Lipids - Lecture Presentation PDF

Chapter 19 Lipids  Chapter 19 Table of Contents 19.1 Structure and Classification of Lipids 19.2 Types of Fatty Acids 19.3 Physical Properties of Fatty Acids 19.4 Energy-Storage Lipids: Triacylglycerols 19.5 Dietary Considerations and Triacylglycero...

Chapter 19 Lipids  Chapter 19 Table of Contents 19.1 Structure and Classification of Lipids 19.2 Types of Fatty Acids 19.3 Physical Properties of Fatty Acids 19.4 Energy-Storage Lipids: Triacylglycerols 19.5 Dietary Considerations and Triacylglycerols 19.6 Chemical Reactions of Triacylglycerols 19.7 Membrane Lipids: Phospholipids 19.8 Membrane Lipids: Sphingoglycolipids 19.9 Membrane Lipids: Cholesterol 19.10 Cell Membranes 19.11 Emulsification Lipids: Bile Acids 19.12 Messenger Lipids: Steroid Hormones 19.13 Messenger Lipids: Eicosanoids 19.14 Protective-Coating Lipids: Biological Waxes 19.15 Saponifiable and Nonsaponifiable lipids Copyright © Cengage Learning. All rights reserved  Section 19.1 Structure and Classification of Lipids Lipids Lipid: An organic compound found in living organisms that is insoluble (or only sparingly soluble) in water but soluble in non-polar organic solvents. Unlike other biomolecules, lipids do not have a common structural features that serves as the basis for defining such compounds. Classification: Based on two methods – Biochemical function – Saponification (hydrolysis under basic conditions) Copyright © Cengage Learning. All rights reserved 3  Section 19.1 Structure and Classification of Lipids Classification based on Biochemical Function For purposes of simplicity of study lipids are divided into five categories based on their biochemical function: – Energy-storage lipids - triacylglycerols triacylglycerols – Membrane lipids - phospholipids, sphingoglycolipids, and cholesterol – Emulsification lipids - bile acids – Chemical messenger lipids - steroid hormones and eicosanoids) – Protective-coating lipids - biological waxes – Transport lipids - lipoproteins Copyright © Cengage Learning. All rights reserved 4  Section 19.1 Structure and Classification of Lipids Classification Based on Saponification Saponification reaction: Hydrolysis reaction that occurs in a basic solution. Based on saponification reactions lipids are divided into two categories : – Saponifiable lipids – triacylglycerols phospholipids, sphingoglycolipids, cholesterol and biological waxes – Nonsaponifiable lipids - bile acids, steroid hormones and eicosanoids) Copyright © Cengage Learning. All rights reserved 5  Section 19.1 Structure and Classification of Lipids Structure Lipids exhibit structural diversity Some are esters, some are amides, and some are alcohols (acyclic and cyclic) and some are polycyclic. Copyright © Cengage Learning. All rights reserved 6  Section 19.2 Types of Fatty Acids Saturated and Unsaturated Fatty Acids Carboxylic acids with linear (unbranched) carbon chain - Fatty acids are naturally occurring monocarboxylic acids Even # of Carbon atoms: – Long chain fatty acids: C 12 - C26 – Medium chain fatty acids: C 6 - C11 – Short-chain fatty acids: C4 - C5 Two Types: – Saturated - all C-C bonds are single bonds – Unsaturated Monounsaturated: one C=C bond Polyunsaturated: 2 or more C=C bonds present - up to six double bonds are present in fatty acids Copyright © Cengage Learning. All rights reserved 7  Section 19.2 Types of Fatty Acids Polyunsaturated Fatty Acids (PUFAs) A polyunsaturated fatty acid is a fatty acid with a carbon chain in which two or more carbon –carbon double bonds are present. Up to six double bonds are found in biochemically important PUFAs. Two types of unsaturated fatty acids. – Omega (ω)-3 fatty acids - An unsaturated fatty acid with its endmost double bond three carbon atoms away from its methyl end. – Omega (ω)-6 fatty acid is an unsaturated fatty acid with its endmost double bond six carbon atoms away from its methyl end. Copyright © Cengage Learning. All rights reserved 10  Section 19.2 Types of Fatty Acids Selected Unsaturated Fatty Acids of Biological Importance Numbering starts from the other end of -COOH Structural notation: it indicates number of C atoms – E.g., 18:2 – 18 carbons, 2 double bonds Copyright © Cengage Learning. All rights reserved 11  Section 19.2 Types of Fatty Acids Omega Acids Essential Fatty Acids: Must be part of diet Nutritionally important Omega-3 and Omega-6 fatty acids – Linolenic acid – Omega-3 – Linoleic acid – Omega-6 Linoleic Acid Deficiency: – Skin redness - becomes irritated – Infections and dehydration – Liver abnormalities – Children need it the most – Human milk has more than cow s milk ’ Copyright © Cengage Learning. All rights reserved 12  Section 19.4 Energy-Storage Lipids: Triacylglycerols Two Types of Triacylglycerols Simple Triacylglycerols: Three identical fatty acids are esterified – Naturally occurring simple triacylglycerols are rare Mixed Triacylglycerols: A triester formed from the esterification of glycerol with more than one kind of fatty acid – In nature mostly mixed triacylglycerols are found and are different even from the same source depending on the feed, e.g., corn, peanut and wheat -fed cows have different triacylglycerols Copyright © Cengage Learning. All rights reserved 20 Section 19.4 Energy-Storage Lipids: Triacylglycerols Figure 19-5 p661 Section 19.4 Energy-Storage Lipids: Triacylglycerols Figure 19-6 p662  Section 19.6 Chemical Reactions of Triacylglycerols Partial Hydrolysis Chemical Properties due to two functional groups: esters and alkenes – Hydrolysis: Partial hydrolysis of triacylglycerols – Breaking of 1-2 ester bonds to give rise to mono- or diacylglycerol and fatty acid(s) – Carried out by enzymes produced by the pancreas Copyright © Cengage Learning. All rights reserved 30 Section 19.6 Chemical Reactions of Triacylglycerols Figure 19-10 p671  Section 19.6 Chemical Reactions of Triacylglycerols Saponification Hydrolysis in basic solution: Produce salt of fatty acid and glycerol O H 2C OH H 2C O C R O + 3NaOH HC OH + 3RCOONa R C O CH O !"#$ H 2C O C R H 2C OH Copyright © Cengage Learning. All rights reserved 32  Section 19.6 Chemical Reactions of Triacylglycerols Halogenation – Addition of halogen halogen across double double (=) bond - increases degree of saturation – a test for unsaturation – the amount of halogen absorbed by a lipid can be used as an index of the degree of unsaturation; the index value is called iodine number , the number of grams of iodine that will add to 100 g of fat or oil – the rule is: high I 2 number indicates a high degree of unsaturation Copyright © Cengage Learning. All rights reserved 38  Section 19.6 Chemical Reactions of Triacylglycerols Oxidation Double bonds in triacylglycerols are subject to oxidation with oxygen in air (an oxidizing agent )-Leads to C=C breakage Remember that oxidation of alkenes may result into two short chain molecules – an aldehydes or a carboxylic acid: – The aldehydes and/or carboxylic acids so produced often have objectionable odors - fats and oils are said to be rancid – To avoid this unwanted oxidation process antioxidants are added as preservatives, e.g., Vitamin C and vitamin E are good antioxidant preservatives. Copyright © Cengage Learning. All rights reserved 39 Section 19.6 Chemical Reactions of Triacylglycerols p673  Section 19.7 Membrane Lipids: Phospholipids All cells are surrounded by a membrane that confines confines their contents. Up to 80% of the mass of a cell membrane can be lipid materials -- dominated by phospholipids. phospholipids. Phospholipid: contains one or more fatty acids, a phosphate group, a platform molecule (glycerol or sphingosine) sphingosine) to which the fatty acid(s) and the phosphate group are attached, and an alcohol that is attached to the phosphate group. ! 1-*'23+,'2. " !"##$ "&'( # $ !"##$ "&'( !"##$ "&'( % & ' )*+,-*"#. /0&+*+0 )*+,-*"#. /0&+*+0 " Copyright © Cengage Learning. All rights reserved 41 Section 19.7 Membrane Lipids: Phospholipids p676  Section 19.7 Membrane Lipids: Phospholipids Glycerophospholipids A glycerophospholipid is a lipid that contains two fatty acids and a phosphate group esterified to a glycerol molecule and an alcohol esterified to the phosphate group. All attachments (bonds) between groups in a glycerophospholipid are ester linkages Glycerophospholipids have four ester linkages as contrasted to three ester linkages in triacylglycerols. Copyright © Cengage Learning. All rights reserved 43 Section 19.7 Membrane Lipids: Phospholipids p677  Section 19.7 Membrane Lipids: Phospholipids Glycerophospholipids Structurally glycerophospholipids are although similar to triacylglycerols, they have different biochemical functions. – Triacylglycerols serve as energy storage molecules – Glycerophospholipids function as components of cell membranes A major structural difference between the two types of lipids is that of their polarity – Responsible for the their “ ” differing biochemical functions. – Triacylglycerols are a non-polar – Glycerophospholipids are polar. Copyright © Cengage Learning. All rights reserved 50  Section 19.7 Membrane Lipids: Phospholipids Sphingophospholipids Structures based on the 18-carbon monounsaturated aminodialcohol sphingosine Contains one fatty acid and one phosphate group attached to a sphingosine molecule and an alcohol attached to the phosphate group 1-*'23+,'2. !"##$ "&'( )*+,-*"#. /0&+*+0 Copyright © Cengage Learning. All rights reserved 51 Section 19.7 Membrane Lipids: Phospholipids p680  Section 19.9 Membrane Lipids: Cholesterol Copyright © Cengage Learning. All rights reserved 59  Section 19.10 Cell Membranes Cell Membrane (Plasma Membrane) Cells are surrounded by plasma membranes: – Separates aqueous interior of a cell from the aqueous environment surrounding the cell – Up to 80% of plasma membrane is lipid material – The membranes are lipid bilayer made up of phospholipids – Bilayer: Nonpolar tails of phospholipids in the middle and polar heads are on the surface 6 - 9 billionths of a meter thick or 6-9 nanometer thick – The membrane is a liquid like structure due to unsaturation in lipid tails Copyright © Cengage Learning. All rights reserved 60 Section 19.10 Cell Membranes Figure 19-17 p685  Section 19.13 Messenger Lipids: Eicosanoids Principal Types of Eicosanoids 2. Thromboxanes: C20-fatty-acid derivative containing a cyclic ether ring and oxygen-containing functional groups ̶ Promote platelet aggregation. 3. Leukotrienes: C20-fatty-acid derivative containing three conjugated double bonds and hydroxyl groups – Promote inflammatory and hypersensitivity (allergy) responses Copyright © Cengage Learning. All rights reserved 78  Section 19.14 Protective-Coating Lipids: Biological Waxes A biological wax: a monoester of a long-chain fatty acid and a long-chain alcohol. The fatty acids found in biological waxes: – Generally are saturated fatty acids – Contain 14 to 36 carbon atoms. The alcohols found in biological waxes: – May be saturated or unsaturated – May contain 16 to 30 carbon atoms. Copyright © Cengage Learning. All rights reserved 79 Section 19.14 Protective-Coating Lipids: Biological Waxes Figure 19-29 p695  Section 19.1 Structure and Classification of Lipids Transport Lipids : Lipoproteins Four major classes: 1. Chylomicrons – transport dietary TAG from the intestine to the liver and to adipose tissue 2. Very-low-density lipoprotein (VLDL) – transport TAG synthesized in the liver to adipose tissue 3. Low-density lipoprotein (LDL) – transport cholesterol synthesized in the liver to cells throughout the body 4. High-density lipoprotein (HDL) – collect excess cholesterol from body tissues and transport it back to the liver for degradation to bile acids Copyright © Cengage Learning. All rights reserved 82  Section 15.15 Saponifiable and Nonsaponifiable Lipids 86)6:1 1/)456!6/789 5451/)456!6/789 ?