Chapter 8 - Lipids-New(2).pptx

CHAPTER 8: LIPIDS AND MEMBRANE PROTEINS Week 10 CHEM311 Chapters 8: Lipids & Membranes Lipids are non-polar (hydrophobic) compounds, soluble in organic solvents. Many important functions of lipids Energy source Energy Storage Cell membranes Hormones Vitamins Insulation/protection Most membrane lipid...

CHAPTER 8: LIPIDS AND MEMBRANE PROTEINS Week 10 CHEM311 Chapters 8: Lipids & Membranes Lipids are non-polar (hydrophobic) compounds, soluble in organic solvents. Many important functions of lipids Energy source Energy Storage Cell membranes Hormones Vitamins Insulation/protection Most membrane lipids are amphipathic, having a non-polar end and a polar end (like detergents). e.g. A 16-C fatty acid: CH3(CH2)14-COO- Non-polar Dr. Salman Ashraf polar 2 Lipids can be divided into many groups Dr. Salman Ashraf 3 Lipids can be divided into many groups Lipids 1. Fatty acids 2. Triacyl glycerols (Triglycerides) & soap 3. Phospholipids 4. Waxes 5. Sphingolipids 6. Glycolipids 7. Steroids 8. Biological membranes Dr. Salman Ashraf 5 Structure and nomenclature of fatty acids Fatty acids consist of a long hydrocarbon tail terminating with a carboxyl group. The fatty acid shown, laurate (or dodecanoate) has 12 carbon atoms and contains no carbon-carbon double bonds. Laurate is called a saturated fatty acid because it contains no double bonds. Fatty acids are more commonly 16 or 18 carbons in length. Fatty acids differ from each other based on the number of C atoms. Dr. Salman Ashraf 6 Fatty Acids (Cont’d) Length of fatty acid plays a role in its chemical character Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized) FA that contain C=C, are unsaturated: If contain only C-C bonds, they are saturated Saturated & Unsaturated Fatty acids Dr. Salman Ashraf 8 Saturated & Unsaturated Fatty acids ( No memorization) Dr. Salman Ashraf Chem 361 – Biochemistry 9 Saturated & Unsaturated Fatty acids Dr. Salman Ashraf 10 Saturated & Unsaturated Fatty acids Dr. Salman Ashraf 11 Effect of Double Bonds on the Conformations of Fatty Acids Kink(s) in hydrocarbon chain Causes disorder in packing against other chains This disorder causes greater fluidity in membranes with cisdouble bonds vs...... saturated FA chains Saturated & Unsaturated Fatty acids cis-double bonds causes a “kink” or “bend” in the structure of the hydrophobic tail of fatty acids, thus reducing Van der Waal’s and hydrophobic interaction between alkyl side chains. Hence, the differences in the melting points between saturated and unsaturated fatty acids Stearate C18:0 Solid at 25 oC mp. 70oC Dr. Salman Ashraf Oleate C18:1 Linoleate C18:2 Liquid at 25 oC! 13 oC -17 oC 13 Saturated & Unsaturated Fatty acids (No memorization) Dr. Salman Ashraf 14 Artificial trans-fatty acids (trans fat) …. Food rich in “trans-fat ….  Trans Fatty Acids and Cardiovascular Disease April 13, 2006 N Engl J Med 2006; 354:1601-1613 DOI: 10.1056/NEJMra054035 Triglycerides: Long-Term Energy Storage Fatty acids may be either unsaturated or saturated. Unsaturated – one or more double bonds between carbons Tend to be liquid at room temperature Example: plant oils Can have chemical groups on the same (cis) or opposite (trans) side of the double bond Saturated – no double bonds between carbons Tend to be solid at room temperature Examples: butter, lard Trans – a triglyceride with at least one bond in a trans configuration Triglycerides: Long-Term Energy Storage Jump to Triglycerides: Long-Term Energy Storage (3) Long Description Triacylglycerols Fatty acids generally exist as triacylglycerols + Dr. Salman Ashraf  20 Soap http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/c_acid/fettsaeuren.vlu/Page/vsc/en/ch/12/oc/c_acid/fatty_acid/fatty_acid.vscml.html Dr. Salman Ashraf 21 Phospholipid (Glycerophospholipid) General formula of a phospholipid Phospholipids have hydrophobic (non-polar) tail + polar head Dr. Salman Ashraf 22 Phospholipids: Membrane Components The structure is similar to triglycerides. It consists of one glycerol molecule linked to two fatty acids and a modified phosphate group. The fatty acids (tails) are nonpolar and hydrophobic. The modified phosphate group (head) is polar and hydrophilic. Function: forms plasma membranes of cells. In water, phospholipids aggregate to form a lipid bilayer (double layer). Polar phosphate heads are oriented towards the water. Nonpolar fatty acid tails are oriented away from water. Nonpolar fatty acid tails form a hydrophobic core. Kinks in the tails keep the plasma membrane fluid across a range of temperatures. Phospholipids Form Membranes Jump to Phospholipids Form Membranes Long Description Waxes (1) Long-chain fatty acids connected to carbon chains containing alcohol functional groups Solid at room temperature Waterproof Resistant to degradation Function: protection Examples: earwax (contains cerumen), plant cuticle, beeswax Waxes (2) (a): ©Harald Theissen/Getty RF; (b): ©IT Stock Free/Alamy RF Jump to Waxes (2) Long Description Steroids Steroids contain four fused rings called A, B, C, and D Much less flexible than fatty acids Almost planar Amphipathic (polar and nonpolar groups) like other lipids Many important hormones are steroid-based Cholesterol, an steroid, is an important constituent of membranes Dr. Salman Ashraf 27 Steroids: Four Fused Carbon Rings They are composed of four fused carbon rings. Various functional groups attached to the carbon skeleton Functions: component of animal cell membrane, regulation Examples: cholesterol, testosterone, estrogen Testosterone and estrogen are sex hormones differing only in the functional groups attached to the same carbon skeleton. Cholesterol is the precursor molecule for several other steroids. Cholesterol Dr. Salman Ashraf 29 Cholesterol Dr. Salman Ashraf 30 LDL Cholesterol and HDL Cholesterol What are LDL and HDL cholesterol? Cholesterol travels through the blood on proteins called “lipoproteins.” Two types of lipoproteins carry cholesterol throughout the body: LDL (low-density lipoprotein) cholesterol, sometimes called “bad” cholesterol, makes up most of your body’s cholesterol. High levels of LDL cholesterol raise your risk for heart disease and stroke. HDL (high-density lipoprotein) cholesterol, sometimes called “good” cholesterol, absorbs cholesterol in the blood and carries it back to the liver. The liver then flushes it from the body. High levels of HDL cholesterol can lower your risk for heart disease and stroke. Lipid Metabolism (just FYI) Lipids are NOT very soluble in water When lipids are put in aqueous environment, they can form various “structures” – monolayers, bilayers, micelles and vesicles Dr. Salman Ashraf 33 Lipid Bilayers  Biological membranes Dr. Salman Ashraf 34 Temperature Transition in Lipid Bilayer With heat, membranes become more disordered; the transition temperature is higher for more rigid membranes; it is lower for less rigid membranes Mobility of the lipid chains increases dramatically Membrane fluidity as a function of temperature Biological Membranes Every cell has a cell membrane (plasma membrane) Eukaryotic cells also have membrane-enclosed organelles (nuclei, mitochondria…etc) Molecular basis of membrane structure is in lipid component(s): polar head groups are in contact with the aqueous environment nonpolar tails are buried within the bilayer the major force driving the formation of lipid bilayers is hydrophobic interaction the arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids) Biological Membranes Plant membranes have a higher percentage of unsaturated fatty acids than animal membranes The presence of cholesterol is characteristic of animal rather than plant membranes Animal membranes are less fluid (more rigid) than plant membranes The membranes of prokaryotes, which contain no appreciable amounts of steroids, are the most fluid Membrane Layers - Lipid bilayer asymmetry Both inner and outer layers of bilayer contain mixtures of lipids Compositions on inside and outside of lipid bilayer can be different lipid bilayer asymmetry Lipid bilayer asymmetry is important in many important physiological cell functions – e.g. apoptosis Fluid Mosaic Model Fluid: there is lateral motion of components in the membrane; proteins, for example, “float” in the membrane and can move along its plane Mosaic: components in the membrane exist side-by-side as separate entities the structure is that of a lipid bilayer with proteins, glycolipids, and steroids such as cholesterol embedded in it Fluid mosaic model of Biological membranes Dr. Salman Ashraf 41 Fluid mosaic model of Biological membranes Dr. Salman Ashraf 42 Cholesterol – an integral part of membranes Dr. Salman Ashraf 43 Cholesterol affects the fluidity of membranes Cholesterol affects the fluidity of membranes https://www.khanacademy.org/test-prep/mcat/cells/cell-membrane-overview/ v/cell-membrane-fluidity Fluid mosaic model of Biological membranes Dr. Salman Ashraf Chem 361 – Biochemistry 46 Fluid Mosaic Model (Cont’d) Figure 8.19 Fluid-mosaic model of membrane structure. Integral membrane proteins span the membrane, but peripheral proteins are associated with one side or the other. (From Russell/ Hertz/Mcmillan, Biology, 3E. © 2014 Cengage Learning.) Kinds of Membrane Proteins Functions: transport substances across membranes; act as receptor sites, and sites of enzyme catalysis Peripheral proteins bound by electrostatic interactions can be removed by raising the ionic strength Found on one side or the other of the membrane Integral proteins bound tightly to the interior of the membrane Integral and peripheral proteins. The integral protein rhodopsin spans the membrane. The heterotrimeric G protein is a peripheral protein. Integral proteins: attaching proteins to membranes Membrane Function: Membrane Transport Passive transport driven by a concentration gradient simple diffusion: a molecule or ion moves through an opening facilitated diffusion: a molecule or ion is carried across a membrane by a carrier/channel protein Active transport a substance is moved against a concentration gradient primary active transport: transport is linked to the hydrolysis of ATP or other high-energy molecule; for example, the Na+/K+ ion pump secondary active transport: driven by H+ gradient Passive vs Active Transport Primary Active transport: movement of molecules against a gradient linked to hydrolysis of high-energy molecule (e.g. ATP) A mechanism for Na+/K+ ATP ase (the sodium–potassium ion pump). The model assumes two principal conformations, Binding of Na+ ions to Enzyme is followed by phosphorylation and release of ADP. Na+ ions are transported and released, and K+ ions are bound before dephosphorylation of the enzyme to Types of Transport Two Types of Active Transport Secondary active transport In secondary active transport, the movement of a driving ion down an electrochemical gradient is used to drive the uphill transport of another ion/molecule against a concentration or electrochemical gradient. Two types of secondary active transport processes exist: cotransport (also known as symport) and exchange (also known as antiport). In cotransport, the direction of transport is the same for both the driving ion and driven molecule/ion, whereas in exchange, the driving ion and driven ion/molecule are transported in opposite directions. X and Y represent transporter substrates. Na+, sodium; K+, potassium; ATP, adenosine triphosphate; ADP, adenosine diphosphate; Pi, inorganic phosphate. Membrane receptors Binding of a biologically active substance to a receptor initiates an action within the cell Membrane Receptors Binding of a biologically active substance to a receptor initiates an action within the cell