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TOPIC 7: MEMBRANE PROTEINS AND THE “FLUID MOSAIC MODEL” Membrane Proteins: The “Mosaic” Part of the Model This “mosaic” aspect of the model has a main component, which are membrane proteins, earlier envisioned by Singer and Nicholson. We will first look at the confirming evidence that microscopes provided to prove mosaic aspect and then consider the major classes of membrane proteins. The Membrane Consists of a Mosaic of Proteins: Evidence from Freeze-Fracture Microscopy Freeze-fracturing is a technique that takes a lipid bilayer or a membrane and freezes it quickly and then usage of a sharp diamond knife to cut the bilayer in 2 plane, as a result the bilayer is split into inner and outer monolayers, revealing the inner surface of each Microscopy evidence shows that proteins are suspended within membranes The inner surface is called E (exoplasmic) and the outer surface called P (protoplasmic) faces Membranes Contain Integral, Peripheral, and Lipid-Anchored Proteins Membrane proteins differ in their affinity for the hydrophobic interior of the membrane where their hydrophobic part attach to the bilayer This determines how easy or hard it is to extract proteins from membranes Membrane proteins fall into three categories: Integral , Peripheral , and Lipid Anchored Integral Membrane Proteins These proteins are amphipathic molecules possessing one or more hydrophobic regions that exhibits an affinity for hydrophobic interior of the lipid bilayer Their hydrophobic region are embedded in the interior of the membrane, therefore cannot be easily removed from the membrane Protein that protrude from only one side of the bilayer are called integral monotopic proteins Transmembrane proteins span the membrane and have hydrophilic regions protruding from the membrane on both sides Singlepass proteins cross the membrane once and Multipass proteins are a single polypeptide that pass the membrane several times Multipass proteins consists of one polypeptide, while Multisibunit proteins have two or more polypeptides Most transmembrane proteins are anchored to the lipid bilayer by one or more hydrophobic transmembrane segments, one for each time a protein crosses the bilayer In most cases, the polypeptide chain appears to span the membrane in an alpha- helical conformation consisting of about 20-30 amino acid residues, most which have hydrophobic R groups
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In the multipass proteins, however, the transmembrane segments are arranged as a beta sheet in the form of a closed beta sheet, which are called beta-barrels which form porins Singlepass proteins have just one transmembrane segment, with a hydrophilic (C-) terminus extending out of the membrane on one side and a hydrophilic amino (N-) terminus protruding on the other side. Same applies for multipass proteins, and C and N terminus vary in direction of
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