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Chapter 24 Lipids

Chapter 24 Lipids. Lipids. Lipids are naturally occurring substances grouped together on the basis of a common property—they are more soluble in nonpolar solvents than in water.

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Chapter 24 Lipids

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  1. Chapter 24Lipids

  2. Lipids • Lipids are naturally occurring substances grouped together on the basis of a common property—they are more soluble in nonpolar solvents than in water. • Some of the most important of them—the ones in this chapter—are related in that they have acetic acid (acetate) as their biosynthetic origin. • In many biosynthetic pathways a substance called acetyl coenzyme A serves as the source of acetate.

  3. 24.1Acetyl Coenzyme A

  4. O R = CCH3; Acetyl coenzyme A Structure of Coenzyme A R = H; Coenzyme A

  5. O O HY •• CH3CSCoA CH3C Y •• Reactivity of Coenzyme A Nucleophilic acyl substitution • Acetyl coenzyme A is a source of an acetyl group toward biological nucleophiles; it is an acetyl transfer agent. + HSCoA

  6. O OH H2C CSCoA CH3CSCoA O CH2CSCoA E Reactivity of Coenzyme A can react via enol • Acetyl coenzyme A reacts with biological electrophiles at its  carbon atom. E+

  7. 24.2Fats, Oils, and Fatty Acids

  8. O CH2OCR' O RCOCH CH2OCR" O Fats and Oils • Fats and oils are naturally occurring mixture of triacylglycerols (also called triglycerides). • Fats are solids; oils are liquids.

  9. O CH2OC(CH2)16CH3 O CH3(CH2)16COCH CH2OC(CH2)16CH3 O Fats and Oils • Tristearin; mp 72°C

  10. O CH2OC(CH2)16CH3 O CH3(CH2)6CH2 CH2(CH2)6COCH C C CH2OC(CH2)16CH3 H H O Fats and Oils • 2-Oleyl-1,3-distearylglycerol; mp 43°C

  11. Fats and Oils • 2-Oleyl-1,3-distearylglycerolmp 43°C H2, Pt Tristearinmp 72°C

  12. O O CH2OCR CH2OH O O HOCR R'COCH HOCH R'COH CH2OCR" CH2OH HOCR" O O Fatty Acids • Acids obtained by the hydrolysis of fats and oils are called fatty acids. • Fatty acids usually have an unbranched chain with an even number of carbon atoms. • If double bonds are present, they are almost always cis. H2O

  13. O CH3(CH2)10COH O CH3(CH2)12COH O CH3(CH2)14COH Table 24.1 Systematic name Common name Dodecanoic acid Lauric acid Tetradecanoic acid Myristic acid Hexadecanoic acid Palmitic acid

  14. O CH3(CH2)16COH O CH3(CH2)18COH Table 24.1 Systematic name Common name Octadecanoic acid Stearic acid Icosanoic acid Arachidic acid

  15. O CH3(CH2)7 (CH2)7COH C C H H Table 24.1 Systematic name: (Z)-9-Octadecenoic acid Common name: Oleic acid

  16. O CH3(CH2)4 (CH2)7COH CH2 C C C C H H H H Table 24.1 Systematic name: (9Z, 12Z)-9,12-Octadecadienoic acid Common name: Linoleic acid

  17. O CH3CH2 (CH2)7COH CH2 CH2 C C C C C C H H H H H H Table 24.1 Systematic name: (9Z, 12Z, 15Z)-9,12,15- Octadecatrienoic acid Common name: Linolenic acid

  18. O OH H H H H H H H H Table 24.1 Systematic name: (5Z, 8Z, 11Z, 14Z)-5,8,11,14- Icosatetraenoic acid Common name: Arachidonic acid

  19. H O O H OR OR H2, cat trans-Fatty Acids • Are formed by isomerization that can occur when esters of cis-fatty acids are hydrogenated.

  20. H O O O H OR OR OR H H H2, cat

  21. 24.3Fatty Acid Biosynthesis

  22. Fatty Acid Biosynthesis • Fatty acids are biosynthesized via acetyl coenzyme A. • The group of enzymes involved in the overall process is called fatty acid synthetase. • One of the key components of fatty acid synthetase is acyl carrier protein (ACP—SH).

  23. O O CH3CSCoA CH3CS—ACP + HSCoA Fatty Acid Biosynthesis • An early step in fatty acid biosynthesis is the reaction of acyl carrier protein with acetyl coenzyme A. + HS—ACP

  24. O O O – – HCO3 CH3CSCoA OCCH2CSCoA Fatty Acid Biosynthesis • A second molecule of acetyl coenzyme A reacts at its  carbon atom with carbon dioxide (as HCO3–) to give malonyl coenzyme A. + Acetylcoenzyme A Malonylcoenzyme A

  25. O O O O ACP—SH – – OCCH2CS—ACP OCCH2CSCoA Fatty Acid Biosynthesis • Malonyl coenzyme A then reacts with acyl carrier protein. Malonylcoenzyme A

  26. O O O CH2CS—ACP CH3C O O – •• CH2CS—ACP O C •• •• Fatty Acid Biosynthesis • Malonyl—ACP and acetyl—ACP react by carbon-carbon bond formation, accompanied by decarboxylation. S—ACP CH3C S-Acetoacetyl—ACP

  27. O O OH O CH2CS—ACP CH2CS—ACP CH3C CH3C H Fatty Acid Biosynthesis • In the next step, the ketone carbonyl is reduced to a secondary alcohol. NADPH S-Acetoacetyl—ACP

  28. O O OH CH2CS—ACP CHCS—ACP CH3CH CH3C H Fatty Acid Biosynthesis • The alcohol then dehydrates.

  29. O O CH3CH2CH2CS—ACP CHCS—ACP CH3CH Fatty Acid Biosynthesis Reduction of the double bond yieldsACP bearing an attached butanoyl group. • Repeating the process gives a 6-carbon acyl group, then an 8-carbon one, then 10, etc.

  30. 24.4Phospholipids

  31. Phospholipids • Phospholipids are intermediates in the biosynthesis of triacylglycerols. • The starting materials are L-glycerol 3-phosphate and the appropriate acyl coenzyme A molecules.

  32. CH2OH O O RCSCoA HO R'CSCoA H CH2OPO3H2 O CH2OCR O R'CO H CH2OPO3H2 + + • The diacylated species formed in this step is called a phosphatidic acid.

  33. O CH2OCR O R'CO H CH2OH O CH2OCR O R'CO H CH2OPO3H2 • The phosphatidic acid then undergoes hydrolysis of its phosphate ester function. H2O

  34. O CH2OCR O R'CO H CH2OH O R"CSCoA O CH2OCR O R'CO H O CH2OCR" • Reaction with a third acyl coenzyme A molecule yields the triacylglycerol.

  35. O CH2OCR O O R'CO H CH2OCR O – CH2OPO2 R'CO H + (CH3)3NCH2CH2O CH2OPO3H2 Phosphatidylcholine • Phosphatidic acids are intermediates in the formation of phosphatidylcholine.

  36. O hydrophobic "tail" CH2OCR O R'CO H hydrophobic "tail" – CH2OPO2 + (CH3)3NCH2CH2O Phosphatidylcholine polar "head group"

  37. Phosphatidylcholine hydrophobic(lipophilic) "tails" hydrophilic "head group"

  38. Cell Membranes water • Cell membranes are "lipid bilayers." Each layer has an assembly of phosphatidyl choline molecules as its main structural component. water

  39. Cell Membranes water • The interior of the cell membrane is hydrocarbon-like. Polar materials cannot pass from one side to the other of the membrane. water

  40. 24.5Waxes

  41. O CH3(CH2)14COCH2(CH2)28CH3 Triacontyl hexadecanoate: occurs in beeswax Waxes • Waxes are water-repelling solids that coat the leaves of plants, etc. • Structurally, waxes are mixtures of esters. The esters are derived from fatty acids and long-chain alcohols.

  42. 24.6Prostaglandins

  43. Prostaglandins Prostaglandins are involved in many biological processes. Are biosynthesized from linoleic acid (C18) via arachidonic acid (C20). (See Table 24.1)

  44. O O OH HO OH O HO OH HO OH Examples: PGE1 and PGF1 PGE1 PGF1

  45. Prostaglandin Biosynthesis PGE2 is biosynthesized from arachidonic acid. The oxygens come from O2. The enzyme involved (prostaglandin endoperoxide synthase) has cyclooxygenase (COX) acitivity.

  46. CO2H CH3 O CO2H PGG2 O CH3 HOO Prostaglandin Biosynthesis Arachidonic acid O2fatty acid cyclooxygenase

  47. O CO2H PGH2 O CH3 HO O CO2H PGG2 O CH3 HOO Prostaglandin Biosynthesis reduction ofhydroperoxide

  48. O CO2H PGH2 O CH3 HO O CO2H CH3 PGE2 HO HO Prostaglandin Biosynthesis

  49. Icosanoids Icosanoids are compounds related to icosanoic acid CH3(CH2)18CO2H. Icosanoids include: prostaglandins thromboxanes prostacyclins leukotrienes

  50. O CO2H PGH2 O CH3 HO O O CO2H TXA2 CH3 HO Thromboxane A2 (TXA2) Thromboxane A2 is biosynthesized from PGH2 TXA2 promotes platelet aggregation and blood clotting

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