1. CHAPTER 21 Lipid Biosynthesis
Key topics:
Biosynthesis of fatty acids and eicosanoids
Biosynthesis of triacylglycerols
Biosynthesis of cholesterol
白麗美 醫學一8F, 5520

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
‘Forehead to forehead I meet thee, this third time, Moby Dick!’ [Ahab (Melville, 1851)]
Head-butting during male–male aggression is a basal behavior for cetaceans
Sinking of Essex (238 ton ship) in 1821 is the first documented case of a sperm whale deliberately striking a ship (Chase, 1821). 2

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. ACC is a bifunctional enzyme Biotin carboxylase Transcarboxylase
Fig The
Acetyl-CoA carboxylase
reaction
ACC is a bifunctional enzyme
Biotin carboxylase
Transcarboxylase
FIGURE 21-1 (part 1) The acetyl-CoA carboxylase reaction. Acetyl-CoA carboxylase has three functional regions: biotin carrier protein (gray); biotin carboxylase, which activates CO2 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction (see Figure 16-16); and transcarboxylase, which transfers activated CO2 (shaded green) from biotin to acetyl-CoA, producing malonyl-CoA. The long, flexible biotin arm carries the activated CO2 from the biotin carboxylase region to the transcarboxylase active site. The active enzyme in each step is shaded blue. 4

5. FIGURE 21-1 The acetyl-CoA carboxylase reaction
FIGURE 21-1 The acetyl-CoA carboxylase reaction. Acetyl-CoA carboxylase has three functional regions: biotin carrier protein (gray); biotin carboxylase, which activates CO2 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction (see Figure 16-16); and transcarboxylase, which transfers activated CO2 (shaded green) from biotin to acetyl-CoA, producing malonyl-CoA. The long, flexible biotin arm carries the activated CO2 from the biotin carboxylase region to the transcarboxylase active site. The active enzyme in each step is shaded blue. 5

6. Fig. 21.2 Addition of two carbons Four steps
FIGURE 21-2 (part 1) Addition of two carbons to a growing fatty acyl chain: a four-step sequence. Each malonyl group and acetyl (or longer acyl) group is activated by a thioester that links it to fatty acid synthase, a multienzyme system described later in the text. 1 Condensation of an activated acyl group (an acetyl group from acetyl-CoA is the first acyl group) and two carbons derived from malonyl-CoA, with elimination of CO2 from the malonyl group, extends the acyl chain by two carbons. The mechanism of the first step of this reaction is given to illustrate the role of decarboxylation in facilitating condensation. The β-keto product of this condensation is then reduced in three more steps nearly identical to the reactions of β oxidation, but in the reverse sequence: 2 the β-keto group is reduced to an alcohol, 3 elimination of H2O creates a double bond, and 4 the double bond is reduced to form the corresponding saturated fatty acyl group. 6

7. FIGURE 21-2 (part 2) Addition of two carbons to a growing fatty acyl chain: a four-step sequence. Each malonyl group and acetyl (or longer acyl) group is activated by a thioester that links it to fatty acid synthase, a multienzyme system described later in the text. 1 Condensation of an activated acyl group (an acetyl group from acetyl-CoA is the first acyl group) and two carbons derived from malonyl-CoA, with elimination of CO2 from the malonyl group, extends the acyl chain by two carbons. The mechanism of the first step of this reaction is given to illustrate the role of decarboxylation in facilitating condensation. The β-keto product of this condensation is then reduced in three more steps nearly identical to the reactions of β oxidation, but in the reverse sequence: 2 the β-keto group is reduced to an alcohol, 3 elimination of H2O creates a double bond, and 4 the double bond is reduced to form the corresponding saturated fatty acyl group. 7

8. Fig. 21.3 Multifunctional Polypeptide Fatty acid synthase thioesterase
FIGURE 21-3a The structure of fatty acid synthase type I systems. The low-resolution structures of (a) the mammalian (porcine; derived from PDB ID 2CF2) and (b) fungal enzyme systems (derived from PDB IDs 2UV9, 2UVA, 2UVB, and 2UVC) are shown. (a) All of the active sites in the mammalian system are located in different domains within a single large polypeptide chain. The different enzymatic activites are: β-ketoacyl-ACP synthase (KS), malonyl/acetyl-CoA—ACP transferase (MAT), β-hydroxyacyl-ACP dehydratase (DH), enoyl-ACP reductase (ER), and β-ketoacyl-ACP reductase (KR). ACP is the acyl carrier protein. The linear arrangement of the domains in the polypeptide is shown in the lower panel. The seventh domain (TE) is a thioesterase that releases the palmitate product from ACP when the synthesis is completed. The ACP and TE domains are disordered in the crystal and are therefore not shown in the structure.
thioesterase 8

9. Palmitate synthesis
FIGURE 21-4 (part 1) The overall process of palmitate synthesis. The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of CO2 at each step. The initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of four, then six, then eight carbons, and so on. The final product is palmitate (16:0). 9

10. First acyl
FIGURE 21-4 (part 2) The overall process of palmitate synthesis. The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of CO2 at each step. The initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of four, then six, then eight carbons, and so on. The final product is palmitate (16:0). 10

11. Acyl carrier protein
FIGURE 21-5 Acyl carrier protein (ACP). The prosthetic group is 4′-phosphopantetheine, which is covalently attached to the hydroxyl group of a Ser residue in ACP. Phosphopantetheine contains the B vitamin pantothenic acid, also found in the coenzyme A molecule. Its —SH group is the site of entry of malonyl groups during fatty acid synthesis. 11

12. FIGURE 21-6 Sequence of events during synthesis of a fatty acid
FIGURE 21-6 Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 12

13. FIGURE 21-6 (part 1) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 13

14. FIGURE 21-6 (part 2) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 14

15. Condensation with acetate -ketoacyl-ACP synthase (KS)
FIGURE 21-6 (part 3) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 15

16. Reduction of carbonyl to hydroxyl -ketoacyl-ACP reductase (KR)
FIGURE 21-6 (part 4) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 16

17. Dehydration of alcohol to alkene -hydroxyacyl-ACP dehydratase (DH)
FIGURE 21-6 (part 5) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 17

18. Reduction of alkene to alkane enoyl-ACP reductase (ER)
FIGURE 21-6 (part 6) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 18

19. Malonyl/acetyl-CoA ACP transferase
Chain transfer
Malonyl/acetyl-CoA ACP
transferase
FIGURE 21-6 (part 7) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 19

20. FIGURE 21-6 (part 8) Sequence of events during synthesis of a fatty acid. The mammalian FAS I complex is shown schematically, with catalytic domains colored as in Figure Each domain of the larger polypeptide represents one of the six enzymatic activities of the complex, arranged in a large, tight "S" shape. The acyl carrier protein (ACP) is not resolved in the crystal structure shown in Figure 21-3, but is attached to the KS domain. The phosphopantetheine arm of ACP ends in an —SH. After the first panel, the enzyme shown in color is the one that will act in the next step. As in Figure 21-4, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps 1 to 4 are described in the text. 20

21. Figure 21-7
Second round of the fatty acid
Synthesis cycle

22. Figure 21-8
Subcellular localization of lipid metabolism
Glycolysis produce NADH, in cytosol NADH/NAD is small

23. Figure 21-9
Production of NADPH
Hepatocyte and adipocyte

24. Figure 21-10
Not from fatty acid

25. Regulation of fatty acid synthesis Acetyl-CoA carboxylase
Figure 21-11
Regulation of fatty acid synthesis
Acetyl-CoA carboxylase
Active-dephosphorylation
1. Allosteric regulation
citrate, palmitoyl CoA
2. hormone-
insulin-dP (activation)
glucagon and epinephrine
P (inactivation)
3. plant: Mg 2+ , pH,
illumination
4. bacteria-cell growth
guanine nucleotide
5. Gene level:
polyunsaturated FA
inhibit lipogenic enzyme in liver

26. Figure 21-12
synthesis of other fatty acids
Elongation: smooth ER and
mitochondria
Acetyl CoA from Malonyl CoA
Coenzyme A as acyl carrier
Essential fatty acid

27. Figure 21-13
Desaturation of fatty acids in vertebrates
Oxidation on both, on smooth ER

28. Box 21-1
Mixed function oxidases (fatty acyl-CoA desaturase),
oxygenases, and cytochrome P450
Oxidase: O2 as electron acceptor
e.g. fatty acid oxidation in peroxisomes
cytochrome oxidase in mitochondria
most: flavoproteins
Oxygenase: O2 into substrate
As hydroxyl or carboxyl group
Dioxygenase

29. Box 21-1
Mixed function oxidases, oxygenases, and cytochrome P450
Monooxygenase (hydroxylases, mix-function oxidases (oxygenase))
Heme protein-cytochrome P-450 (SER)
Hydroxylation –adrenocortical hormone
Drug-soluble , Detoxification
Figure, 21-13, 16, 30, 37, 47

30. Figure, 21-14 Action of plant desaturation
ER and chloroplast
Membrane fluidity

31. Figure, 21-15 (a) the cyclic pathway
Eicosanoids:
biological signal molecules
Local hormone
G-protein linked receptor
(SER)
COX:
Prostaglandin H2 synthase

32. Figure, 21-15 (b) the cyclic pathway
Cyclooxygenase:
Cox-1: prostaglandins-gastric mucin (protect)
Cox-2: prostaglandins-inflammation, pain, fever

33. Figure, 21-15 (b) nonsteroidal anti-inflammatory drug (NSAID)
minicking the structure of the substrate or an intermediate
Thromboxane synthase
PGH2 to thromboxane A2-constriction of blood vessels
and platelet aggregation (under COX1 pathway)
Low doses of aspirin reduce heart attacks and strokes

34. Figure21-15C
Cox-2-specific drugs , 1,000 times difference to Cox-1 and Cox-2
Reduce prostacyclin (COX2)
Dilates blood vessels)

35. Figure, 21-16 the linear pathway
lipooxygenase (mix-function oxidases)
Leukocytes, heart, brain, lung, and spleen
Use cytochrome P-450 (SER)

36. Figure, 21-17
Biosynthesis of phosphatidic acid
Liver and kidney

37. Figure, 21-18
Biosynthesis of triacylglycerol

38. Figure, 21-19
Regulation of triacylglycerol synthesis by insulin
Reduce cAMP- reduce lipolysis: reduce free fatty acid in blood
Citrate lyase
ACC

39. Figure, 21-20 the triacylglycerol cycle –low when other fuels available
75% released to form TAG in liver

40. Figure, 21-21Glyceroneogenesis
In adipocyte: glucagon and epinephrine
Suppress glycolysis
Little DHAP is available
no glycerol kinase in adipocyte
No glucose synthesis
DHAP

41. Glucocorticoil hormones regulate PEP carboxykinase
In the liver and adipose tissue

42. Figure, 21-22 regulation of glyceroneogenesis
Increase flux of TAG cycle

43. High levels of free fatty acids in the blood
Interfere with glucose utilization in muscle
Promote insulin resistance-type 2 diabetes
Thiazolidinediones-reduce fatty acids in the blood
Increase sensitivity to insulin

44. Figure, 21-22 regulation of glyceroneogenesis
This drug binds to and activate a nuclear hormone receptor
-peroxisome proliferator activated receptor g
Increase PEP carboxykinase in adipose tissue

45. Synthesis of malonyl-CoA, and fatty acyl chain
2. Regulation of fatty acid synthesis
3. Mixed function oxidase
4. Eicosanoids (NSAID)
5. Biosynthesis of triacylglycerol and the TAG cycle