Lecture 4 LIPIDS BIOL 158.pdf

LIPIDS  Structure and function of Lipids Lipids are class of biological molecules defined by low solubility in water and high solubility in nonpolar solvents. As molecules that are largely hydrocarbon in nature, lipids represent highly reduced forms of carbon and, upon oxidat...

LIPIDS  Structure and function of Lipids Lipids are class of biological molecules defined by low solubility in water and high solubility in nonpolar solvents. As molecules that are largely hydrocarbon in nature, lipids represent highly reduced forms of carbon and, upon oxidation in metabolism, yield large amounts of energy. Lipids are thus the molecules of choice for metabolic energy storage. Lipids in living things: Fatty acids and their derivatives, Fats and oils (triacylglycerols), Wax esters, glycerophospholipids, sphingolipids and isoprenoids (terpenoids and steroids) What is the structure and chemistry of fatty acids? What is the structure and chemistry of triacylglycerols? What is the structure and chemistry of glycerophospholipids and sphingolipids? What are steroids, and what are their cellular functions? What is the structure and chemistry of fatty acids? Fatty acid: a long “tail” (hydrocarbon chain) and a “head” (carboxyl group) The tail (hydrocarbon chain): – Even number of carbon atoms (usually 14 ~24) – Saturated and unsaturated – Monounsaturated and polyunsaturated Saturated fatty acids: – Free rotation around C-C bonds, extremely flexible, fully extended conformation (most stable).  Unsaturated fatty acids: Slightly more abundant in nature than saturated – Written form: 18:1(9): 18C, C9=C10 – Double bonds: cis form – hydrocarbon chain: bend form Prevent close packing, produce flexible fluid aggregates 18:1(9) Chapter 8. Lipids Palmitic acid 16 C Stearic acid 18 C Oleic acid 18:1(9) α- Linoleic acid Arachidonic acid Linoleic acid 18:2(9,12) 18:3(9,12,15) 20:4(5,8,11,14)  What is the structure and chemistry of Triacylglycerols? Triacylglycerols: triglycerides (TG) Insoluble in water, soluble in benzene, chloroform, ether, and hot ethanol. Fatty acid groups: simple or mixed triglycerides. Abundant reduced carbons: Major energy reserve in animals Oxidation: 1g TG→38kJ ; 1g Pro./Carbohydrates→17kJ  What is the structure and chemistry of Glycerophospholipids? Glycerophospholipids: one kind of phospholipids 1 2 3 One esterified phosphate group 2C ( 2-position): asymmetric carbon  Glycerophospholipids are the most common phospholipids phosphatidylcholine Polar head Phosphatidyl- ethanolamine First found in heart serine glycerol inositol Glycerophospholipids In most cases, C1: saturated fatty acid; C2: unsaturated fatty acid Ether Glycerophospholipids Platelet-activating factor (PAF) A soluble messenger in signal transduction Physiological Functions: Choline Stimulate production of fetal lung surfactant (in newborn infant) Pathological Functions: C2: 2C acetyl group Toxic shock syndrome, Blood pressure Blood volume C1: 16C-ether linkage pumped by heart Plasmalogens Ethanolamine; Serine; Choline They are found in all mammalian tissues, with ethanolamine plasmalogens 10- fold higher than choline plasmalogens except in muscles. The enol ether double bond at the sn-1 position makes plasmalogens more susceptible to oxidative stress than the corresponding ester-bonded glycero-phospholipids. Plasmalogens are not only structural membrane components and a reservoir for second messengers but may also be involved in membrane fusion, ion transport, and cholesterol efflux. Plasmalogens may also act as antioxidants, thus protecting cells from oxidative stress. The Neuroscientist, Vol. 7, No. 3, 232-245 (2001)  What are Sphingolipids, and how are they important for higher animals? Sphingolipids: Sphingosine and ceramide derivates, frequently found in biological membranes Sphingosine: 18C amino alcohol; Ceramide: fatty acid (amide linkage) Sphigomyelins: especially important in the nervous tissue of higher animals. C1 of Ceramide: Phosphorylethanolamine Phosphorylcholine Glycosphingolipids: another class of ceramide-based lipids, in muscle and nerve membranes in animals. Cerebroside C1 of Ceramide: One or more sugar residues in a β-glycosidic linkage; Functions: at cell surfaces determine certain elements of tissue and organ specificity (cell-cell recognition, tissue immunity) Gangliosides: in nerve endings, important in nerve impulse transmission. What are Steroids, and their cellular functions? Cholesterol Terpene-based lipids Alkyl chain: 8~10-C Steroid ring Found in membrane and lipoproteins of blood Cholesterol is the most common steroid in animals and the precursor for all other steroids  Types of Cholesterol Low-density lipoprotein (LDL) This is also called “bad” cholesterol because it blocks your blood vessels and increases your risk for heart disease. High-density lipoprotein (HDL) This is considered “good” cholesterol because it helps protect you from heart disease. The higher your HDL, the better. Finally, total cholesterol includes a triglycerides count. Steroid hormones are derived from cholesterol Glucocorticoids Androgen Progestin Estrogen Bile acids Key points: Structure of Fatty acid, Triglycerides. Structure of Glycerophospholipids and Sphingolipids. Structure of Cholesterol and Steroids.