Unit 5 1 UNIT 5 MEMBRANES AND MEMBRANE PROTEINS Note: The test for this unit is combined with the test for Unit 4. Membranes define the external boundary of cels and regulate it's molecular trafic. They divide the internal space of the cel into discrete compartments. Membranes are not merely pasive bariers but house an aray of proteins specialized for promoting or catalyzing a variety of molecular events. Pumps move specific organic solutes and inorganic ions across the membrane against a concentration gradient; energy transducers convert one form of energy into another, and receptors on the plasma membrane sense extracelular signals. These ideas wil be reviewed in this unit and studied in more depth in Unit 12. Asignment: Nelson & Cox, pp. 343 - 345, 349 (start at section 10.2) - 350 (stop at "Some glycerophospholipids have ether linked faty acids"), 351, 355 - 357 ("sterols have four fused carbon rings"), 371 - 386 (stop at "caveolins.."), 389 - 391 (stop at "The Glucose Transporter?."), 396 - 399 (P-type ATPases). 1. Given the total number of carbons, and the number and positions of double bonds, draw the structure of a straight-chain faty acid (pp. 343 - 345; note the conventions for positions of double bonds). Unit 5 2 2. Representing the structure of a faty acid as shown below, draw the structure of a phosphatidate (diacylglycerol 3-phosphate; p. 351). R-C-OH O a. What kind of linkage joins the faty acids to the glycerol part of the molecule? b. What kind of linkage joins the phosphate to the glycerol part of the molecule? 3. Review phospholipid structures (Fig. 10-9, p. 351). The reason why so many diferent lipids are found in membranes is stil not clear. In eukaryotes the various internal membranes al have diferent lipid compositions, and none are formed from a single type of lipid. The shape (size of head) of diferent lipids varies and lipids can be asorted to obtain the required curvature. Charge is another important factor. Some lipids also have important signaling functions. a. Given the structure of the head group, draw the structure of any of the glycerophospholipids listed in Fig. 10-9 (p. 351). b. What is the net charge on each lipid listed below? 1) Phosphatidyl choline 2) Phosphatidyl serine 3) Phosphatidyl ethanolamine 4. Cholesterol (pp. 355 - 357) Cholesterol is a flat, stif molecule which modulates fluidity and may have other functions related to its distribution in eukaryotic cel membranes (Fig. 11-2, p. 372). Do bacteria make sterols (p. 356)? Unit 5 3 5. The Lipid Bilayer (p. 374) Using Fig. 11-4 (p. 374), discuss the structures of bilayers, vesicles (liposomes), and miceles. a. What is the major driving force for formation of these structures? b. Detergents, including soaps (i.e. ionized faty acids), form iceles and phospholipids form bilayers. Use Fig. 11-4 (p. 374) to discuss the diference in structure that could acount for the formation of miceles vs bilayers? c. Liposomes, which can be made by several methods, such as sonication, have a wide variety of sizes. They have been used extensively to study transporters and receptors, lipid permeability, and lipid interactions. 6. Membrane proteins can be categorized as integral, peripheral, or amphitropic (p. 375). a. Use Fig. 11-6 (p. 375) to define these terms. b. Use Fig. 11-6 (p. 375) to discuss how detergents "solubilize" membranes and membrane proteins. Note that detergents also "solubilize" lipids by forming mixed miceles with them. 7. Membrane protein structure (pp. 375 - 381) a. The structures of many integral membrane proteins have been determined. Make some generalizations about these structures in terms of ?-helices and ?-sheets (Figs. 11-9 [p. 377], 11-13 [p. 379]). Note also the distribution of Trp, Tyr, and charged residues in membrane proteins (Fig. 11-12, p. 379). b. When the primary sequence but not the structure of a membrane protein is known, how can a model of the structure be made (se Figs. 11-11 [p. 378] and 11-29a [ p. 392]). Unit 5 4 8. The "Fluid - mosaic" model of membrane structure proposed that both the lipid and protein components of membranes are mobile (pp. 381 - 384). a. Use Table 10-1 (p. 344) and p. 381 to discuss the following: 1) What is the efect of unsaturated faty acids on the fluidity of a membrane? 2) What is the relationship betwen faty acid length and fluidity? 3) Distinguish betwen the gel phase of membrane lipids, the liquid-disordered and the liquid-ordered states (p. 381). b. What is the major barier to movement of phospholipids and proteins from one side of a membrane to the other (pp. 381 - 383)? 1) The asymmetric distribution of lipids betwen the two sides of the bilayer as shown in Fig. 11-5 (p. 375) is presumably created by flippases of diferent specificity (p. 382)? 2) Distinguish betwen a "flippase", a "floppase", and a "scramblase" (pp. 382 - 383). c. Use Fig. 11-17 (p. 383) to name and describe the experiment typicaly used to measure the rate of lateral difusion of lipids and proteins in membranes. Note that fluorescence is not typicaly used to visualize single molecules. 9. Describe "microdomains", or "rafts", in the context of plasma membranes (p. 384). 10. In general terms, discuss how Atomic Force Microscopy can be used to visualize membrane proteins (Box 11-1, p. 385). 11. Solute transport across membranes (pp. 389 - 391, 396 - 399) a. Discuss the two components of the electrochemical potential that determine the direction of movement of charged solutes across membranes (pp. 389 - 391). Unit 5 5 b. Use Fig. 11-25 (p. 389) to diferentiate betwen the diferent types of transport. Facilitated difusion is also known as uniport, and secondary active transport as symport (shown) or antiport. c. P-Type ATPases (pp. 396 - 399) Recently there has been considerable progres in working out the mechanisms of active transport systems, largely from the determination of structures using x-ray difraction. As an example, we wil focus on the Ca 2+ ATPase. Compare the structure (Fig. 11-35, p. 397) with the postulated mechanism (Fig. 11-36, p. 398) which comes from many measurements of ion binding and formation of the covalent phosphorylated intermediate, E-P. How is the energy of ATP hydrolysis coupled to the generation of the calcium gradient? In Unit 12, you wil learn about another ion-transporting ATPase, ATP synthase, which is very diferent. Jim Blankenship Microsoft Word - U05_F08.doc
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