Chapter 13

1. Chapter 13 Lipids

2. Lipids • Lipids:a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties • insoluble in water, but soluble in organic solvents including diethyl ether, dichloromethane, and acetone • Lipids include • fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, and glycolipids • lipid-soluble vitamins • prostaglandins, leukotrienes, and thromboxanes • cholesterol, steroid hormones, and bile acids

3. Triglyceride • Triglyceride:an ester of glycerol with three fatty acids

4. Fatty Acids • Fatty acid: an unbranched-chain carboxylic acid derived from hydrolysis of animal fats, vegetable oils, or membrane phospholipids • nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain • the three most abundant are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1) • in most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare • unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

5. Fatty Acids

6. Triglycerides • Physical properties depend on the fatty acid components • melting point increases as the number of carbons in the hydrocarbon chains increases and as the number of double bonds decreases • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats

7. Triglycerides • The lower melting points of triglycerides rich in unsaturated fatty acids are related to differences in their three-dimensional shape • hydrocarbon chains of saturated fatty acids can lie parallel with strong London dispersion forces between their chains; they pack into well-ordered, compact crystalline forms and melt above room temperature • because of the cis configuration of the double bonds in unsaturated fatty acids, their hydrocarbon chains have a less ordered structure and London dispersion forces between them are weaker; these triglycerides have melting points below room temperature

8. Hydrogenation • Hardening: reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/catalyst • in practice, the degree of hardening is carefully controlled to produce fats of a desired consistency • the resulting fats are sold for kitchen use (Crisco, Spry, Dexo, and others) • margarine and other butter substitutes are produced by partial hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils

9. Soaps • Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap)

10. Soaps • Soaps clean by acting as emulsifying agents • their long hydrophobic hydrocarbon chains cluster so as to minimize their contact with water • their polar hydrophilic carboxylate groups remain in contact with the surrounding water molecules • driven by these two forces, soap molecules spontaneously cluster into micelles

11. Soaps • soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)

12. Complex Lipids • Phospholipids • contain an alcohol, two fatty acids, and a phosphate ester • in glycerophospholipids, the alcohol is glycerol • in sphingolipids, the alcohol is sphingosine • Glycolipids • complex lipids that contain a carbohydrate

13. Complex Lipids

14. Membranes • Complex lipids form the membranes around cells and small structures within cells • In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers • 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)

15. Fluid Mosaic Model

16. Glycerophospholipids • Glycerophospholipids are the second most abundant group of naturally occurring lipids • they are also called phosphoglycerides • found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phosphoacylglycerols and 50% - 60% proteins • the most abundant phosphoacylglycerols are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid • the three most abundant fatty acids in phosphatidic acids are palmitic (16:0), stearic (18:0), and oleic (18:1)

17. Glycerophospholipids • A phosphatidic acid • the fatty acid on carbon 2 is always unsaturated • further esterification with a low-molecular-weight alcohol gives a glycerophospholipid

18. Glycerophospholipids

19. Glycerophospholipids • a lecithin

20. Sphingolipids • Found in the coatings of nerve axons (myelin) • contain the long-chain aminoalcohol, sphingosine, from which this class of compounds in named

21. Glycolipids • Glycolipid: a complex lipid that contains a carbohydrate • the carbohydrate is either glucose or galactose • the cerebrosides are ceramide mono- or oligosaccharides

22. Steroids • Steroids: a group of plant and animal lipids that have this tetracyclic ring structure

23. Cholesterol • Cholesterol is the most abundant steroid in the human body, and also the most important • it is a component in plasma membranes in all animal cells • it is the precursor of all steroid hormones and bile acids

24. Lipoproteins • Cholesterol, along with fats, are transported by lipoproteins

25. Lipoproteins • Schematic of a low-density lipoprotein

26. Steroid Hormones • Androgens: male sex hormones • synthesized in the testes • responsible for the development of male secondary sex characteristics

27. Steroid Hormones • Among the synthetic anabolic steroids are

28. Steroid Hormones • Estrogens: female sex hormones • synthesized in the ovaries • responsible for the development of female secondary sex characteristics and control of the menstrual cycle

29. Steroid Hormones • Progesterone-like analogs are used in oral contraceptives

30. Bile Salts • Bile salts, the oxidation products of cholesterol • synthesized in the liver, stored in the gallbladder, and secreted into the intestine where they emulsify dietary fats and aid in their absorption and digestion

31. Prostaglandins • Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers • one such polyunsaturated fatty acid is arachidonic acid

32. Prostaglandins • Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid

33. Prostaglandins • Some prostaglandins are always present in the body and they have a variety of functions: • lower blood pressure by relaxing the muscles around the blood vessels - Stimulate uterine contractions and induce labor. • Other short-lived prostaglandins are produced by injured tissues and are responsible for various physiological responses, including pain, swelling and fever.

34. Prostaglandins • Medicines that treat pain, inflammation and fever (aspirin, ibuprofin, etc.) work by inhibiting prostaglandin synthesis.

35. Prostaglandins

36. Thromboxanes • Thromboxanes are also derived from arachidonic acid • thromboxane A2 induces platelet aggregation and vasoconstriction • aspirin and other NSAIDs inhibit the synthesis of thromboxanes by inhibiting the COX enzyme

37. Leukotrienes • Leukotrienes are also synthesized from arachidonic acid • they occur mainly in leukocytes • they produce muscle contractions, especially in the lungs and thereby can cause asthma-like attacks • in this regard, they are 100 times more potent than histamine • several recently-developed anti-asthma drugs inhibit the synthesis of leukotrienes

38. Lipids End Chapter 13