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Published bySharyl Harvey Modified over 8 years ago
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Membrane structure & function
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Integral proteins Can have any number of transmembrane segments –Multiple transmembrane segments: often small molecule transport
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Transport example: Bacteriorhodopsin Proton pump: establish H + gradient Subsequent ATP generation 7 transmembrane segments of ~20 AA – -helical –Hydrophobic interaction anchor –Pore for H + movement –Interior of helices has some polar/charged character
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1° structure: predict transmembrane segments “Hydropathy plots” Predict whether sequence is Hydrophobic enough to cross membrane –Measure the G when AA transferred from Hydrophobic into H 2 O –Calculate a ‘hydropathy index’ for a particular segment –If index of region > 0 → transmembrane segment
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Tyr and Trp Higher presence at membrane interface in integral proteins –Can interact both with lipids and H 2 O –Tyr (orange), Trp (red), charged (purple)
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How do molecules cross the membrane? Membrane fusion –uptake and release without “crossing a membrane” –Endocytosis: internalization of a vesicle –Exocytosis Requires –Two bilayers recognize each other –Bilayers become closely ‘apposed’ In position to fuse –Local disruption of bilayers –Fusion of bilayers to form a continuous surface Mediated by fusion proteins –Recognition and local distortion
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Regulated exocytosis
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Simple diffusion: permeable divider (ie. solute able to diffuse through the membrane) Uncharged species (polar or nonpolar) –Based on concentration gradient Solute: net diffusion toward dilute side At equilibrium: no net diffusion
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Simple diffusion Charged species –Concentration gradient and electrical gradient (membrane potential V m ) Drives ions to reduce V m –Ion movement depends on the electrochemical potential Tend to equalize concentration AND equalize charge
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Facilitated diffusion Transporters or permeases that decrease E a –Span lipid bilayer at least once –Movement only in thermodynamically favored direction –Affinity/specificity through weak forces Classes –Carriers Bind with high specificity Saturable Not very efficient Monomers –Channels Rapid transport Less stereospecificity Oligomers
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Glucose transporter of erythrocytes Required for metabolism Transport of glucose from plasma into cells Uniport system (one solute) 50000 x faster transport ~12 transmembrane regions (hydropathy plots) – helices that have an polar/electrostatic channel for transport
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Glucose transporter of erythrocytes Behaves like a MM enzyme –Saturation effects –Model one glucose binds at a time No covalent bonds Fully reversible process –Concentration gradient dependent –Passive transport
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Active transport Solute accumulation against equilibrium –movement from low to high [solute] Thermodynamically unfavorable: requires energy 1° active transport –Coupled to exergonic chemical reaction –Commonly ATP hydrolysis P-type, F-type, V-type, multidrug type 2° active transport –Coupling of endergonic and exergonic transport of 2 different solutes Exergonic process drives endergonic transport
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Transport ATPases P-type –Cation transporters –Reversible phosphorylation by ATP conf. change V-type & F-type –H + transport –Acidification of intracellular compartments (lysosomes) –Drives ATP synthesis Multidrug transporters –Clinical significance –Transports drugs out of tumor cells or microbial cells –‘multi-drug resistance’
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P-type ATPases Na + K + ATPase [Na + ] intra low [K + ] intra high Cells accumulate K + and release Na + Control of cell volume, action potentials, sugar and AA transport Each ATP hydrolyzed 3Na + out and 2 K + in –Membrane potential (V m ) -50-70 mV Maintenance 20-40% metabolic energy of most cells
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Na + K + ATPase –Model EnzI has high Na + affinity Enz II has high K + affinity
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Na + K + ATPase Inhibitors –Ouabain –Digitoxigenin –Used as cardiac glycosides to treat congestive heart failure –Stabilize the E 2 -P complex Na + accumulation in cells Antiporter (Ca 2+ in and Na + out) is activated elevated cytosolic Ca 2+ stimulates and strengthens contractions of heart muscles Digitoxigenin-foxglove Strophanthus gratus
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