Lipids and Membranes

1. Lipids and Membranes

3. Learning Objectives • What Is the Definition of a Lipid? • What are the functions of lipids ? • What Are the Chemical Natures of the Lipid Types? • What are phospholipids & what are their functions ? • Which Are the Lipid-Soluble Vitamins, and What Are Their Functions? • What are waxes, sphingolipids & glycolipids ? • What Are Steroids & their derivatives ?

4. 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 nonpolar organic solvents including diethyl ether, chloroform, methylene chloride, and acetone • Amphipathic in nature • Lipids include Open Chain forms • fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, glycolipids, • lipid-soluble vitamins • prostaglandins, leukotrienes, and thromboxanes Cyclic forms • cholesterol, steroid hormones, and bile acids

5. What Are Functions of Lipids ?

6. Functions of Lipids • Source of energy • Cellular membranes • Excellent insulators • Thermal insulators • Electrical insulators (in membranes) • Special roles: • Signals • Hormones, Mediators, or growth factors. • Coenzymes • Vitamins

7. Fatty Acids • Fatty acid: an unbranched-chain carboxylic acid, most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or phospholipids of biological membranes • Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized) • FA that contain C=C, are unsaturated: If they contain only C-C bonds, they are saturated • Length of fatty acid plays a role in its chemical character

8. Fig. 8-1a, p. 194

9. Fig. 8-1b, p. 194

10. Fig. 8-1c, p. 194

11. Fig. 8-1d, p. 194

12. Fig. 8-1e, p. 194

13. Fig. 8-1f, p. 194

14. 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

15. Fatty Acids

16. Fatty Acids

17. Fatty acid composition of three food fats

18. In the shorthand notation for fatty acids • saturated FA: the number of carbons then (:) then (0) are shown • unsaturated FA: the number of carbons then (:) then the number and location of double bonds in the chain raised above delta symbol (Δ) are shown, separated by a colon • Note: Counting starts from C of COOH • Examples: palmitic (16:0), stearic (18:0), and oleic acids (18:1Δ9) = double bond between C9 and C10

19. Essential Fatty Acids • They have to be supplied in the diet without exception. • They are all polyunsaturated fatty acids: the C20 fatty acid arachidonic acid (20:4 Δ 5,8,11,14) and the two C18 acids linoleic acid (18:2 Δ 9,12) and linolenic acid (18:3 Δ 9,12 ,15 ). • The animal organism requires arachidonic acid to synthesize eicosanoids.

20. Nutritional fats contain palmitic, stearic, oleic acid, and linoleic acid particularly often. Unsaturated fatty acids are usually found at the central C atom of glycerol.

21. Triacylglycerols • Triacylglycerol (triglyceride):an ester of glycerol with three fatty acids • natural soaps are prepared by boiling triglycerides (animal fats or vegetable oils) with NaOH, in a hydrolysis reaction called saponification (Latin, sapo, soap)

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

25. Hydrolysis of Triglycerides • Chemical hydrolysis, by saponification as mentioned earlier • Enzymatic digestion by lipase enzyme

26. Phosphoacylglycerols • Phosphoacylglycerols (= phospholipids) are the second most abundant group of naturally occurring lipids • found almost exclusively in plant and animal cell 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 linoleic (18:1)

27. Phosphatidic acid

28. Phosphatidic acid

29. Phosphatidic acid

30. Phosphatidyl esters • further esterification with a low-molecular-weight alcohol with phosphate group gives a phosphoacylglycerol or phosphatidyl ester • the most common of these low-molecular-weight alcohols are:

31. Phosphatidyl esters

32. Phosphatidyl esters

33. Phosphatidyl esters

34. Phosphatidyl esters • A lecithin

35. Pulmonary surfactant Molecular Dynamics simulation of DPPC lipid bilayer formation in two phase systems reduces surface tension

36. Waxes A complex mixture of esters of long-chain carboxylic acids and alcohols Found as protective coatings for plants and animals Biological wax Beeswax, is an ester of palmitic acid with the alcohol triacontanol.

37. Sphingolipids • contain sphingosine, a long-chain (18 C) aminoalcohol from which this class is named • Found in plants and animals • Abundant in nervous system choline

38. Glycolipids • Glycolipid: a compound in which a carbohydrate is bound to an -OH of the lipid • many glycolipids are derived from ceramides • Ceramide + simple sugar= cereboside (found in nerve and brain cells)

39. Fig. 8-8, p.190

40. Multiple sclerosis It is the most common autoimmune disorder affecting the central nervous system. The insulating covers of nerve cells in the brain and spinal cord are damaged.

42. Steroids • Steroids: a group of plant and animal lipids that have this tetracyclic ring (four fused rings) structure • Composed from three six-membered rings (A, B and C) and one five-membered ring (D) • The features common to the ring system of most naturally occurring steroids are illustrated here

43. Cholesterol • The steroid of most interest is cholesterol • Cholesterol is the major component of biological membranes (especially animal membranes) • The precursor of other steroids (sex hormones and vitamin D) cholesterol

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

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

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