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Transcript
BIOLOGICAL MEMBRANES Associate Professor Dr. Wipa Suginta School of Biochemistry, Institute of Science References 1. Lehninger Principles of Biochemistry by David L. Nelson, Michael M. Cox. 2. Biochemistry by Jeremy M. Berg, John L. Tymoczko, Lubert Stryer. 3. Biochemistry by Donald Voet, Judith G. Voet. Membranes_WipaSuginta_2/2556 1 Overview of cellular membranes Red-blood-cell plasma membranes Courtesy of Dr. Vincent Marches The boundaries of cells are formed by plasma membranes Plasma membranes prevent molecules inside the cell from leaking out and unwanted molecules from diffusing in. Membranes_WipaSuginta_2/2556 2 Membrane permeability Lipid bilayers are highly impermeable to ions and most polar molecules. Membranes_WipaSuginta_2/2556 3 Isolation of cellular membranes by ‘differential centrifugation’ Membranes_WipaSuginta_2/2556 4 Isolation of cellular membranes by ‘density gradient centrifugation’ Membranes_WipaSuginta_2/2556 5 Lipid vesicles can be formed from phospholipids Membranes_WipaSuginta_2/2556 6 Liposome for drug delivery Membranes_WipaSuginta_2/2556 7 Common features of biological membranes 1. Membranes are sheet-like structure. The thickness is between 60 – 100 Å (6-10 nm). 2. Membrane lipid bilayers are barriers to the polar molecules. 3. Membranes provide selective permeability around the cell. 4. Specific proteins mediates distinct functions, such as pumps, channels, receptors, energy transducers, and enzymes. 5. Membranes are non-covalent assemblies. 6. Membranes are asymetric and fluidic. 7. Most cell membranes are electrically polarized. For example, the intracellular face has negative potential of ~60 mV. 8. Membrane potentials play a key role in transport, energy conversion, and excitability. Membranes_WipaSuginta_2/2556 8 Membrane transporters Membranes_WipaSuginta_2/2556 9 Energy transducer on the outer membrane of mitochondria ATP synthase (EC 3.6.3.14) is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate (ATP). ATP is the most commonly used "energy currency" of cells from most organisms. ATP synthase consists of 2 regions • The FO portion is within the membrane. • The F1 portion of the ATP synthase is above the membrane, inside the matrix of the mitochondria. Membranes_WipaSuginta_2/2556 10 Photosynthesis on the thylakoid membranes of plant cells Photosynthesis is a process used by plants and other autotrophic organisms to convert light energy, normally from the sun, into chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as sugars, are synthesized from carbon dioxide during the process. In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to synthesize ATP and NADPH. In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars, Membranes_WipaSuginta_2/2556 11 which are later combined to form sucrose and starch. Membrane receptors The insulin receptor is embedded in the cell membranes of muscle, fat cells and certain types of other cells. Its function is to facilitate their uptake of glucose from the blood stream through special glucose transport proteins that are normally present inside the cell in an inactive form. When the insulin molecule binds to the alpha subunits of the receptor, it triggers a chain reaction within the cytosol that activates GLUT4 and causes it to be translocated and inserted into the cell membrane. Membranes_WipaSuginta_2/2556 12 Chemical composition of cellular membranes Three common types of membrane lipids • Phospholipids • Glycolipids • Cholesterols Membrane consists mainly of lipids and proteins (mass ratio ranging from 1:4 to 4:1). Membranes also contain carbohydrates that are linked to lipids or proteins and cholesterols. Membranes_WipaSuginta_2/2556 13 Asymmetry of carbohydrates in plasma membranes Carbohydrates – Total asymmetry Membranes_WipaSuginta_2/2556 14 Asymmetry of lipids in plasma membranes Lipids – partial asymmetry Membranes_WipaSuginta_2/2556 15 Different membranes have different lipid compositions Membranes_WipaSuginta_2/2556 16 Phospholipids are major membrane lipids Membranes_WipaSuginta_2/2556 17 Effect of temperature on membrane fluidity Types of phospholipids that affect membrane fluidity Membranes_WipaSuginta_2/2556 18 Bleaching experiments to demonstrate membrane movement Fluorescence Loss in Photobleaching (FLIP) Membranes_WipaSuginta_2/2556 19 Immunological experiments that demonstrate lateral movement of lipids in membranes Membranes_WipaSuginta_2/2556 20 Movement of phospholipids in a bilayer Membranes_WipaSuginta_2/2556 21 Cholesterol in plasma membranes Membranes_WipaSuginta_2/2556 22 Asymmetry of proteins in plasma membranes Proteins – Total asymmetry Association of proteins with the cell membranes Membranes_WipaSuginta_2/2556 23 Plasma membrane proteins have a variety of functions Membranes_WipaSuginta_2/2556 24 Membrane proteins are covalently attached to lipids by various acyl groups Membranes_WipaSuginta_2/2556 25 Transport of proteins to endoplasmic reticulum (ER) Proteins incorporated into the plasma membrane, enzymes in lysosomes and proteins secreted to the outside of the cell are synthesized in ribosomes attached to the ER membrane. Membranes_WipaSuginta_2/2556 26 See movie: http://csls-text.c.u-tokyo.ac.jp/flash/0544_1.html Transport vesicles transport the membrane components and secretory proteins. The destination of a transport vesicle is determined by the type of SNARE protein. When the transport vesicle is fused with the plasma membrane, proteins on the membrane stay on the cell surface, while those inside the Membranes_WipaSuginta_2/2556 27 transport vesicle are released to the outside of the cell. Topology prediction of integral membrane proteins Hydropathy plot of glycophorin Membranes_WipaSuginta_2/2556 28 Membranes_WipaSuginta_2/2556 29 Hydropathy plot of bacteriorhodopsin Membranes_WipaSuginta_2/2556 30 Channel proteins can be formed from beta-strands Structure of bacterial porin Membranes_WipaSuginta_2/2556 31 Visualization of membrane proteins by atomic force ) microscopy (AFM Membranes_WipaSuginta_2/2556 32 Solubilization of membrane proteins by detergents Membranes_WipaSuginta_2/2556 33 Properties of detergents Membranes_WipaSuginta_2/2556 34 A common experimental system for studying the functions of transport proteins is liposomes Membranes_WipaSuginta_2/2556 35 containing a purified transport protein. Liposome swelling assay is the technique to study permeation of molecules across membranes Membranes_WipaSuginta_2/2556 36