CHAPTER 21 Lipid Biosynthesis

1. CHAPTER 21Lipid Biosynthesis • Biosynthesis of fatty acids and eicosanoids • Biosynthesis of isoprenes and cholesterol • Cholesterol regulation • Biosynthesis of triacylglycerols, and membrane lipids Key topics:

2. Lipids Fulfill a Variety of Biological Functions • Storage of energy • Constituents of cellular membranes • Anchors for membrane proteins • Cofactors for enzymes • Signaling molecules • Pigments • Detergents • Transporters • Antioxidants

3. Catabolism and Anabolic of Fatty Acids Proceed via Different Pathways • Catabolism of fatty acids • produced acetyl-CoA • reducing power to NADH • location: mitochondria • Anabolism of fatty acids • requires malonyl-CoA and acetyl-CoA • reducing power from NADPH • location: cytosol in animals, chloroplast in plants

4. Lipid Catabolism and Anabolism in Animal and Plant Cells

5. Overview of Fatty Acid Synthesis • Fatty acids are built in several passes processing one acetate unit at a time • Acetate from activated malonate in the form of malonyl-CoA • In each pass involves reduction of a carbonyl carbon to a methylene carbon

6. Overview of Fatty Acid Synthesis

7. Synthesis of Malonyl-CoA (1) • The three-carbon precursor for fatty acid synthesis is made from acetyl-CoA and CO2 • The reaction is catalyzed by acetyl-CoA carboxylase (ACC) • ACC is a bifunctional enzyme • Biotin carboxylase • Transcarboxylase • ACC contains biotin, nature’s carrier of CO2 • Biotin shuttles between the two active sites

8. Synthesis of Malonyl-CoA (2) • Bicarbonate reacts with the terminal phosphate of ATP to give carbamoyl phosphate • Biotin carries out a nucleophilic attack to carbamoyl phosphate • The product is a good donor of a carboxylate group

9. Synthesis of Malonyl-CoA (3) • The arm swing moves carboxybiotin to the transcarboxylase site • Terminal methyl of acetyl-CoA probably deprotonates to give a resonance-stabilized carbanion • The carbanion picks up the carboxylate moiety from biotin

10. Fatty Acid Synthesis • Overall goal is to attach a two-carbon acetate unit from malonyl-CoA to a growing chain and then reduce it • Reaction involves cycles of four enzyme-catalyzed steps • Condensation of the growing chain with activated acetate • Reduction of carbonyl to hydroxyl • Dehydration of alcohol to trans-alkene • Reduction of alkene to alkane • The growing chain is initially attached to the enzyme via a thioester linkage • During condensation, the growing chain is transferred to the acyl carrier protein • After the second reduction step, the elongated chain is transferred back to fatty acid synthase

11. Acyl Carrier Protein • Contains a covalently attached prothetic group 4’-phospho-pantethiene • The acyl carrier protein delivers acetate (in the first step) or malonate (in all the next steps) to the fatty acid synthase • The acyl carrier protein shuttles the growing chain from one active site to another during the four-step reaction

12. Charging the Acyl Carrier Protein and Fatty Acid Synthase • Two thiols participate in the fatty acid synthesis • Thiol from 4-phosphopantethine in acyl carrier protein • Thiol from cysteine in fatty acid synthase • Both thiols must be charged for the condensation reaction to occur • In the first step, acetyl from acetyl-CoA is transferred to acyl carrier protein • Acyl carrier protein passes this acetate to fatty acid synthase • Acyl carrier protein is then re-charged with malonyl from malonyl-CoA

13. Assimilation of Two-Carbon UnitsCondensation and First Reduction • 1 Condensation of an activated acyl group • 2 the β-keto group is reduced to an alcohol

14. Assimilation of Two-Carbon UnitsDehydration and Second Reduction • 3 elimination of H2O creates a double bond, and • 4 the double bond is reduced

15. Enzymatic Activities in Fatty Acid Synthase • Condensation with acetate • -ketoacyl-ACP synthase (KS) • Reduction of carbonyl to hydroxyl • -ketoacyl-ACP reductase (KR) • Dehydration of alcohol to alkene • -hydroxyacyl-ACP dehydratase (DH) • Reduction of alkene to alkane • enoyl-ACP reductase (ER) • Chain transfer • Malonyl/acetyl-CoA ACP transferase

16. Sequence of Events in Synthesis of Fatty Acids

27. Regulation of Fatty Acid Synthesis in Vertebrates

28. Insulin in Regulation of Fatty Acid Synthesis

29. C16, C18 and of Un-saturated Fatty Acids • Animals can readily introduce one double bond to palmitate and stearate • Vertebrates cannot introduce additional double bonds between C10 and methyl-terminal • We must obtain linoleate and -linolenate with diet; these are essential fatty acids • Plants, algae, and some insects synthesize linoleate from oleate

30. PC-Oleate Acts as A Substrate for Plant Desaturases • Oleic and Linoleic Acids are essential Fatty acids

31. Vertebrate Fatty Acyl Desaturase • Non-Heme Iron -- Mixed Function Oxidase • O2 accepts four electrons from two substrates • Two electrons come from saturated fatty acid • Two electrons come from ferrous state of Cytochrome b5

32. Oxidases, Monooxygenase, Dioxygenase • Molecular oxygen can serve as an electron acceptor • Oxidases do not incorporate oxygen atoms into the organic product • Oxygen atoms usually end up in hydrogen peroxide • Often use flavin as redox cofactors • Monooxygenases incorporate one of the oxygen atoms into the product • The other oxygen ends up in water • Often use iron as redox cofactor • Cytochrome P450 • Dioxygenases incorporate both oxygen atoms into the organic product

33. Cytochrome P450 (Mono-oxygenase)

34. Eicosanoids - Signalling functions • Prostaglandins • Gastric Mucin (Cox-1) • Inflammation, Pain Fever (Cox-2) • Thromboxanes • Blood Clotting • Leukotrienes • Signal through G-protein Coupled receptors • Asthma and Bronchodilation

35. Synthesis of Eicosanoids • Cyclooxygenase is a target for many anti-inflammatory drugs

36. The "linear" pathway from arachidonate to leukotrienes.

37. COX-2–specific cyclooxygenase inhibitors

38. Biosynthesis of Cholesterol • Summary 1. Acetyl CoA => Mevalonate 2. Mevalonate => Isoprenes 3. 6 isoprenes => squalene 4. Squalene to lanosterol

39. 1. Formation of Mevalonate • HMG-CoA reductase is a target for some cardiovascular drugs

40. Formation of Activated Isoprene • Pyrophosphate is a good leaving group in these nucleophilic substitution reactions

41. Formation of Squalene (1)

42. Formation of Squalene (2)

43. Ring closure converts squalene to the steroid nucleus

44. Cholesteryl esters. Esters more hydrophobic for storage and transport

45. Human Plasma Lipoproteins

46. Apolipoproteins of the Human Plasma Lipoproteins

47. Lipoproteins and lipid transport

48. LCAT on HDL stimulated by apoA-I.

49. Receptor-mediated endocytosis

50. Sterol regulatory element-binding proteins (SREBPs) released in response to membrane sterol content