Lipids

1. Lipids

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 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.

5. Acetyl Coenzyme A

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

7. 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

8. 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 a carbon atom. E+

9. Fats, Oils, and Fatty Acids

10. 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.

12. Saturated fat

13. Unsaturated fat

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

15. 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

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

17. 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

18. O CH3(CH2)10COH O CH3(CH2)12COH O CH3(CH2)14COH Fatty Acids Systematic name Common name Dodecanoic acid Lauric acid Tetradecanoic acid Myristic acid Hexadecanoic acid Palmitic acid

19. O CH3(CH2)16COH O CH3(CH2)18COH Fatty Acids Systematic name Common name Octadecanoic acid Stearic acid Icosanoic acid Arachidic acid

20. O CH3(CH2)7 (CH2)7COH C C H H Fatty Acids Systematic name: (Z)-9-Octadecenoic acid Common name: Oleic acid

21. O CH3(CH2)4 (CH2)7COH CH2 C C C C H H H H Fatty Acids Systematic name: (9Z, 12Z)-9,12-Octadecadienoic acid Common name: Linoleic acid

22. O CH3CH2 (CH2)7COH CH2 CH2 C C C C C C H H H H H H Fatty Acids Systematic name: (9Z, 12Z, 15Z)-9,12,15- Octadecatrienoic acid Common name: Linolenic acid

25. O OH H H H H H H H H Fatty Acids Systematic name: (5Z, 8Z, 11Z, 14Z)-5,8,11,14- Icosatetraenoic acid Common name: Arachidonic acid

26. Fatty Acid Biosynthesis

27. 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).

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

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

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

31. 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

32. 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

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

34. 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.

35. Phospholipids

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

41. 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.

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

43. O CH2OCR O R'CO H CH2OH O CH2OCR O R'CO H CH2OPO3H2 • Reaction with a third acyl coenzyme A molecule yields the triacylglycerol. H2O

44. 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.

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

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

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

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

49. 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