Lipids and Membranes

1. Lipids and Membranes Chapter 12 (pp. 382 – 454) Read topics covered in lectures

2. What are Lipids? (Greek: lipos, fat) • Fourth major group of molecules found in cell • Lipids are not polymeric • (unlike nucleic acids, proteins & polysaccharides) • Water insoluble • (soluble in organic solvents such as methanol & chloroform) • Includes fats, oils, certain vitamins and hormones

3. Lipids, a broad class of organic products found in living systems. • Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material. • Major classes of lipids include the fatty acids, the glycerol-derived lipids (including the fats and oils and the phospholipids), the sphingosine-derived lipids (including the ceramides, cerebrosides, gangliosides, and sphingomyelins), the steroids and their derivatives, the terpenes and their derivatives, certain aromatic compounds, and long-chain alcohols and waxes. • In living organisms lipids serve as the basis of cell membranes and as a form of fuel storage. • Often lipids are found conjugated with proteins or carbohydrates, and the resulting substances are known as lipoproteins and lipopolysaccharides. • The fat-soluble vitamins can be classified as lipids. • Liposomes are spherical vesicles formed by mixing lipids with water or water solutions. They have found applications in the oral administration of some drugs (e.g., insulin and some cancer drugs), since they retain their integrity until they are broken down by the lipases in the stomach and small intestine. http://www.allrefer.com/

4. Lipid Classification Fatty acids Triacylglycerols Glycerophospholipids Sphingolipids Steroids Other lipids(Example: Eicosanoids)

5. Biological Membranes • Organized sheet-like assemblies consisting mainly of proteins & lipids • Functions carried out by membranes are indispensable for life • Plasma membrane give cells their individuality • Eukaryotic cells contain internal membranes that form the boundaries for organelles

6. Common Features of Biological Membranes • Sheet-like structures with thickness of about 6 - 10 nm • Consists of mainly lipids and proteins (Carbohydrates may be linked to proteins or lipids) • Membranes are fluid and asymmetric • Highly selective permeability barriers-- not impervious walls • Control flow of information and material between cells or organelles & their environments • Specific proteins mediate distinctive functions of membranes • Proteins in the membrane carry out important functions Example: Mitochondrial inner membrane where ATP production takes place

7. Schematic drawing of a prokaryotic cell Page 4

8. Schematic diagram of an animal cell Page 7

9. Drawing of a plant cell Page 11

10. What are membranes made of? Biological membranes are composed of proteins associated with a lipid bilayer matrix Oligosaccharide Lipid (bilayer) Protein Cholesterol

11. Lipid composition of the plasma membrane & organelle of a rat hepatocyte Note the diversity of various membranes From Lehninger Principles of Biochemistry

12. Fatty Acids • Carboxylic acids with long hydrocarbon side group (R-COOH) • Usually occur in esterified form (R-COOR) • Can be either saturated or unsaturated (contain double bonds) • Often are polyunsaturated (2 or more double bonds) • Double bonds have cis configuration and are non conjugated Fully extended lowest energy conformation Oleic acid CH3(CH2)7 CH=CH(CH2)7 COOH Stearic acid CH3(CH2)16COOH

13. Structural formulas of some C18 Fatty acids Stearic acid Stearic acid, alternative representation 18 1 Oleic acid 18 9 1 a-Linolenic acid 18 15 12 9 1

14. From Lehninger Principles of Biochemistry

15. Packing of fatty acids into stable aggregates Polar Presence of one or more cis double bonds interferes with tight packing Results in less stable aggregates Pack Efficiently Stabilized by hydrophobic interactions

16. Melting points of fatty acids • Increase with increase in chain length (or molecular mass) • For Example: • Melting Point (oC) • 12:0 Lauric acid 44.2 • 18:0 Stearic acid 69.1 • Decrease with the degree of unsaturation • (or increase in double bonds) • For Example: • Melting Point (oC) • 18:0 Stearic acid 69.1 • 18:1 Oleic acid 13.2 • 18:2 Linoleic acid -9

17. Triacyl Glycerols (triglycerides) • Fatty acid triesters of glycerol • Non polar, water insoluble • Fats & oils are mixtures of triacyl glycerol • Function as energy reservoirs in animals • Usually not part of biological membranes • Fats are good way to store • metabolic energy • More energy than sugars or proteins which are partially oxidized • Fats are non-polar and are stored in anhydrous form (unlike glycogen which binds twice its weight of water) From Lehninger Principles of Biochemistry

18. The principal class of storage and membrane lipids From Lehninger Principles of Biochemistry

19. Glycerophospholipids (or phosphoglycerides) Major lipid components of biological membranes Derived from glycerol-3-phosphate whose C1 and C2 positions are esterified with fatty acids Phosphoryl group is linked to a group “X” Amphiphilic molecule: non-polar hydrocarbon tail polar phosphoryl X “heads” Glycerol-3-phosphate

20. Page 385 Table 12-2 The Common Classes of Glycerophospholipids.

21. Plasmalogen • Glycerophospholipids • At C1 position there is a, b-unsaturated ether linkage instead of ester linkage • Functions of most plasmalogen not well understood Choline Ethanolamine, choline & serine form the most common plasmalogen head groups From Lehninger Principles of Biochemistry

22. Sphingolipids • Major membrane components • Derived from C18 amino alcohol, sphingosine • Double bond of sphingosine has transconfiguration From Lehninger Principles of Biochemistry

23. The specificities of phospholipases hydrolytic enzymes Phospholipids & sphingolipids are degraded in lysosomes

24. Steroids • Structure consists of three 6-membered rings & one 5-membered ring, all fused together • Cholesterol is the most common steroid in animals (& precursor for all other steroids in animals) • Steroid hormones serve many functions in animals - including salt balance, metabolic function and sexual function

25. Cholesterol

26. Lipids as intracellular signals From Lehninger Principles of Biochemistry

27. Eicosanoids carry messages to nearby cells From Lehninger Principles of Biochemistry

28. Steroids derived from cholesterol Steroids are oxidized derivatives of sterols From Lehninger Principles of Biochemistry