Lipids Function of Lipids Energy storage molecules in animals [release 9 kcal/g versus 4 kcal/g by carbohydrates] Form membranes (cellular or sub-cellular) that separate compartments in cells [lipids are insoluble in water so help generate membranes] Serve as chemical messengers [steroid hormones are primary messengers, while prostaglandins and thromboxanes are secondary messengers mediating hormonal response] MEDC 527 Fall 2008

Simple and Complex Lipids Glycerophospholipids Classification of Lipids by Structure Lipids can be classified into four groups based on their structure 1) Simple lipids, e.g., fats; 2) Complex lipids 3) Steroids; 4) Prostaglandins, thromboxanes, and leukotrienes Simple and Complex Lipids Simple Complex Phospholipids Glycolipids Glycerophospholipids Sphingolipids Fatty acid Glycerol Alcohol PO4 choline Sphingosine Glup / Galp MEDC 527 Fall 2008

Lipids Triglycerides Animal fats and vegetable oils are triglycerides Triesters of glycerol and long chain carboxylic acids called fatty acids Glycerol component cannot vary, but the fatty acid component may vary Fatty acids are unbranched carboxylic acids having 10 to 20 carbons in even number Some fatty acids have double bond(s). Typically these double bond(s) are cis n + A triglyceride p Glycerol m palmitate (16:0) oleate (18:1) stearate (18:0) MEDC 527 Fall 2008

Properties of Triglycerides Generally fats from animals are solids at room temperature. Most fats from plants and fish are liquids. Liquid fats are referred to as oils. Presence or absence of unsaturation makes a triglyceride solid or liquid Saturated and trans fatty acids stack well Unsaturated cis fatty acids do not …. (greater hydrophobic interactions; (lesser hydrophobic interactions; greater order in arrangement; lesser order in arrangement; carries over to triglyceride structure) carries over to triglyceride structure) MEDC 527 Fall 2008

Properties of Triglycerides Saturated Fatty Acids Tm (OC) 12:0 CH3(CH2)10COOH lauric acid 44 14:0 CH3(CH2)12COOH myristic acid 58 16:0 CH3(CH2)14COOH palmitic acid 63 18:0 CH3(CH2)16COOH stearic acid 70 20:0 CH3(CH2)18COOH arachidic acid 77 Unsaturated cis Fatty Acids 16:1 CH3(CH2)5 CH=CH(CH2)7COOH palmitoleic acid 1 18:1 CH3(CH2)7 CH=CH(CH2)7COOH oleic acid 16 18:2 CH3(CH2)4 (CH=CHCH2)2(CH2)6COOH linoleic acid -5 18:3 CH3CH2(CH=CHCH2)3(CH2)6COOH linolenic acid -11 20:4 CH3(CH2)4 (CH=CHCH2)4(CH2)2COOH arachidonic acid -49 MEDC 527 Fall 2008

Mixture of fatty acid sodium salt Properties of Triglycerides Pure fats and oils are colorless, odorless and tasteless. Fats are not soluble in water. They are hydrophobic molecules and soluble in organic solvents The presence of double bonds, especially in plant fats, makes them susceptible to oxidation. Heavily oxidized fats are not edible. These generate rancidity. The ester group in fats is susceptible to hydrolysis releasing acidic groups, e.g., butyric acid, which have bad taste and odor. NaOH _ _ _ + + + + Mixture of fatty acid sodium salt (Soap) Saponification MEDC 527 Fall 2008

Complex Lipids Complex lipids constitute the main component of membranes. Two main types of complex lipids – phospholipids and glycolipids. Phospholipids contain a phosphate group, while glycolipids contain a carbohydrate group Phospholipids can be made from glycerol or from sphingosine + choline _ Phosphatidylcholine (lecithin) (A glycerophospholipid) A sphingomyelin (A sphingolipid) sphingosine + + _ choline MEDC 527 Fall 2008

The Fluid Mosaic Model of Cell Membranes Complex lipids containing unsaturated fatty acids constitute cell membranes. Membranes are made up of lipid bi-layers. The Fluid Mosaic Model of Cell Membranes MEDC 527 Fall 2008

Cyclopentanoperhydrophenanthrene Steroids Steroids constitute the third major class of lipids Steroids are cyclopentanoperhydrophenanthrenes. Cholesterol is the commonest steroid. It is present cell membranes. It is also present in lipoprotein particles, e.g., LDL, HDL, and others A B C D Cyclopentanoperhydrophenanthrene (Steroid skeleton) Cholesterol MEDC 527 Fall 2008

A Model of a Lipoprotein Particle Lipoprotein Particles Lipoprotein particles transport cholesterol in blood Contain a core of hydrophobic lipid molecules surrounded by a shell of hydrophilic molecules such as proteins and phospholipids At least four kinds of particles: HDL, LDL, VLDL and Chylomicrons Apolipoprotein Cholesterol Phospholipids Cholesterol ester A Model of a Lipoprotein Particle MEDC 527 Fall 2008

Prostaglandins, Thromboxanes and Leukotrienes Prostaglandins are derived from arachidonic acid They have a wide variety of effects of the body; they can lower or raise blood pressure, cause inflammation, induce blood clotting and induce labor Aspirin and other NSAIDs inhibit the enzyme (cyclooxygenase COX) involved in the synthesis of prostaglandins Arachidonic Acid PGE2 PGF2a Thromboxane A2 Leukotriene B4 MEDC 527 Fall 2008