1 / 28


PFOS-012 MEMBRANE TRANSPORT. Overview of transport mechanisms Channels Facilitated diffusion Concepts of active transport Active transporters Co-transport systems Final remarks. (1) Overview of transport mechanisms. Plasma membrane is a semi-permeable membrane.

Télécharger la présentation


An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.


Presentation Transcript

  1. PFOS-012 MEMBRANE TRANSPORT • Overview of transport mechanisms • Channels • Facilitated diffusion • Concepts of active transport • Active transporters • Co-transport systems • Final remarks

  2. (1) Overview of transport mechanisms Plasma membrane is a semi-permeable membrane Small hydrophobic molecules: O2, CO2, N2, benzene Small uncharged polar molecules: H2O, ethanol, glycerol Lipid bilayer Larger uncharged polar molecules: glucose, amino acid, nucleotides Ions: H+, Na+, HCO3- , K+, Ca+, Mg2+, Cl- , etc. Overview of transport mechanisms

  3. DIFFUSION (hydrophobic/ lipophilic molecules only): a slow process. FACILITATED DIFFUSION, uniport (e.g. glucose transporters) CO-TRANSPORT: symport (same direction) or antiport (opposite directions CHANNELS, e.g. sodium channel, water channel (aquaporin) ATP ACTIVE TRANSPORTER, against the concentration gradient ADP + Pi Overview of transport mechanisms

  4. Structure of Aquaporin1 Adapted from Ren et al., 2001. PNAS 98(4):1398-1403. Recently, water channels (aquaporins) are found (2003 Nobel Price, Peter Agre and Rod Mackinnon) for transport of water molecules through plasma membrane. Overview of transport mechanisms

  5. 2. Channels 2.1 Voltage gated channels NH2 COOH NH2 COOH Sodium channel consists of FOUR transmembrane domain, each has SIX transmembrane αhelices, the forth helice is believed to be the voltage sensor. Potassium channel has ONE molecule of only SIX transmembrane αhelices 3. Channels

  6. 2.2 Many marine toxins block sodium channel • Tetrodotoxin from pufferfish: found mainly in the liver and gonads. • Saxitoxin from algae, bio-accumulated from algae to bivalves such as oyster and mussels. Red-tide (algal bloom) could be dangerous. They have sodium channel blockers. When accumulated in mussels or oysters and consumed by human, paralytic shell-fish poisoning resulted. • Sodium channel blockers can cause suffocation when the nervous system controlling respiration is blocked. At low dose, paralytic effects observed in patients intoxicated with these toxins. 3. Channels http://www.neuro.wustl.edu/neuromuscular/mother/chan.html#chtoxins

  7. Ciguatera toxins produced from a dinoflagellate Gamberdiscus but accumulated in reef fish, however, open ions channels. • Vomiting, Diarrhea, Abdominal pain, and many neurological defects from Sensory loss, Paresthesias & Pruritis, "Reversal" of hot & cold sensation to Hallucinations.

  8. Acetylcholine-gated cation channel (excitatory) consists of five polypeptides each has a protein with four transmembrane domains. The gate is specific for certain ions according to their hydration energy to go through the channel to be opened by activation of chemical messenger such as acetylcholine. CORBA snake toxins inhibit Ach receptor g d acetylcholine binding site a a channel b pore lipid bilayer 4 nm CYTOSOL gate 3. Channels 2.3 Transmitter- gated channels

  9. 3. Facilitated diffusion • Molecules with low permeability coefficients can go through the membrane faster than normal diffusion process. • The uptake of glucose into erythrocyte is a good example. It is rapidly moved across the membrane down the concentration gradient with the help of a membrane protein called permease. • The velocity of transport is saturable in this facilitated diffusion. • Permease is a multipass transmembrane protein to facilitate the diffusion of specific molecules across biological membrane. 2. Facilitated diffusion

  10. Facilitated Diffusion: with molecule specific permease (passive transport) to facilitate the transport down the gradient (not going against) Rate of Glucose Uptake Saturable Vmax Transport velocity Facilitated diffusion Half Vmax Simple diffusion Glucose transporter (permease) Km P External concentration of glucose, mM ADP ATP Transportersmust have a specific binding site for the solute, e.g. glucose. Once they get in the cells, they are usually phosphorylated to go into metabolic pathway and hence the outer concentration of glucose is higher than inside. 2. Facilitated diffusion

  11. Passive transports • Direction of diffusion is always from a higher to a lower concentration (down the gradient) across the membrane. • As the concentration of the solute on one side is increased, there will be an increasing initial rate of diffusion until at equilibrium (concentrations at both side the same across the membrane). After that, there will be a continued exchange of solute. • Permeases and channels are passive transporters, transport of molecules follow the rules of diffusion; solutes going down the gradients. • They have substrate specificities, e.g. glucose transporters for glucose, water channel for water.

  12. 4. Concepts of active transport • Use ATP hydrolysis directly or indirectly (secondary active transport) to move molecules across the membrane UP or against the concentration gradient. • Active transporters are membrane proteins specifically bind and move the molecules across the membrane to a unique direction using ATP hydrolysis as an energy source. • These are primary active transporters mainly for ions and chemicals, creating a chemical gradient using ATP energy. • The gradients created maintain many life processes. 4. Concepts of active transport

  13. Maintaining a typical cellular ion concentrations(mammalian cells with enzymes are in favor of low Na+, high K+ and low Cl – concentrations). 4. Concepts of active transport

  14. 5. Active transporters Classified according to their protein sequence homology and structures. • P-type: Na+-K+-ATPase, Ca2+ and H+ pump, P means they have phosphorylation and they all sensitive to vanadate inhibition. • V-type: inner membrane ATPase to regulate H+ and adjust proton gradients, v means vacuole type for acidification of lysosomes, endosomes, golgi, and secretory vesicles. • F-type: ATP synthase to generate ATP energy from moving the proton across; F means energy coupling factor. There are F1 and F0 subcomplexes: F1 generates ATP, F0 lets H+ go through the membrane. • ABC transporters: ATP-binding cassette protein for active transport of hydrophobic chemicals and Cl-. 5. Active transporters

  15. 5.1 Three types of ATPases: P, V & F. 1997 Nobel prize of Physiology and Medicine went to Paul Boyer and John Walker for their work on ATP synthase (F type), and Jens Skou for his work on Na-K-ATPase, which uses one-third of the ATP made by ATP synthase. V-type ATPase F-type ATPase F1 complex P-type ATPase V1 complex α α β β F0 complex V0 complex ATP synthase Proton pump 5. Active transporters

  16. Structure of Na+-K + -ATPase: 2α and 2β subunits join together on cell membrane β β α α Oligosaccharide chains The αsubunit has 12 transmembrane domains (TMDs) COOH Theβsubunit is glycosylated and has one TMD. Adapted from Garrett and Grisham, 1995. Molecular Aspects of Cell Biology. Sauders College Pub. COOH Phosphorylation site NH2 NH2 5. Active transporters

  17. P-type ATPase: Ca2+ATPase Ca2+ Ca2+ binding site Aspartate ATP binding site COOH NH2 ATP ADP Other divalent ion transporters have similar structure with this Ca2+-ATPase and the αsubunit of Na+-K+-ATPase. 5. Active transporters

  18. Menkes and Wilson Diseases are caused by mutated copper ion transporters • Copper ion transporters (ATP 7A and 7B) are essential to homeostasis of copper contents in our body. • Menkes disease: copper gets into the intestine but cannot transport further (mutated ATP7A), leading to copper deficiency. Copper histidine is needed for infiltration treatment. • Wilsons disease: copper in the liver cannot get into ceruloplasmin to excrete (mutated ATP7B), leading to copper accumulation in kidney, brain, and cornea. Penicillamine is needed for treatment to remove excessive copper, with zinc supplement.

  19. 5.2 ABC (ATP-Binding Cassette) transporters: 6 or 12 trans-membrane helices with 2 ATP binding sites, drug or ligand- binding sites yet to be clearly identified. Chloride channel: the cystic fibrosis transmembrane conductance regulator, CFTR, has an extra R domain P-glycoprotein Oligosaccharide chains NH2 NH2 COOH COOH R domain ATP binding domains ATP binding domains 5. Active transporters

  20. Removal of cancer drugs by P-gp using ATP energy Multiple drug resistance is related to P-glycoprotein (P-gp) which is a chemical pump using ATP energy to actively remove the hydrophobic drugs out of the cells. The P-gp is a membrane protein with two nucleotide (ATP) binding site. After ATP hydrolysis, change in protein structure facilitate the movement of hydrophobic drugs. N ATP binding sites C Cancer drugs: adriamycin, colchicine, vinblastine, etc. Adapted from Ling 1989. Scientific American. 5. Active transporters

  21. CL - Agonist CFTR and Cystic Fibrosis MembraneReceptor Genetic defects of CFTR leads to CF (Cystic fibrosis). CF is the most common genetic diseases in Caucasians (1/1000). Cell death in the lung’s epithelial due to lack of ion control, leading to malfunction of cells with mucus obstruction of gas exchanges and thus lethal to juveniles having CF. P G AC ATP ADP PKAa ATP cAMP PKAj CFTR-Cl channel is an ATP- and cAMP dependent Cl channel on epithelial cell membrane. Adapted from Sperlakis (ed)., 1998. Cell Physiology Source Book. Academic Press. 5. Active transporters

  22. 6. Co-transport Systems and their coupling solute K+ Na+ Na+ Na+ -K+ ATPase Na+ -driven symport ATP ADP + Pi solute K+ = 4 mM Na+ = 145 mM Na+ K+ Na+ lysosome K+ = 140 mM Na+ = 12 mM H+ H+ ATPase ++++ ---- ADP + Pi ATP H+ Primary and secondary active transporters work coordinately in animal cells. They generate membrane potential, generate proton gradient, maintain acidity, etc…..

  23. Glucose uptake from lumen to the capillaries using glucose transporter (permease), glucose-sodium symport and Na+-K+-ATPase. Glucose gets into the cell andsimultaneously transported by glucose transporter into the capillaries. Brush border cell Capillaries Intestinal lumen Glucose transporter: permease Glucose Glucose Glucose Glucose-Na+ symport is driven by intracellular Na+ levels using Na+ –K+ -ATPase. ATP Na+ Na+ Na+ K+ Na+-Glucose Symport K+ Na+-K+-ATPase ADP + Pi

  24. 6.3 Na+-K+-ATPase and Ca2+-ATPase Ca 2+ Ca-channel Sarcoplasmic reticulum Ca-ATPase Ca 2+ Ca 2+ Ca-Na Antiport Na+ Na+ Ouabain inhibits Na-K-ATPase. Na-K ATPase Ttoponin C K+ Adapted from Sperlakis (ed)., 1998. Cell Physiology Source Book. Academic Press. 6. Co-transport Systems and their coupling

  25. Ouabains for treatments of angina pectoris and myocardial infarction • Ouabain blocks Na + -K + -ATPase. By blocking the Na+-K+-ATPase, Intracellular Na+ remains high • Hence, the Na+- Ca2+ antiport cannot remove Ca2+ ions out from the cardiac muscle cells. • Eventually, the Ca2+ ion level is restored to maintain the contraction power of cardiac muscle.

  26. 6.4 Anion antiport in parietal cells of stomach with H+- K+ -ATPase to produce stomach acid. Cl -channel Anion antiport Cl - Cl - Cl - H+ H+ HCO3 HCO3 H+-K+-ATPase ATP Carbonic anhydrase K+ ADP + Pi Omeprazole inhibits the proton pump. H2O CO2 K+ CO2 + OH - K+ K+channel Basolateral membrane Apical membrane

  27. Omeprazole and Cimetidine stop stomach acid • H+-K+-ATPase is an electroneutral antiport. K+ is removed by K+ channel and concurrently Cl- channel removes Cl- to the same direction. • HCl is the overall transport product in the stomach lumen. • Omeprazole inhibits the proton pump directly. • Cimetidineresembles histamine to block the binding of histamine to its receptor thus inhibit the activation of H+K+-ATPase by histamine receptor.

  28. 7. Final Remarks • Communications among different compartments for specific function of cellular reactions. • Transporters (passive and active) are needed to efficiently move things around at the right time and place. • Mutations in these transporter lead to diseases, many are related to transporters in mitochondria. • These transporters or cell surface receptors are tissue specific and thus some drugs could be used for heart failure with minimal side effects observed in other tissues. • Proper excitation or inhibition of these transporters is needed for treatment of diseases. e.g. Sulfonylureas such as tolubutamide and glibenclamide inhibit K channel to stimulate insulin secretion in diabetes..

More Related