Lecture23_Membranes and Transport.pdf

Membranes and Membrane Transport Dr. Chirico Biomedical Sciences Room 434 [email protected] 1 CMSRU Disclosure In accordance with the ACCME Essentials and Standards, everyone involved in planning and presenting this CMSRU educational lectur...

Membranes and Membrane Transport Dr. Chirico Biomedical Sciences Room 434 [email protected] 1 CMSRU Disclosure In accordance with the ACCME Essentials and Standards, everyone involved in planning and presenting this CMSRU educational lecture has no relevant commercial relationships or conflicts of interest. There is no commercial support for this program. 2 Instructional materials contained in this slide set may include copyrighted material. The copyright law of the U.S. (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Please do not reproduce, or transmit, any portion of this content. 3 Learning Objectives 1. Describe the structure and composition of lipid bilayers 2. Explain how membrane composition, temperature, and cholesterol influence fluidity & permeability 3. Describe asymmetrical distribution of phospholipids and the role of enzymes in the establishment and maintenance of these distributions 4. Understand the mechanisms for transporting various substances through the membrane 5. Describe the mechanisms of primary and secondary active transport 6. Understand how transporters maintain balance between intracellular and extracellular environments 4 USMLE Content Outline General Principles of Foundational Science Biochemistry and molecular biology Structure and function of proteins and enzymes Biology of cells Cell/tissue structure, regulation, and function, including cytoskeleton, organelles, glycolipids, channels, gap function, extracellular matrix, and receptors 5 Cellular Membranes 1. Defines the cell boundaries 2. Acts as a barrier between the cytoplasm and the aqueous environment outside of the cell 3. Regulates the movement of substances (e.g., ions, water, proteins, nutrients, etc.) into and out of the cell 4. Contributes to intra- and inter-cellular communication 5. Forms membrane junctions linking adjacent cells together Berne & Levy. Fig 1.1 6  LO#1 Structure & Composition 5nm- thick lipid bilayer with associated proteins Proteins are integrated into bilayer or attached to inner or outer surface of membrane Fluid-mosaic model 7  LO#2 Membrane Fluidity Molecules within membrane can move and change places Membrane acts as 2D fluid Depends on Lipid composition Length of tail Number of double bonds (saturation) Temperature Cholesterol content (maintains fluidity) Increase fluidity = short tails, fewer double bonds (unsaturated), high temperature, cholesterol at Odile et al. Up-to-Date Insight About Membrane Remodeling as a Mechanism of Action for Ethanol-Induced Liver Toxicity, Trends in low temps Alcoholic Liver Disease Research 8  LO#3 Membranes are asymmetrical Sphingomyelin (SM) Outer Phosphatidylcholine (PC) Leaflet Phosphatidylserine (PS) Inner Phosphatidylethanolamine (PE) Leaflet Phosphatidylinositol (PI) Cholesterol (CL) 9 Role of enzymes in lipid asymmetry 1. Flippases: Utilizes ATP to move the phospholipids PS and PE from the outer leaflet to the inner leaflet of the membrane against a concentration gradient 2. Floppases: Utilizes ATP to move PC and SM from the inner leaflet to the outer leaflet against a concentration gradient 3. Scramblases: non-specific ATP-independent enzymes that facilitate the movement of lipids down their concentration gradients 10  LO#4 Membrane Proteins Integral membrane proteins Embedded in, and anchored to, cell membrane Peripheral membrane proteins Adhere to integral proteins on either side of membrane Serve as receptors Serve as adhesion molecules Serve as transporters Carry out movement of water soluble substances Pores, channels, carriers, and pumps Serve as enzymes 11 Permeation of Lipid Bilayers Gasses and small molecules permeate lipid bilayers Hydrophobic core of lipid bilayers acts as a barrier to the diffusion of large and/or charged molecules Membrane permeation of large polar molecules and ions is mediated by transport proteins 12 Principles of Membrane Transport Ion concentrations differ inside and outside of the cell Distribution of ions inside and outside cell is controlled: activity of transport proteins Permeability characteristic of lipid bilayer 13 Transport Pathways Protein molecules have different properties for transporting substances Channel proteins: have open spaces and allow movement of molecules based on charge or size Carrier proteins: bind with substance and carry substance to other side Function via 2 processes Diffusion Active transport 14 Simple Diffusion Lipid-Soluble Substances Dissolve in lipid bilayer and diffuse through Large amounts of oxygen can easily be transported Lipid-Insoluble Molecules Passes through channels E.g. Pores called aquaporins Rapid passage of water through membrane Selectively permeable Transport 1 or more specific ions or molecules Potassium channels- highly selective for potassium Sodium channels- highly selective for sodium Flow down concentration gradient 15 Ion channels Integral, membrane-spanning proteins Gated vs Non-Gated Selective Size of channel Charge of lining (-) lined channels Cations, no anions (+) lined channels Anions, no cations Non-gated = always open Gated = opens when “needed” 16  Gated Ion Channels Gated channels controls ion permeability When gate is open, ions flow through by passive diffusion Voltage gated Conformation of gate responds to electrical potential Basis of action potentials Chemical gated 2nd-messenger-gated Signalling molecules cAMP IP3 Ligand-gated Hormones, neurotransmitters ACh 17 Facilitated Diffusion “Carrier-mediated diffusion” Occurs down concentration gradient Required membrane carrier Molecule enters pore within protein; binds to “receptor” Conformational change in carrier protein Pore opens to opposite end; molecule released Examples: glucose (GLUT), amino acids 18 Limiting Rate of Facilitated Diffusion Rate of transport limited by time of conformational changes Rate of facilitated diffusion reaches a maximum, Vmax 19  LO#5 Active Transport Maintains ion concentrations Goes “uphill” against gradient 2 types according to source of energy used Primary active transport Energy derived from ATP Secondary active transport Energy is derived secondarily from energy created originally by primary active transport Depends on carrier proteins 20  LO#5/6 Sodium-Potassium Pump (1◦) During 1 cycle Na+/K+ ATPase 3 Na+ ions out of the cell 2 K+ ions into the cell Consuming 1 molecule ATP Binding of ions stimulates phosphorylation of pump Against concentration gradient Maintains low Na+ and high K+ inside cells Critical for cell function 21  LO#5/6 Secondary Active Transport When ions are transported by primary active transport, a large concentration gradient forms Gradient is a storehouse of energy Diffusion energy of ion can pull other substances along with it Require carrier proteins 22 Sodium-Glucose Co-Transporter As Na+ moves down concentration gradient, carries along glucose Sodium-dependent glucose transporters (SGLT) are found in the epithelial cells lining the small intestine and proximal tubule of the nephron 23 Costanzo. Fig 1.7. 2014  Sodium Counter-Transporter (Antiport) Counter-transport: transport in opposite direction of primary ion Na+-Ca2+ counter-transport occurs though almost all cell membranes Na+-H+ counter-transport occurs in several tissues such as kidneys Na+ into tubular cell H+ into tubular lumen Can transport large number of H+ at once key to maintain body fluids 24 Transporters Comparison Type of Transport Active or Passive Carrier-Mediated Uses metabolic Dependent on Na+ energy gradient Simple diffusion Passive No No No Facilitated diffusion Passive Yes No No Primary active transport Active Yes Yes, direct No Secondary active Active Yes Yes, indirect Yes Cotransport Secondary active Active Yes Yes, indirect Yes Counter-transport 25  LO#6 Review: Na+/K+ ATPase What does this pump do? 26 Review: Na+/K+ ATPase What happens to the pump if an ATPase-inhibitor (ouabain, glycosides) is used? 28 Clinical Relevance A rise in intracellular Ca2+ concentration causes muscle cells to contract. In the heart, a Na+/Ca2+ antiport pumps Ca2+ out of the cell allowing the heart to relax. Ouabain & Digitalis are given to patients with heart disease in order to make the heart pump more strongly. These drugs work by partially inhibiting the Na+- K+ pump in the heart. Explain the mechanism behind the effectiveness of these drugs. 30 Review Questions (1st order) 1. Membrane fluidity depends on which characteristics? 2. What is the function of cholesterol? 3. Which enzyme flips phospholipids to the outside? Inside? Both inside and outside? 4. What type of diffusion includes carrier molecules? 5. Name 3 ways an ion channel can be gated. 6. What limits the rate of diffusion? 7. What is the difference between primary and secondary active transport? 32  2 order Statements/Questions nd (Food for thought!) Cholesterol plays a regulatory function in antibiotic resistance. How might its role in membrane structure prevent cell damage? MDR cells possess higher levels of total cholesterol Membranes of neoplastic and metastatic cells are more fluid due to reduced cholesterol composition. What “benefit” might this have for cancer cells? PS and PE (normally on inner leaflet) have increased surface expression on outer membrane in tumor cells. How might this influence cancer- targeting drugs? 33 thank you questions?? Erica Chirico [email protected] Office 434