Harvey Lodish Arnold Berk Paul Matsudaira Chris A
Harvey Lodish • Arnold Berk • Paul Matsudaira • Chris A. Kaiser • Monty Krieger • Matthew P. Scott • Lawrence Zipursky • James Darnell Molecular Cell Biology Fifth Edition Chapter 7: Transport of Ions and Small Molecules Across Cell Membranes Copyright © 2004 by W. H. Freeman & Company
Cell membrane Barrier to the passage of most polar molecule ·Maintain concentration of solute
Aquaporin, the water channel, consists of four identical transmembrane polypeptides
Relative permeability pf synthetic lipid bilayer to different classes of molecule
Diffusion rate depends on : 1. Concentration gradient or electrochemical gradient 2. Hydrophobicity 3. i. e. higher partition coefficient 4. 3. Particle size
Three main class of membrane protein 1. ATP- power pump( carrier, permease) couple with energy source for active transport binding of specific solute to transporter which undergo conformation change 2. Channel protein formation of hydrophilic pore allow passive movement of small inorganic molecule
3. Transporters uniport symport antiport
1. All transmembrane 2. proteins 2. ATP binding sites 3. Move molecules uphill against its gradient
Kinetics of simple diffusion and carrier mediated diffusion
Unique features for Uniport transport: 1. Higher diffusion rate for uniport 2. Irrelevant to the partition coefficient 3. Transport rate reach Vmax when each uniport working at its maximal rate 4. Each uniport transports only a single species of molecules or single or closely related molecules
Liposome containing a single type of ytransport protein are useful in studying functional properties of transport protein
Families of GLUT proteins( 1 -12) GLUT 1 GLUT 2: express in liver cell ( glucose storage) and ß cell( glucose uptake) pancrease GLUT 4: found in intracellular membrane, increase expression by insulin, lowers the blood glucose
ATP powered pump 1. P- class 2. 2 , 2 subunit 3. i. e. Na+-K+ ATP ase, Ca+ATP ase, H+pump 4. 2. F-class 5. locate on bacterial membrane , chloroplast and mitochondria 6. pump proton from exoplasmic space to cytosolic for ATP synthesis 7. 3. V-class 8. maintain low p. H in plant vacuole
Operational model of the Ca+-ATP ase in the SR membrane of skeletal muscle cells Higher Ca+2 Lower Ca+2
Structure of the catalytic subunit of the muscle Ca+2 ATP ase
-helix Phosphorylation site
Operational model of the Na+/K+ ATP ase in the plasma membrane Higher affinity for Na+
V-class H+ ATP ase pump protons across lysosomal and vacuolar membrane
Effect of proton pumping by V-class ion pumps on H+ concentration gradients and electric potential gradients across cellular membrane
Generation of electrochemical gradient Electrochemical gradient combines the membrane potential and concentration gradient which work additively to increase the driving force
ABC transporter 2 T ( transmembrane ) domain 6 - helix form pathways for transported substance · 2 A ( ATP- binding domain) 30 -40% homology for membranes
i. e. bacterial permease use ATP hydrolysis transport a. a , sugars, vitamines, or peptides inducible, depend on the environmental condition i. e. mammalian ABC transporter ( Multi Drug Resistant) export drug from cytosol to extracellular medium mdr gene amplified by drus stimulation mostly hydrophobic for MDR proteins
Structural model for E. coli flippase 6 - helix
Flippase model of transport by MDR 1 and similar ABC proteins
Diseases linked with ABC proteins 1. ALD( X-link adrenoleukodestrophy) 2. 3. defect in ABC transport protein( ABCD 1) located on peroxisome, used for transport for very long fatty acid 4. 2. Tangiers disease 5. Dificiency in plasma ABCA 1 proteins, which is used for transport of phospholipis and cholesterol 6. 3. Cystic fibrosis 7. mutation of CTFR( cyctic fibrosis transmenbrane regulator; a Cl- transporter in the apical membrane of lung, sweat gland pancrease)
Ion Channel Generation of electrochemical gradient across plasma membrane i. e. Ca+ gradient regulation of signal transduction , muscle contraction and triggers secretion of digestive enzyme in to exocrine pancreastic cells i. e. Na+ gradient uptake of a. a , symport, antiport
Q: how does the electrochemical gradient formed? Selective movement of Ions Create a transmembrane electric potential difference
Measuring the electrochemical gradient
Structure of resting K+channel from the bacterium Streptomyces lividans
Important for selection
Smaller Na+ does not fit perfectly Replacement of carboxyl backbone from P segment
Oocyte expression assay is useful in comparing the function of normal and mutant forms of channel proteins
Cotransport: Use the energy stored in Na+ or H+ electrochemical gradient to power the transport of another subatance Symport: the transportd molecules and cotransported ion move in the same direction Antiport: the transported molecules move in opposited direction
Operation Model for the two-Na+/one glucose symport Glucose transport against its gradient in the epithelial cells of intestine 1 glucose in 2 Na+ in G=0
Na+ linked antiport Exports Ca+2 from cardiac Muscle Cells 3 Na+ out+ Ca+2 in 3 Na+ in+ Ca+2 out maintenance of low cytosolic Ca+2 concentration i. e. inhibition of Na+/K+ ATPase by Quabain and Digoxin raises cytosolic Na+ lowers the efficiency of Na+/Ca+2 antiport increases cytosolic Ca+2 ( used in cogestive heart failure)
Cotransporters that regulate cytosolic p. H H 2 CO 3 H+ H+ + HCO- can be neutrolized by 1. Na+/HCO 3 -/Cl- antiport 2. Cabonic anhydrase HCO 33. Na+/H+ antiport CO 2+OH-
The activity of membrane transport proteins that regulate the cytosolic p. H of mammalian cells changes with p. H
Plant vacuole membrane p. H 3— 6 Low acidity maintained by V-class ATP-powered pump PPi -powered pump
Concentration of ions and sucrose by the plant vacuole
Movement of water Osmosis: movement of water across semipermeable membrane Osmotic pressure: hydrostatic pressure uses to stop the net flow of water
Osmotic pressure =RT( C -C ) B A
Expression of aquaporin by frog oocytes increases their permeability Aquaporin 1 erythrocyte Aquaporin 2 kidney cells
Water channel pprotein( aquaporin)
tetrameric · 6 -helices for each subunit
· 2 -nm-long water selective gate 0. 28 nm gate width ·Highly conserved arginine and histidine in the gate H 2 O for HO bonding with cystein
Transepithelial transport Import of molecules on the lumen side of intestinal epithelial cells and their export on the blood facing sides
Transcellular transport of glucose from the intestinal lumen into the blood Cholera toxin activated by Cl-
Acidification of the stomach lumen by parietal cells in the gastric lining
Typical morphology of two types of mammalian neurons 100 m/sec
Neurotransmitters Receptors 1. Ligand gated ion channels 2. G-protein coupled receptors Synaptic vesicle: Storage of neurotransmitter. Low p. H of vesicle lumen powers entry of neuritransmitter into lumen by H+/protein antipoter
Structures of small molecules function as neurotransmitters
Exocytosis of synaptic vesicle 1. Action potential 2. Influx of Ca+2 triggers release of neurotransmitter
Cycling of nuerotransmitters and of synaptic vesicles in axon terminals H+/protein antiport
Signaling at synapse id terminated by degradation or reuptake of neurotransmitter 1. degradation i. e. acetyocholine hydrolyzed by acetyocholineaterase 2. reuptake i. e. transport into axon terminals by Na+/linked symport transporters for GABA, norepinephrine, dopamine, and serotonin
Synaptic vesicles in the axon terminal near the region where neurotransmitter release
Sequential activation of gated ion channels at a neurotransmuscular junction
Incoming signals must reach the threshold potential to trigger an action potential in post synaptic cells
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