Chapter 11 Biological Membranes and Transport Membranes define the external boundaries of cells and regulate the molecular traffic across that boundary o In eukaryotic cells divide the internal space into discrete compartments to segregate processes and components o Organize complex reaction sequences o Are central to both biological energy conservation and cell to cell Membranes are flexible self sealing and selectively permeable to polar o Flexibility permits shape changes accompanying cell growth and communication solutes movement Two membranes can fuse or undergo fission Membranes contain proteins o Transporters that move specific organic solutes and inorganic ions o Receptors that sense extracellular signals and trigger molecular across the membrane changes o Adhesion molecules that hold neighboring cells together Inside the cell membranes organize cellular processes Membranes are very thin 11 1 The Composition and Architecture of Membranes Each Type of Membrane Has Characteristic Lipids and Proteins The proportions of protein and lipid vary with different types of membranes The protein composition varies more than lipid composition o Some proteins are covalently linked to oligosaccharides Influence folding stability and intracellular destination Play a role in binding of ligands to glycoprotein surface receptors All Biological Membranes Share Some Fundamental Properties Membranes are impermeable to most polar or charged solutes but a model that developed due to the permeability and the permeable to nonpolar compounds Fluid mosaic model motion of individual protein and lipid molecules within membranes Proteins are held by hydrophobic interactions Membranes have functional asymmetry due to their proteins Membranes are fluid because most of the interactions among its components are noncovalent A Lipid Bilayer Is the Basic Structural Element of Membranes Hydrophobic interactions among lipid molecules provide the thermodynamic driving force for the formation and maintenance of micelles Three types of lipid aggregate can form when amphipathic lipids are mixed with water o Micelles spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules Favored when the cross sectional area of the head group is greater than that of the acyl side chain s Free fatty acids Lysophospholipids phospholipids lacking one fatty acid Detergents two lipid monolayers leaflets form a two dimensional o Bilayer sheet Favored if the cross sectional areas of the head group and acyl side chain s are similar Glycerophospholipids Sphingolipids Relatively unstable because the hydrophobic regions at its edges are exposed to water so it spontaneously folds back on itself to form a hollow sphere vesicle Plasma membrane lipids are asymmetrically distributed between the two layers of the bilayer but not absolutely like proteins o Changes in this distribution have biological consequences Three Types of Membrane Proteins Differ in Their Association with the Membrane very firmly associated with the lipid bilayer Integral membrane proteins and are removable only by agents that interfere with hydrophobic interactions detergents organic solvents denaturants Peripheral membrane proteins electrostatic interactions and hydrogen bonding associate with the membrane through o Released by relatively mild treatments that interfere with electrostatic interactions or break hydrogen bonds carbonate at high pH Amphitropic proteins found both in the cytosol and in association with membranes o Noncovalent association in some cases o Covalent attachment to lipids in other cases o Reversible association regulated by phosphorylation or ligand binding Many Membrane Proteins Span the Lipid Bilayer Membrane protein topology the localization of protein domains relative to the lipid bilayer o Determined with reagents that react with protein side chains but cannot cross membranes Glycophorin erythrocyte glycoprotein o Spans the membrane with its amino terminal domain on the outer surface and the carboxyl terminus protruding on the inside of the cell Membrane proteins have specific orientations in the bilayer Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids Annular lipids Some proteins only have one hydrophobic region while others have multiple lipids that lie on the protein surface with their head groups interacting with polar amino acid residues at the inner and outer membrane water interfaces and their side chains o Form a bilayer shell annulus around the protein The Topology of an integral membrane Protein Can Sometimes Be Predicted from its Sequence The presence of unbroken sequences of more than 20 hydrophobic residues in a membrane protein is evidence that these sequences traverse the lipid bilayer o 20 30 of all proteins are integral membrane proteins A polypeptide chain surrounded by lipids having no water molecules will Hydropathy index tend to form helices or sheets of an amino acid side chain from a hydrophobic solvent in water the free energy of transfer accompanying the movement o Ranges from very exergonic for charged or polar residues to very endergonic for amino acids with aromatic or aliphatic hydrocarbon side chains o A region with more than 20 residues of high hydropathy index is presumed to be a transmembrane segment Many transmembrane proteins have Tyr and Trp residues at the interface the positively charged Lys His and Arg residues of between lipid and water Positive inside rule membrane proteins occur more commonly on the cytoplasmic face of membranes Barrel which 20 or more transmembrane segments form sheets that line a cylinder Porins membrane of gram negative bacteria proteins that allow certain polar solutes to cross the outer a structural motif common in bacterial membrane proteins in o Have many stranded barrels lining the polar transmembrane Hydropathy plot is not useful in predicting transmembrane segments for passage proteins with barrel motifs Covalently Attached Lipids Anchor Some Membrane Proteins Several types of covalently linked lipids o Long chain fatty acids o Isoprenoids o Sterols o Glycosylated derivatives of phosphatidylinositol Attached lipids provide a hydrophobic anchor that inserts into the lipid bilayer and holds the protein at the membrane surface Most proteins have more than one attached lipid 11 2 Membrane Dynamics Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees The structure and flexibility of the lipid
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