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biochem midterm study notes.docx

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Western University
Biochemistry 2280A
Derek Mc Lachlin

Topic 7- Lipids and Biological Membranes 10/22/2012 9:40:00 AM Topic Seven- Lipids and Biological Membranes Important Functions: Separate the content of a cell or organelle from the surrounding environment Control import and export of molecules; using proteins that span the membrane. Contain sensors or receptors that allow the cell to respond to external stimuli Involved in cell movement Membranes are based on lipid molecules. Lipids: biological molecules with little or no solubility in water but are soluble in organic solvents. Lipids have many functions: Energy storage Enzyme cofactors Signaling molecules Pigments Classes of Lipids: Fatty acids: Hydrocarbon chains ending in a carboxylic acid group Contain an even number of carbon atoms- range from 4-36 carbons No double bonds = saturated 1 double bond = monounsaturated. 2+ double bonds = polyunsaturated In an unsaturated molecule- the first double bond is between C9 & C10 o COUNTING FROM THE COOH END Almost always cis, causing a kink Any additional double bond is usually every three carbons O HOO 9, 12 C18:2 cis,cis- C number of carbons: number of double bonds if they are cis/trans, and the number they occur on. LDL = low-density lipoprotein, carries cholesterol to tissues HDL = high-density lipoprotein, scavenges cholesterol from tissues Triacylglycerols: Energy storage molecules Glycerol is a three-carbon molecule with hydroxyl group at each carbon Obtained by attaching a fatty acid to each hydroxyl via an ester linkage Most contain 2-3 different types of fatty acids Used to store fatty acids as energy reservoirs in adipocytes Glycerophospholipids: Used in membranes Like triacylglycerols except that the fatty acid on one end of glycerol is replaced with phosphate Phosphate group is usually conjugated to a polar alcohol like ethanolamine Spingolipids: OHH CH 2 CHH Used in membranes CH CH CH2 OH OHH Based on sphingosine CH NH + OH gycer 3 If a fatty acid is attached to the nitrogen of sphingosine via an amide linkage, the molecule is called a ceraminde HC Sphingomyelins: Terminal hydroxide group can be modified with CH 212 phosphoethanolamine or phosophocholine CH 3 etnoiiin Glycosphingolipids: Terminal hydroxide group is instead modified with sphingosnein carbohydrates Steroids: Hormones (cholesterol: membranes) Based on a system of four fused rings, three with six carbons and one with five; almost a planar system Steroids that have a hydroxyl group at C3 are called sterols Other Lipids: Some lipids have structures that dont fit into these classes Amphipathic: One part of the molecule is hydrophobic, the other part is hydrophilic Glycerophospholipids, sphingolipids and the steroid cholesterol all are amphipathic Well suited for components of biological membranes In water assemble into bilayers. Hydrophobic tail inside, hydrophilic heads outside Flexible, and will spontaneously close to form vesicles or liposomes Lipids Can Diffuse and Move within the Plane of the Bilayer: Diffusion rate determined by: 1. Temperature 2. Length of chains 3. Degree of unsaturation 4. Cholesterol concentration At high temperature- cholesterol decreases fluidity At cold temp- cholesterol interferes lipids packing together, so it increases fluidity Bilayers are asymmetrical Glycerophospholipids, sphingomyelins and glycolipids are distributed asymmetrically Glycolipids are not usually found on the cytosolic face of the membrane Cholesterol distribution is symmetric Topic 8- Membrane Proteins 10/22/2012 9:40:00 AM Proteins make up about 50% of the mass of typical biological membranes Fluid Mosaic Model: proteins are free to diffuse laterally within the bilayer, unless their movement is restricted by cellular components Functions of Membrane Proteins 1. Transporters: a. Ions and most polar molecules do not readily cross the hydrophobic membrane. Required to move ions and polar molecules between compartments of the cell or between the inside and the outside. Most transporter proteins are specific for a particular substrate or a small set of substrates. 2. Anchors: a. Structural proteins that provide stability to the membrane and help control the shape of the cell and its position relative to other cells. Bind to macromolecules on one or both sides of the membrane. 3. Receptors: a. Sense chemical signals on the outside of the cell and carry the message to the inside of the cell so that the cell can react to its environment. Most recognize a very specific molecular signal. 4. Enzymes: a. Catalyze chemical reactions can be associated with the membrane. Many receptors also have enzymatic activity. Integral: Cannot be removed from the membrane without disrupting the lipid bilayer. Transmembrane: Completely crosses the membrane at least once. Proteins usually span membranes as -helices o Single -helix of about 20 hydrophobic aa o Multiple -helices, often with hydrophilic centre Proteins can also span membranes as -sheets o Hydrophobic exterior, hydrophilic interior
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