Chapter 11 – Biological Membranes and Transport- Membranes define the external boundaries of cells and regulate the molecular traffic across that boundaryo In eukaryotic cells: divide the internal space into discrete compartments to segregate processes and componentso Organize complex reaction sequenceso Are central to both biological energy conservation and cell-to-cell communication- Membranes are flexible, self-sealing, and selectively permeable to polar soluteso Flexibility permits shape changes accompanying cell growth and movement- Two membranes can fuse or undergo fission- Membranes contain proteins:o Transporters that move specific organic solutes and inorganic ions across the membraneo Receptors that sense extracellular signals and trigger molecular changeso Adhesion molecules that hold neighboring cells together- Inside the cell, membranes organize cellular processes- Membranes are very thin11.1 The Composition and Architecture of MembranesEach 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 compositiono Some proteins are covalently linked to oligosaccharides Influence folding, stability, and intracellular destination Play a role in binding of ligands to glycoprotein surface receptorsAll Biological Membranes Share Some Fundamental Properties- Membranes are impermeable to most polar or charged solutes but permeable to nonpolar compounds- Fluid mosaic model – a model that developed due to the permeability and themotion 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 noncovalentA Lipid Bilayer Is the Basic Structural Element of Membranes- Hydrophobic interactions among lipid molecules provide the thermodynamicdriving 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)- Detergentso Bilayer – two lipid monolayers (leaflets) form a two-dimensional 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 consequencesThree Types of Membrane Proteins Differ in Their Association with the Membrane- Integral membrane proteins – very firmly associated with the lipid bilayer and are removable only by agents that interfere with hydrophobic interactions (detergents, organic solvents, denaturants)- Peripheral membrane proteins – associate with the membrane through electrostatic interactions and hydrogen bondingo 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 membraneso Noncovalent association in some caseso Covalent attachment to lipids in other caseso Reversible association regulated by phosphorylation or ligand bindingMany Membrane Proteins Span the Lipid Bilayer- Membrane protein topology – the localization of protein domains relative to the lipid bilayero Determined with reagents that react with protein side chains but cannot cross membranes- Glycophorin – erythrocyte glycoproteino 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 bilayerIntegral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids- Some proteins only have one hydrophobic region while others have multiple- Annular lipids – 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 chainso Form a bilayer shell (annulus) around the proteinThe 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 bilayero 20%-30% of all proteins are integral membrane proteins- A polypeptide chain surrounded by lipids, having no water molecules, will tend to form helices or sheets- Hydropathy index – the free energy of transfer accompanying the movement of an amino acid side chain from a hydrophobic solvent in watero Ranges from very exergonic for charged or polar residues to very endergonic for amino acids with aromatic or aliphatic hydrocarbon side chainso 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 between lipid and water- Positive-inside rule – the positively charged Lys, His, and Arg residues of membrane proteins occur more commonly on the cytoplasmic face of membranes- Barrel – a structural motif common in bacterial membrane proteins in which 20 or more transmembrane segments form sheets that line a cylinder- Porins – proteins that allow certain polar solutes to cross the outer membrane of gram-negative bacteria o Have many-stranded barrels lining the polar transmembrane passage- Hydropathy plot is not useful in predicting transmembrane segments for proteins with barrel motifsCovalently Attached Lipids Anchor Some Membrane Proteins- Several types of covalently-linked lipids:o Long-chain fatty acidso Isoprenoidso Sterolso 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
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