MCB110 FINAL May 20, 2002 . Your name and student ID QUESTION POINTS 1 (20 points) 2 (15 points) 3 (10 points) 4 (40 points) 5 (10 points) 6 (15 points) 7 (10 points) 8 (25 points) 9 (10 points) 10 (15 points) 11 (10 points) 12 (10 points) 13 (10 points) 14 (25 points) 15 (25 points) 16 (50 points) TOTAL (300 points) WARNING: Your exam will be taken apart and each question graded separately. Therefore, if you do not put your name and ID# on every page or if you write an answer for one question on the backside of a page for a different question you are in danger of irreversibly LOSING POINTS!1.a – A bacterial colony is shifted from 25 to 37 ˚C. How do you expect this will affect the activity of membrane desaturases? (5 p.) As the temperature goes up there will be less need of unsaturated bonds to maintain the membrane fluidity, so the activity of desaturases will go down. 1.b – Why are glycosylated lipids less likely to flip-flop than other lipids (5 p.) Glycosylated lipids will have an oligosaccharide chain, made of large, polar sugar molecules. It will be very energetically unfavorable for this oligosaccharide chain to go through the hydrophobic core of the lipid bilayer 1.c – Give an example of an application of liposomes in the study of ion channels, and one of their application in medicine (10 p.) Liposomes are used with inserted channels in patch clamping experiments. In medicine they can be used for drug delivery, especially when the drug is hydrophobic or crystalline.2. a – On the basis of the hydropathy plots on the right, predict which of the three proteins will be an integral membrane protein and why (5 p.) Only protein in c) is likely to have 4 helical transmembrane regions, three N-terminal and one C-terminal. 2.b – State a type of transmembrane domain/secondary structure that would not be predicted by this method and explain why. (5 p.) Integral membrane proteins made of beta sheets forming a barrel (like porins) will not be detected by this method, which is based on finding hydrophobic patches of about 20 a.a. corresponding to the length of an alpha helix across the hydrophobic part of the lipid bilayer. In beta barrels the hydrophobic and hydrophilic a.a. alternate. 2.c – Indicate two reasons why integral membrane proteins are difficult to purify and to characterize biochemically. (5 p.) They are generally very rare (just localize to the 2-D space of the membrane), and they have to be solubilized. 3 extra points if they indicate that heterologous overexpression of membrane proteins is difficult.3. – What is the free energy cost of pumping Ca2+ out of the cell when the cytosolic concentration is 0.1 μM, the concentration outside is 1 mM, and the membrane potential is –70 mV? (2.3RTroom = 1.4 kcal/mol; F = 23.06 kcal/mol.V; log10 (1)=0; log10 (10)=1; log10 (100)=2, etc) (10 p.) G = 2.3RT log10Co[]Ci[]+ zFE = 1.4 log10 (104) + (2) (23.06)(0.07) = = 5.6 + 3.2 = 8.8 kcal/mol 4.a – The K+ and Na+ channels have basically the same structure and are oriented in the same way in the plasma membrane. Yet Na+ channel allows movement of ion inside the cell and the K+ channel allows movement of ions to the outside of the cell. Why do you think that the Na+ and K+ channels allow flow of charge in opposite directions? (10 p.) Both channels let ions move in the direction down their electrochemical gradients. The Na+/K+ ATPases uses energy to generate gradients of these ions. K+ is concentrated inside the cell, so open of K+ channels will lead to exit of ions, while Na+ is concentrated inside the cell, so open Na+ channels will let ions in. 4.b – Batrachotoxin is a frog poison that causes the persistent opening of Na+ channel when the membrane is depolarized, but they close when the membrane is repolarized. Which structural process is blocked by the poison? (10 p.) The deactivation of the channel through the ball-chain mechanism by which the N-terminal, cytosolic domain of the channel blocks the ion path following channel opening.4.c – If you were to inject a squid giant axon with a tiny volume of solution containing 0.1 M NaCl and 0.1 M KCl in which both the Na+ and K+ ions are radioactively labeled, which of the radioactively labeled ions would you expect to appear within the sea water medium when the neuron is at rest? And after stimulation? (10 p.) At rest K+ will come out through leaking K+ channels and Na+ will come out through the Na+/K+ ATPase. After stimulation K+ will rapidly come out through voltage-gated channels causing repolarization. 4.d. – The extracellular concentration of Cl- is 123 mM and the intracellular concentration is 4 mM. What will be the effect of Cl- channel opening on the excitability of a neuron? Justify why you think the ions will move in or out of the cell (10 p.) 1.4 log 4/123 + 1 (23.06) 0.07 = - 2.08 + 1.7 = - 0.38 kcal/mol thus Cl- will get into the cell. The entrance of negative charge will cause the hyperpolarization of the membrane, making it more difficult a depolarization event. Therefore the opening of Cl- channels causes the inhibition of the neuron. 5. – How would you compare the rate of movement of ions passing through a channel with those transported actively through a P-type pump? Explain why. (10 p.) Transport through a channel is many orders of magnitude faster. Once the channels opens many ions pass through. In a P-type pump typically one ion goes trough per ATP cycle, which involves ATP binding, hydrolysis and several conformational changes in the pump, which take much longer to occur. 6.a – If you were to use a drug that interferes with binding of ribosomes to mRNA, what effect would this have on the appearance of the RER? (5 p.) Can you think of a mutation in SRP that would have a similar effect? (5 p.)The effect will be the disappearance of the “roughness” that is cause by the binding of ribosomes to the cytosolic side of the RER membrane. An equivalent effect will be obtained if the SRP is mutated so that it cannot release GDP and bind to GTP, and step necessary for its binding to the ribosome and for the recruitment of the ribosome to the RER. Other possibilities are a mutation that inhibits SRP binding to the signal sequence, to the ribosome, or to the SRP receptor. For those that are
View Full Document
Unlocking...