Membrane structure & function
This overview details the structure and function of integral proteins within cellular membranes, particularly their role in transport mechanisms. Integral proteins can have multiple transmembrane segments, facilitating the transport of small molecules, as demonstrated by bacteriorhodopsin, a proton pump crucial for ATP generation. The document explores methods to predict transmembrane segments using hydropathy plots, mechanisms behind membrane fusion via endocytosis and exocytosis, and contrasts simple diffusion, facilitated diffusion, and active transport, including the significance of P-type ATPases in maintaining ion gradients vital for cellular functions.
Membrane structure & function
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Presentation Transcript
Integral proteins • Can have any number of transmembrane segments • Multiple transmembrane segments: often small molecule transport
Transport example: Bacteriorhodopsin • Proton pump: establish H+ gradient Subsequent ATP generation • 7 transmembrane segments of ~20 AA • a-helical • Hydrophobic interaction anchor • Pore for H+ movement • Interior of helices has some polar/charged character
1° structure: predict transmembrane segments“Hydropathy plots” • Predict whether sequence is Hydrophobic enough to cross membrane • Measure the DG when AA transferred from Hydrophobic into H2O • Calculate a ‘hydropathy index’ for a particular segment • If index of region > 0 → transmembrane segment
Tyr and Trp • Higher presence at membrane interface in integral proteins • Can interact both with lipids and H2O • Tyr (orange), Trp (red), charged (purple)
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
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
Simple diffusion • Charged species • Concentration gradient and electrical gradient (membrane potential Vm) • Drives ions to reduce Vm • Ion movement depends on the electrochemical potential • Tend to equalize concentration AND equalize charge
Facilitated diffusion • Transporters or permeases that decrease Ea • 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
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) • a helices that have an polar/electrostatic channel for transport
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
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
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’
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 (Vm) -50-70 mV • Maintenance 20-40% metabolic energy of most cells
Na+K+ ATPase • Model • EnzI has high Na+ affinity • Enz II has high K+ affinity
Na+K+ ATPase Digitoxigenin-foxglove • Inhibitors • Ouabain • Digitoxigenin • Used as cardiac glycosides to treat congestive heart failure • Stabilize the E2-P complex • Na+ accumulation in cells • Antiporter (Ca2+ in and Na+ out) is activated • elevated cytosolic Ca2+ stimulates and strengthens contractions of heart muscles Strophanthus gratus
