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UA BIOC 460 - Membrane Transport

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BIOC 460, Spring 2008LEC 20-21: Membranes 3-4, MembraneTransport (corrected slides 37-38, p. 19,3-6-08) 1Lectures 20-21Membranes 3-4:Membrane TransportReading: Berg, Tymoczko & Stryer, Chapter 13, pp. 351-376Problems in textbook: chapter 13, pp. 379-380: #1, 3, 14a, 20Jmol structure of valinomycin:http://www.biochem.arizona.edu/classes/bioc462/462a/jmol/valinomycin/vali.htmJmol structure of gramicidin:http://www.biochem.arizona.edu/classes/bioc462/462a/jmol/gramicidin/gram1.htmAnimations of valinomycin (mobile carrier) and gramicidin (small molecule channel-former):http://www.biochem.arizona.edu/classes/bioc460/spring/460web/lectures/LEC20-21_MembraneTransport/MembraneCarriersPores.htmlKey Concepts• Free energy of transporting material across membrane depends onconcentration gradient across membrane:• For uncharged solutes,• Solutes move spontaneously (ΔGt < 0) from compartment of higher concentration to compartment of lower concentration. Equilibrium: ΔG = 0 when C1 = C2 • Charged solutes: presence of a membrane potential as well as the chemical concentration gradient influences the distribution of ions: Passive transport: spontaneous passage of solute "down" concentration and/or electrical potential gradient -- no input of free energy required• Simple diffusion (no assistance)• Facilitated diffusion (rate enhanced by carrier or channel, generally an integral membrane protein (transporter or permease)–rapid diffusion, "down" a concentration gradient–saturable (max. velocity depends on transporter concentration)–specific (depends on interaction of solute with transporter)–Example: GLUT1 glucose transporter in erythrocytesBIOC 460, Spring 2008LEC 20-21: Membranes 3-4, MembraneTransport (corrected slides 37-38, p. 19,3-6-08) 2Key Concepts, continuedPassive transport, continued:• GLUT1 transporter of erythrocytes– Example of characteristics of a transporter proteins:• works by conformational changes linked to ligand binding• Rapid transport “down” concentration gradient• Saturable (shows a maximum velocity; can measure Kt analogousto Km)• Specific• Gated ion channels (ligand-gated or voltage-gated)– VERY rapid, ~107-108 ions/sec, "down" a concentration gradient– not saturable– degree of specificity/ion selectivity varies– Examples:• Acetylcholine receptor of motor neurons• bacterial potassium channel• Eukaryotic sodium, potassium, and calcium channels Terminology applying to all transporter proteins, passive or active:• Uniport (system in which one solute transported)• Cotransport (system in which transport of one solute is coupled to transport of another solute)– Symport (different solutes transported in same direction)– Antiport (different solutes transported in opposite directions)Key Concepts, continuedActive transport (transport of solute against its concentration gradient)– requires an exergonic process to drive the “uphill” transport• Primary active transport (transport of solute against its concentrationgradient, coupled directly to an exergonic chemical reaction, e.g.,ATP hydrolysis)– Examples:• P-type ATPases– Ca2+ ATPase of muscle cell sarcoplasmic reticulum– Na+-K+ ATPase of animal cell plasma membranes• ABC transporters• Secondary active transport (transport/"flow" of one solute "down" itsconcentration gradient is used to drive transport of a different soluteagainst its concentration gradient energy– Concentration gradient of solute that “drives” the unfavorable processcomes from ATP hydrolysis– Examples• E. coli lactose permease (H+-lactose symporter)• Na+-glucose symporter (in some animal cells• Mechanisms of transport processes involving membrane proteinsusually involve protein conformational changes.BIOC 460, Spring 2008LEC 20-21: Membranes 3-4, MembraneTransport (corrected slides 37-38, p. 19,3-6-08) 3Learning Objectives• Terminology: membrane potential, passive transport, simple diffusion,facilitated diffusion, ionophore, gated channel, P-type ATPase, activetransport (primary, and secondary), cotransport, symport, antiport,uniport• What defines equilibrium for a transport process involving an unchargedsolute? (Express it in words, and in terms of concentrations at the"origin" and at the "destination", C1 and C2, and also in terms ofΔGtransport.)• Explain free energy changes in biological transport reactions in terms ofwhether they are "favorable" (solute is moving in direction to go towardsequilibrium, ΔGtransport < 0) or "unfavorable" (would have to go in otherdirection to go towards equilibrium, ΔGtransport > 0).• Given the equation for free energy of transport, be able to calculateΔGtransport for an uncharged solute (e.g., glucose), given theconcentrations on both sides of the membrane and the direction oftransport.• Explain the 2 terms involved in calculating ΔGtransport for a charged solute,under what conditions the first term (concentration gradient term) wouldbe favorable, and under what conditions the 2nd term (electrical gradientterm) would be favorable.• Briefly explain ionophores, and the difference between mobile carriersand channel-forming compounds, with one example of each.Learning Objectives, continued• Explain whether ion channels mediate passive or active transport, anduse the acetylcholine receptor to explain how one ion channel cancontrol the flow of ions.• Describe the GLUT1 erythrocyte glucose transporter with respect to– its biological function– kinetics of transport ("saturation" behavior: plots of Vo vs. [glucose]and 1/Vo vs. 1/[glucose])– proposed mode of action.• Name 2 P-type ATPases and briefly explain their biological functions.Which one consumes over 30% of the ATP in animal cells in order toestablish/maintain the [Na+] and [K+] concentration gradients andelectrical potential across the plasma membrane? Which oneestablishes/maintains the Ca2+ gradient involved in control of musclecontraction?• Outline the proposed mechanism by which P-type ATPases couple ATPhydrolysis with "uphill" transport of solutes, with the sarcoplasmicreticulum Ca2+-ATPase as the example.• Briefly explain how the E. coli lactose permease couples H+ transportand lactose transport, as an example of a membrane protein thatfunctions as a secondary active transporter, coupling "downhill" transportof one solute to "uphill" transport of another solute.BIOC 460, Spring 2008LEC 20-21: Membranes 3-4,


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UA BIOC 460 - Membrane Transport

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