradient. 1. Two approaches, see Fig

30 a) 1E: direct energy coupled movement of 1 species b) 2E: energy coupling through movement of 2nd species 2. Back to thermo: ΔG = ΔGNE + RT ln ([P]'[S]) Recall ΔG = ΔGNE + RT ln Q from CHM 112? 3. Transported species not ionic: ΔG = RT ln (C 2 'C 1 ) a) The variables? ΔG R T C 2 & C 1 (Transport region 1 to region 2.) If you understand where ΔGNE went, you are starting to 30

31 get this. b) What happens if S is an ion? Z ö Δψ ΔG = RT ln (C 2 'C 1 ) + Zö Δψ Make sure you can do problems like the worked examples (11-1 & 11-2) with great alacrity. 31

32 F. P-type ATPases undergo phosphorylation during their catalytic cycles. 1. Cellular outcome re. memebrane potential, [Na + ], [K + ] Fig Mechanism, Fig Na + 'K + ATPase function in animals Fig G. V-type & F-type ATPases are ATP-driven H + pumps. Read on your own. 32

33 H. ABC transporters use ATP to drive the active transport of a wide variety of substrates. See Box 11-2 re. Cystic fibrosis (CF) and the CFTR. I. Ion gradients provide the energy for 2E active transport. See Table 11-4 & worked Example J. Aquaporins form hydrophilic transmembrane channels for the passage of H 2 O. This is important. 33

34 K. Ion-selective channels allow rapid movement of ions across membranes. L. Ion-channel movement is measured electrically. M. The structure of the K + channel reveals the basis for its specificity. 34

35 N. Ion gated channels. 1. Neuronal Na + channel is a voltage-gated ion channel. 2. The acetylcholine receptor is a ligand-gated ion channel. Read on your own. O. Defective ion channels can have adverse physiological consequences. 35

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