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Ch. 10 - Membrane Proteins Summary of chapter and lecture on membrane proteins. Includes illustrations and graphics.

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BIOL 2021
Julie Clark

Biol 2021 March 12 2009- lecture- Chapter 10 Proteins in membrane  Membrane proteins perform most of the membranes specific tasks and therefore give each type of cell membrane its characteristic functional properties.  Amounts of protein vary according to membrane function as well  Typical plasma membrane has amount half its mass as protein Types of membrane proteins Membrane association with proteins 2 classes: I. Intergral - Mainly defined by “cannot be washed off with high salt soltutions”; membrane must be disrupted with detergent in order to be accessed and purified - Protein that is retained in a membrane by virtue of one or more domains that span or are embedded in the lipid bilayer - Inside membrane, integrated tightly - Transmembrane proteins and many proteins held in the bilayer by lipid groups or hydrophobic polypeptide regions that insert into the hydrophobic core of the lipid bilayer - Single pass transmembrane protein: polypeptide chain crosses only once - Multipass transmembrane proteins: polypeptide chain crosses multiple times  Peptide chain can form B barrel (b sheet rolled into barrel) ie. In porin proteins a. -(1,2,3): transmembrane proteins: proteins that extend through the lipid bilayer with part of their mass on either side  Amphiphilic, with hydrophobic regions that pass through the membrane and interact with the hydrophobic tails of lipid molecules in the interior of the bilayer where they are sequestered away from water. Also has hydrophilic region exposed to water on either side of the membrane  The covalent attachment of a fatty acid chain that inserts into the cytosolic monolayer of the lipid bilayer increasesthe hydrophobicity of some of these transmembrane proteins (ie, protein 1. In figure) - Protein #1: single alpha helix, transmembrane; covalently attached fatty acid chain inserted in the cytosolic lipid monolayer - Protein #2: multiple alpha helix- multipass transmembrane protein - Protein #3: B sheet- beta barrel - Protein #4: Alpha helix on cytosolic monolayer by amphiphilic alpha helix - Protein #5: associated with cytosolic side by covalently attached lipid (that anchors it to cytosolic monolayer and keeps it in place) - Glycossylphosphatidylinositol (GPI) anchor  Anchor protein to membrane by glycosyl linkage  Found in signalling  Protein can be released with enzyme called phosphatidlinositol-specific phospholipase C, which cuts proteins free from their anchors - Protein #6: covalently attached via ogliosaccharide linker to phosphatidylinositol in the non cytosolic monolayer (GPI anchor) - ALPHA HELIX - - Figure 1 Segment of tranmembrane polypeptide chain crossing the lipid bilayer- green and yellow= hydrophobic amino acids BETA BARREL:  Abundant in outer membrane of mitochondria, chloroplast and bacteria  Porin barrel: polar amino acids line inside of channel while non polar side chains of amino acids project from the outside of the barrel to interact with the hydrophobic core of lipid bilayer.  Loop of polypeptide chains can protrude into the channel causing it to be narrow and more selective.  Not all beta barrel proteins are transport proteins- can also function as receptors or enzymes  Conformational change is unlikely because of hydrogen bond to neighbours. II. PERIPHERAL PROTEINS - Attached by non covalent interactions to integral membrane protein. i.e. Via ionic interactions - Peripheral proteins do not extend into the hydrophobic lipid bilayer at all; they are instead bound to either face of the membrane by noncovalent interactions with other membrane proteins Protein 7: peripheral protein on cytosolic side Protein 8: peripheral protein on extracellular side - Peripheral proteins can be washed off with high salt solutions ; salt disrupts ionic interactions - Detergents have hydrophobic tails and hydrophilic heads and form micelles in water. Detergent
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