1. 1 … Úrsula on the other hand, held a bad memory of that visit, for she had entered the room just as Melquíades had carelessly broken a flask of bichloride of mercury. “It’s the smell of the devil,” she said. “Not at all,” Melquíades corrected her. “It has been proven that the devil has sulphuric properties and this is just a little corrosive sublimate.” … Gabriel García Márquez, One Hundred Years of Solitude.

2. LIPID METABOLISM Lipids serve as a source of energy and for the synthesis of membranes and biologically active molecules Fatty acids are the preferred source of energy for the heart, the liver and for the skeletal muscles (during rest and prolonged activity) Fatty acids can be obtained from the diet or they can be synthesized from excess carbohydrates and proteins (mainly in the liver and the adipose tissue) and stored in the adipose tissue advantages to having triacylglycerides as storage lipids:  they have high energy content because they are highly reduced  they form aggregates because of their non-polar nature; osmolality is not raised and there is no solvation  stability of the C-C bonds 2

3. 3  human milk, on the other hand, is rich in long-chain fatty acids which are essential for the development of the brain  some amount of lipases is also released along with the milk The insolubility of lipids in water poses a problem for their digestion  lipid-digesting enzymes are water-soluble  they can access only the outer surface of the lipid aggregates  this problem is solved by bile salts which emulsify the lipids and increase the surface area that is exposed to the enzymes Bile salts are amphipathic molecules synthesized from cholesterol by the liver and stored in the gall bladder

4. 4 The digestion and transport of lipids Although the main lipids in the diet are triacylglycerides, small amounts of phospholipids, cholesterol and cholesterol esters are present Most of the digestion of lipids in the intestine and produces free fatty acids, 2-monoacylglycerols, cholesterol and lysophospholipids (phospholipids with a fatty acid removed from carbon 2) Lingual and gastric lipases are relatively active in infants and children  the enzymes cleave short and medium-chain ( < 12 carbons) fatty acids from triacyglycerides  short and medium-chain fatty acids are abundant in cow milk

5. 5 Secretion of bile salts and cholesterol into the bile is the only way of excreting cholesterol Bile salts enclose pieces of triacylglyceride Colipase binds with TAG and lipase; removes bile salts and the lid that was covering the active site of lipase Lipase breaks triacyglycerides down into two free fatty acids and 2-monoacyglycerol Cholesterol esterase produces free cholesterol and a fatty acid molecule Phospholipase A 2 gives a lysophospholipid and a fatty acid  lysophospholipids are powerful detergents  assist in the emulsification process  Some amount of lecithin (phosphatidylcholine) is secreted in the bile  Bee and cobra venoms contain phospholipase A 2

6. 6

7. 7 Short and medium-fatty acids are absorbed directly into the intestinal epithelial cells Once in the cells, they are bound with intestinal fatty acid binding protein (I-FABP)

8. 8 From the intestine they pass to the portal veins and they travel to the liver bound with albumin  the function of I-FABP and albumin is to increase solubility and prevent the damage to cell membranes that could be brought by the detergent properties of free fatty acids The absorption of the remaining products of lipid digestion is through the formation of mixed micelles that are made up of amphipathic molecules: bile salts, lysophospholipids, long- chain fatty acids, 2- monoacyglycerol, cholesterol and fat- soluble vitamins Except for the bile salts, the rest are absorbed by the duodenal and jejunal epithelial cells  in the epithelium triacylglycerides, phospholipids and some cholesterol ester are resynthesized > 95 % of the bile salts reabsorbed in the ileum and returned to the liver (enterohepatic circulation) to be recycled; the rest is excreted

9. 9 Absorption of the products of lipid digestion

10. 10 Enterohepatic circulation of bile salts

11. 11 o Steatorrhea or the excretion of undigested lipids could result from the deficiency of pancreatic lipase or bile salts the digestion and absorption of fat-soluble vitamins is also impaired If the resynthesized TAG were to enter the blood directly, they would have formed aggregates and blood flow would have been blocked This problem is solved through the synthesis of chylomicrons – one type of lipoproteins The idea behind lipoproteins is to transport hydrophobic lipids by sequestering them in the core of the particle and covering them with a hydrophilic layer made up of proteins The core of chylomicrons consists of TAG, cholesterol esters and vitamins; the surface is made of proteins and the polar groups of cholesterol and phospholipids

12. 12 The resynthesis of TAG The main apoprotein of chylomicrons is Apo B-48 and is produced by the rough endoplasmic reticulum TAG resynthesized by the smooth ER; TAG and Apo B-48 are packed into chylomicrons in the Golgi complex Other proteins in chylomicrons include Apo-CII and Apo-E Of the lipoproteins, chylomicrons are the biggest in the size and the lowest in density (smallest amount of proteins)

13. 13

14. 14 Nascent chylomicrons (containing only Apo B-48) are exocytosed into the lymph and then join the blood stream While in the circulation, they receive the other apoproteins from high density lipoprotein (HDL) and become mature chylomicrons a fat-rich meal gives the blood a milky appearance Chylomicrons are destined mainly for the adipose, muscles (especially cardiac muscles) and lactating mammary glands These tissues produce lipoprotein lipase (LPL) LPL is found associated with the proteoglycans in the basement membranes of the endothelial cells of the capillaries serving these tissues The LPL isozyme from the adipose has a higher K m and is most active after meals when there is much TAG in the blood insulin stimulates the synthesis of adipose LPL

15. 15 LPL is activated by Apo CII and removes fatty acids from the TAG of the chylomicrons The free fatty acids travel bound with albumin until they are absorbed The fatty acids are used as energy sources in the muscles In the adipose they are reesterified and stored while in the mammary glands they are incorporated into milk fat The glycerol that was produced through the action of LPL travels to the liver and could be used for TAG synthesis in the fed-state When most of the TAG has been removed, chylomicrons shrink and become chylomicron remnants Chylomicron remnants are endocytosed by the liver (through recognition of Apo E) and their contents are degraded in the lysosomes

16. 16

17. 17 Mobilization of triacylglycerides from the adipose The adipose stores mainly TAG and steroid hormone – synthesizing tissues like the adrenal cortex, ovaries and testes store cholesterol esters The lipids are stored in the form of lipid droplets: a core of TAG or cholesterol esters surrounded by a single layer of phospholipids and perilipin proteins Perilipins prevent the untimely mobilization of lipids The release of glucagon in the fasting state or epinephrine in the fight or flight response increases the level of cAMP – a secondary messenger – in the adipose cAMP activates protein kinase A which in turn phosphorylates perilipin The phosphorylated perilipin allows hormone-sensitive lipase (HSL) to move to the surface of the lipid droplet

18. 18 HSL also is phosphorylated by protein kinase A; but the main factor for the massive increase in mobilization is the phosphorylation of perilipin HSL breaks TAG down into fatty acids and glycerol The glycerol travels to the liver and is used for gluconeoge. Some of the fatty acids travel-bound with albumin-to the cardiac and skeletal muscles and the renal cortex; provide energy About 75 % of the fatty acids released by lipolysis are reesterified This is known as the triacylglycerol cycle the fatty acids are reesterified while they are still in the adipose or after they have travelled to the liver The cycle operates even during starvation

19. 19

20. 20 In the adipose tissue, dihydroxyacetone from glycolysis is changed to glycerol-3-phosphate for TAG synthesis; the liver can make use of glycerol But glucagon and epinephrine inhibit glycolysis and DHAP would not be available in the adipose for TAG synthesis o What is the source of the DHAP used for the synthesis of TAG in the adipose during starvation?  glyceroneogenesis is a shortened version of gluconeogenesis that produces DHAP from gluconeogenic substrates like alanine, aspartate and malate  the adipose has two gluconeogenic enzymes – pyruvate carboxylase and PEP carboxykinase – even though it does not produce glucose

21. 21 Glyceroneogenesis supports TAG synthesis in the liver during starvation Natural and artificial glucocorticoids induce the synthesis of the liver isozyme of PEP carboxykinase while inhibiting the adipose isozyme  little reesterification in the adipose; more fatty acids will be released to the blood stream  but esterification in the liver will be more active and reestablish the 75 % Excess levels of fatty acids in the blood have been implicated in the development of insulin resistance  thiazolidinediones are drugs that induce the synthesis of the adipose isozyme of PEP carboxykinase  used in the treatment of type 2 diabetes mellitus

22. 22 The triacylglycerol cycle Thiazolidinediones

23. 23 FATTY ACID CATABOLISM/ β- OXIDATION Fatty acids released from the adipose have their origins in the diet or synthesis in the liver The most common dietary fatty acids are palmitate (16:0), stearate (18:0), oleate (18:1) and linoleate (18:2) Animal fat contains mostly saturated and monounsaturated fatty acids while vegetable oils contain linoleate and longer polyunsaturated fatty acids Fatty acids enter cells both by diffusion and sodium- dependent transport As shown by Lehninger and Kennedy, the oxidation of fatty acids takes place in the mitochondria Short and medium-chain fatty acids can freely enter into and be activated in the mitochondria

24. 24 long chain fatty acids, on the other hand, are first activated in a reaction with CoA; form a high energy thioester  the acyl-CoA synthetase is present in the ER and the outer membranes of the mitochondria and the peroxisomes. It shows little or no activity with short, medium and very long chain (>22C) fatty acids

25. 25 The fatty acyl-CoA can be used either in the synthesis of TAG and membrane lipids or transported into the mitochondria and used for energy production Transport by the carnitine shuttle  Carnitine reacts with fatty acyl-CoA and gives acyl- carnitine; commits the fatty acids to oxidation  the enzyme responsible is carnitine acyltransferase I (CAT I) which is located on the outer mitochondrial membrane  Acylcarnitine crosses the inner membrane through a translocase  In the mitochondria CAT II changes acylcarnitine to fatty acyl-CoA  Carnitine returns to the cytosol through the translocase

26. 26 Acylcarnitine

27. 27 As the name indicates, β- oxidation is a set of reactions aimed at forming a carbonyl group on the β -carbon The presence of two carbonyl groups weakens the otherwise stable C α -C β bond and acetyl-CoA groups are removed successively One cycle of β- oxidation consists of four reactions 1.The oxidation of the C α -C β bond Catalyzed by acyl-CoA dehydrogenases - a family of three isozymes specific for short, medium and long-chain FA Contain non-covalently (but tightly) attached FAD FADH 2 transfers the electrons to electron transfer flavoprotein (ETF) ETF is oxidized by ETF:UQ oxidoreductase and the electrons are delivered to coenzyme Q

28. 28 o The double bond is in the trans form

29. 29 2.Addition of water across the double bond Trans-Δ2-enoyl-CoA is acted upon by enoyl-CoA hydratase 3.Oxidation of the β –hydroxyl group β –hydroxyacyl-CoA is oxidized by L-hydroxyacyl-CoA dehydrogenase; NAD is the electron carrier

30. 30 4.cleavage of the C α -C β bond β –ketoacyl-CoA cleaved by β –ketoacyl thiolase An acetyl-CoA molecule and an acyl-CoA two carbons shorter are produced o Steps 2-4 of the β- oxidation of long chain fatty acids are carried out by a multienzyme complex known as trifunctional protein (TFP)  individual enzymes for short and medium chain FA

31. 31 A cycle of β- oxidation releases one molecule each of FADH 2,NADH and acetyl-CoA the four steps are repeated n/2 - 1 times ( n = number of carbons) until the final acetyl-CoA is released n/2 acetyl-CoA and n/2 - 1 each of FADH 2 and NADH are produced by the complete oxidation of a fatty acid with an even number of carbons taking the oxidation of palmitate (16:0): Palmitoyl-CoA+ 7 CoA+ 7 FAD+ 7 NAD + +7 H 2 O → 8 acetyl-CoA+ 7 FADH 2 +7 NADH+ 7H + 8 acetyl-CoA ≈ 8o ATP, 7 FADH 2 ≈ 10.5 ATP and 7 NADH ≈ 17.5 ATP ⇒ total = 108 ATP the equivalent of two ATP is spent on activation of FA ⇒ net production of 106 ATP o The oxidation of fatty acids releases a large amount of metabolic water

32. 32 the β- oxidation of odd-chain fatty acids Odd-chain FA are common in plants and marine organisms β- oxidation proceeds in the normal way but the last product to be released is not acetyl-CoA but propionyl-CoA The propionyl-CoA is first changed to D-methylmalonyl-CoA through the action of propionyl-CoA carboxylase  biotin is the coenzyme D-methylmalonyl-CoA is epimerized L-methylmalonyl-CoA undergoes intramolecular rearrangement to give succinyl-CoA  this is one of the two reactions in humans where vitamin B 12 is involved Succinyl-CoA can be used for energy production or the synthesis of glucose

33. 33

34. 34 the β- oxidation of unsaturated fatty acids Double bonds naturally occurring in fatty acids are in the cis form Enoyl-CoA hydratase cannot act on cis double bonds In order for β- oxidation to proceed, either the cis form has to be changed to trans or the double bond should be reduced Two additional enzymes are needed – an isomerase and a reductase For monounsaturated fatty acids, the enoyl-CoA isomerase suffices Polyunsaturated fatty acids need the participation of both enzymes NADPH is used as the reductant

35. 35 This slide: the oxidation of a monounsaturated fatty acid – oleate Next slide: the oxidation of a polyunsaturated fatty acid–linoleate

36. 36

37. 37 the regulation of β- oxidation The concentration of mitochondrial CoA may be limiting to the formation of fatty acyl-CoA The formation of malonyl-CoA is the first step during the synthesis of fatty acids Malonyl-CoA inhibits CAT I making sure that fatty acids do not get degraded at the same time they are synthesized High levels of FADH 2 and NADH respectively inhibit acyl-CoA dehydrogenase and β-hydroxyacyl-CoA dehydrogenase; high levels of acetyl-CoA inhibit thiolase

38. 38 Disorders of fatty acid oxidation Inherited defects in the carnitine-assisted transport or the enzymes of β- oxidation are manifested as serious heart disease CAT I/II, carnitine translocase, acyl-CoA dehydrogenases, … may be defective Carnitine obtained from the diet or synthesized in the body  carnitine deficiency is unlikely to be seen in adults  carnitine related enzymes and transporters are more commonly deficient leading to the accumulation of long chain acyl/ acyl carnitine  weak muscles; should avoid prolonged exercise and fasting Acyl-CoA dehydrogenase deficiency results in the excretion of *derivatives of fatty acids in the urine and *non-ketotic hypoglycemia

39. 39 Alternate routes of fatty acid oxidation Minor, yet important pathways of fatty acid oxidation 1. peroxisomal oxidation Human diet usually contains VLCFA and branched-chain fatty acids from chlorophyll degradation VLCFA are also produced in high amounts in the nervous system and incorporated into the myelin sheath VLCFA undergo β- oxidation but the first step is unique in that it transfers electrons from acyl-CoA to FAD and then to O 2 - production of H 2 O 2 Acyl-CoA oxidase

40. 40 The three remaining reactions of β- oxidation proceed in the normal way Acetyl groups move out of the peroxisome by themselves or carried by carnitine When the acyl-CoA is shortened to about 8 carbons, it is transported out of the peroxisomes (with carnitine) and its oxidation is completed in the mitochondria

41. 41 Another type of oxidation takes place in the peroxisomes – α-oxidation of branched-chain fatty acids Mainly on phytanic acid and pristanic acid phytanic acid is first changed to pristanic acid by hydroxylation of the α carbon followed by decarboxylation The shortening by one carbon puts methyl groups on the α rather than the β carbon – normal β oxidation continues Acetyl-CoA and propionyl-CoA are alternatively removed from the acyl-CoA When a middle-chain acyl-CoA is reached, it is transported to the mitochondria Refsum’s disease - deficiency of phytanic acid α hydroxylase  accumulation of phytanic acid in the body  leads to serious neurological problems

42. 42

43. 43 Zellweger syndrome: the absence of functional peroxisomes ; X-linked adrenoleukodystrophy (XALD): lack of tansporters for VLCFA into the peroxisomes  accumulation of VLCFA in the body, especially C26:0 and C26:1  Affect mainly the liver and the brain; fatal in the very early years of life ω-oxidation In the endoplasmic reticulum, the end farthest from the carboxyl group is oxidized first, the methyl group is changed to hydroxyl by cytochrome p 450 enzymes Then comes dehydrogenation which yields a dicarboxylic acid

44. 44 Either end can be attached to CoA and undergo β oxidation β oxidation produces smaller dicarboxylic acids such as adipic and succinic acids Dicarboxylic acids are highly water-soluble and can be removed through the urine ω-oxidation is normally involved in the metabolism of xenobiotics that have structures similar with fatty acids ω-oxidation becomes more prominent when β oxidation is defective and the need arises to excrete the unmetabolized fatty acids

45. 45

46. 46 Metabolism of ketone bodies Most of the acetyl-CoA produced from hepatic fatty acid oxidation enters the TCA cycle The remaining acetyl-CoA molecules undergo ketogenesis – the production of the ketone bodies: acetone, acetoacetate and β- hydroxybutyrate The ketone bodies are released from the liver and provide energy for the heart, skeletal muscles, renal cortex and brain  changed back into acetyl-CoA and join the TCA cycle Ketogenesis occurs in the mitochondrial matrix  two acetyl-CoA condense to give acetoacetyl-CoA  another acetyl-CoA is added to acetoacetyl-CoA to give β-hydroxyl-β –methyl –glutaryl –CoA (HMG-CoA)  HMG-CoA is changed to acetoacetate and acetyl-CoA  Acetoacetate is reduced to β- hydroxybutyrate or decarboxylated to acetone

47. 47

48. 48 The thiolase that catalyzes the first reaction of ketogenesis is the same enzyme that is involved in the last reaction of β oxidation Ketogenesis and *cholesterol synthesis share the reactions up to the formation of HMG-CoA Acetoacetate and β- hydroxybutyrate are exported to other tissues through the blood  ketone bodies are easily transportable forms of fatty acids  the conversion of acetoacetate and β- hydroxybutyrate back to acetyl-CoA takes place in the mitochondria  two reactions are retained from ketogenesis and one new reaction is introduced – the activation of acetoacetate by the transfer of CoA from succinyl-CoA  β -ketoacyl transferase is absent in the liver Most of the acetone is exhaled

49. 49 2 acetyl CoA = 20 ATP changing succinyl CoA to succinate without the production of GTP ⇒ subtract 1 ATP for the activation  19 ATP starting from β- hydroxybutyrate ⇒ 21.5 ATP (additional energy from NADH)

50. 50 o The degradation of *ketogenic amino acids provides acetoacetyl-CoA and acetyl-CoA The abundance of fatty acids in the blood during the fasting state increases the level of ketone body synthesis  Oxaloacetate is diverted to gluconeogenesis  More acetyl-CoA enters ketogenesis because of the small levels of oxaloacetate to condense with (for TCA) In type I diabetes, the inhibitory effect of insulin on lipolysis is relieved  fatty acids released by lipolysis in turn produce excess amounts of acetyl-CoA  even though glucose is abundant in the blood gluconeogenesis continues unabated – oxaloacetate is consumed for glucose production

51. 51 In addition, glucose will not be used by the liver for fatty acid synthesis  The levels of malonyl-CoA, which is the starting material for fatty acid synthesis fall  The inhibition of malonyl-CoA on CAT I is relieved  β -oxidation produces excess acetyl-CoA which feeds ketogenesis The production of acetoacetate and β- hydroxybutyrate brings about an associated decrease in extracellular pH If the amount of ketone bodies is more than the amount that can effectively be buffered by bicarbonate ( > 7mM), ketoacidosis results  the amount of ketone bodies can be measured from the urine or the blood  “Acetone breath” can serve as an indication of excess ketogenesis

52. 52

53. 53 FATTY ACID SYNTHESIS Fatty acids are used for the synthesis of storage, membrane and other important lipids The raw material (acetyl-CoA) for the biosynthesis of fatty acids comes from the excess intake of carbohydrates or proteins The main site of synthesis is the liver and the adipose also carries out some degree of synthesis It is a cytosolic process:  the acetyl-CoA from the PDC reaction condenses with oxaloacetate and citrate is exported from the mitochondria  citrate lyase provides cytosolic acetyl-CoA  the NADPH provided by the malic enzyme reaction (and the NADPH from PPP) is used for the biosynthesis  protein degradation provides mainly cytosolic acetyl CoA

54. 54 The committed step of fatty acid synthesis is the synthesis of malonyl-CoA : acetyl-CoA is acted upon by acetyl-CoA carboxylase (ACC) with the help of biotin The first half of the ACC reaction

55. 55 The second half of the ACC reaction

56. 56 ACC is the rate limiting enzyme of fatty acid synthesis ACC is an octamer made up of inactive monomers The final product of fatty acid synthesis is palmitoyl-CoA The accumulation of palmitoyl-CoA leads to the dissociation of the polymers Citrate, on the other hand, favors the association The local control exerted by citrate and palmitoyl –CoA acts in concert with the global control of hormones  dephosphorylated ACC has got a higher affinity for citrate and small amounts of citrate can activate it  phosphorylated ACC has got a higher affinity for palmitoyl- CoA and small amounts of palmitoyl-CoA can inhibit it  the enzyme responsible for the phosphorylation is AMP- dependent protein kinase (AMPK) – activated by AMP and inhibited by ATP

57. 57 Glucagon and epinephrine activate protein kinase A which phosphorylates and inactivates phosphatase 2A Insulin activates the phosphatase and favors fatty acid synthesis Adaptive control of ACC based on the diet - high carbohydrate foods induce the synthesis of ACC and fatty acid synthase Fatty acid synthase (FAS) During fatty acid synthesis, two carbons from malonyl-CoA are added to a growing acyl chain that began with acetyl-CoA as a primer The series of reactions until the stage of palmitoyl-CoA is reached is catalyzed by FAS FAS is a dimeric protein with identical subunits Each subunit has seven enzymatic activities and an acyl carrier protein (ACP) FAS is a multifunctional protein

58. 58 Glucagon, epinephrine _ The regulation of ACC

59. 59 The subunits of FAS are arranged in a “head-to-tail” manner Each subunit has three domains The first domain is responsible for the binding and condensation of acetyl and malonyl groups: contains acetyl transferase (AT), malonyl transferase (MT) and β -ketoacyl synthase (KS) The second domain reduces the intermediate produced by the first domain using NADPH; contains ACP, β -ketoacyl reductase (KR), dehydratase (DH) and enoyl-CoA reductase (ER) The third domain contains thioesterase (TE) which releases the final product, palmitate Growing acyl chains are transferred between the cysteinyl sulfhydryl group of the KS of one subunit and the phosphopantetheinyl sulfhydryl group of the ACP of the other subunit

60. 60 Fatty acid synthase

61. 61 The mechanism of fatty acid synthase Domain 1 of one subunit interacts with domains 2 and 3 of the other subunit Acetyl-CoA and malonyl-CoA attach to serine residues on AT and MT, respectively (the CoA is removed) Acetyl is then transferred to the ACP of the other subunit and ACP takes acetyl to the KS of the previous subunit The now free ACP carries the malonyl and takes it to the KS Condensation takes place on KS with the removal of CO 2  the CO 2 removed is the same one that was introduced during the synthesis of malonyl-CoA  decarboxylation favors nucleophilic attack of the methylene carbon of malonyl on carbonyl of acetyl on KS; makes the process of the condensation of acyl groups exergonic

62. 62

63. 63 After the condensation, the four carbon ketoacyl – acetoacetyl – would be carried by ACP ACP takes acetoacetyl to domain 2 where reduction, dehydration and saturation take place (step 6 on the figure) ACP gives the product to KS and is once again ready to accept a malonyl group from MT Steps 4-6 are repeated until the 16 carbon acyl, palmitate is on KS; KS then gives palmitate to ACP Palmitate is released from KS by thioesterase and synthesis on FAS stops The acetyl group added to AT in the beginning becomes the ω- carbon of palmitate o FAS synthesizes two fatty acids simultaneously

64. 64 β -ketoacyl reductase (KR) dehydratase (DH) enoyl-CoA reductase (ER)

65. 65 The balance sheet of fatty acid synthesis The activation of acetyl-CoA: 7 acetyl-CoA + 7 CO 2 + 7 ATP → 7 malonyl-CoA + 7 ADP + 7P i Seven cycles of FAS reactions: acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H + → palmitate + 7 CO2 + 14 NADP + + 8 CoA + 6 H 2 O *6 H 2 O because one water molecule is consumed by thioesterase Overall reaction: 8 acetyl-CoA + 7 ATP + 14 NADPH + 14 H + → palmitate + 8 CoA + 7 ADP + P i + 14 NADP + + 6 H 2 O Assuming all of the cytosolic malate produced by citrate lyase is changed to pyruvate, 8 NADPH would be formed for 8 cytosolic acetyl-CoA molecules The remaining 6 NADPH molecules would come from the pentose phosphate pathway

66. 66 Elongation of fatty acids Shorter chain fatty acids can be synthesized by terminating the reaction before reaching the level of palmitate The synthesis of longer chain fatty acids takes place on the smooth endoplasmic reticulum and in the mitochondria First, palmitate is activated by CoA In the SER, malonyl-CoA attacks the carboxyl end of palmitoyl-CoA  decarboxylation, reduction and dehydration take place in the same way as the corresponding FAS reactions In the mitochondria, acetyl-CoA molecules are successively added  the reactions are nearly the reverse of β- oxidation except that NADPH is used instead of FADH 2 for the reduction of the double bond

67. Mitochondrial elongation of fatty acids 67

68. 68 The main elongation reaction in the body is the synthesis of stearate,18:0 Desaturation of fatty acids The most common unsaturated fatty acids in the body are palmitoleate,16:1 ( △ 9 ) and oleate, 18:1 ( △ 9 ) Desaturation takes place in the SER O 2, NADH, cytochrome b 5 and the flavoprotein cytochrome b 5 reductase are involved The reaction is catalyzed by fatty acyl-CoA desaturase The fatty acyl-CoA and NADH/H + donate two electrons and two protons each – the synthesis of two water molecules  NADH gives its electrons to cytochrome b 5 reductase  cytochrome b 5 reductase passes the electrons to cytochrome b 5  fatty acyl-CoA desaturase accepts electrons from cyt. b 5

69. 69 Fatty acyl-CoA desaturase donates the electrons from cytochrome b 5 and fatty acyl-CoA to oxygen  a cis double bond formed; water released o humans get trans fatty acids from butter, milk and partially- hydrogenated vegetable oils - margarine excessive consumption of trans fatty acids is a risk factor for heart disease

70. 70 The synthesis of polyunsaturated fatty acids in addition to the △ 9 position, hepatocytes and adipocytes can introduce double bonds at the △ 4, △ 5 and △ 6 positions △ 9 unsaturated fatty acids can be elongated to give fatty acids with double bonds at positions beyond △ 9 Fatty acids with double bonds three (ω-3) or six (ω-6) carbons away from the methyl end are required for the synthesis of important molecules known as eicosanoids : “eicosa” twenty Humans cannot synthesize these fatty acids de novo (even with the combination of desaturation and elongation) Precursors of ω-3 and ω-6 fatty acids must be provided in the diet and these precursors are known as essential fatty acids  linoleate,18:2 ( △ 9,12 ) - an ω-6 fatty acid  α -linolenate,18:3 ( △ 9,12,15 ) - an ω-3 fatty acid

71. 71 Linoleate and α –linolenate are mainly obtained from plant oils In the body essential fatty acids are elongated and unsaturated further:  Linoleate is changed first to dihomo γ– linolenate (DGLA; eicosatrienoate), 20:3 ( △ 8, 11,14 ) and then to arachidonate (eicosatetraenoate), 20:4 ( △ 5,8, 11,14 )  α–linolenate is changed to eicosapentaenoate (EPA), 20:5 ( △ 5,8, 11,14,17 )  Fish oils contain ω-3 and ω-6 fatty acids e.g. EPA and docosahexaenoate (DHA), 22:6 ( △ 4,7,10,13,16,19 )  DHA is found in high concentrations in the retina, cerebral cortex, testis and sperm; provided by the placenta and milk

72. 72 Eicosanoid synthesis Eicosatri/tetra/pentaenoate are utilized in the synthesis of different kinds of eicosanoids – series 1, 2 and 3 eicosanoids resp.  the series number depends on the number of double bonds found outside of the ring structure of the eicosanoids Arachidonate obtained in the diet or synthesized in the body is found esterified in phospholipids of the membranes The activation of phospholipase A 2 or C (by cytokines and histamine) lyses arachidonate from phospholipids Arachidonate is then used for the synthesis of eicosanoids Properties of eicosanoids:  Local hormones that act in an autocrine or paracrine manner  Exert their effects at very low concentrations  They are not stored; rapidly inactivated

73. 73 There are three major ways of synthesis of eicosanoids: 1.The cyclooxygenase pathway Produces prostaglandins and thromboxanes Prostaglandin endoperoxide synthase acts on arachidonate to give PGH 2 which is the precursor for the remaining prostaglandins and thromboxanes

74. 74

75. 75 Prostaglandin endoperoxide synthase is a membrane-bound enzyme that has cyclooxygenase and gluthathione-dependent hydroperoxidase activities Letters (E,H,…) are assigned based on ring substituents The numbers refer to the series of eicosanoids Thromboxanes are structurally similar to prostaglandins except that they have a six-membered ring containing oxygen The functions of prostaglandins and thromboxanes:  PGD 2, PGE 2, PGI 2 (prostacyclin) – induce vasodilation and inhibit platelet aggregation and the action of immune cells  PGF 2 α - constriction of bronchi and blood vessels; smooth muscle contraction  Thromboxane A 2 – favors vasoconstriction, platelet aggregation and immune action

76. 76 thromboxane A 2 and prostacyclin have antagonistic effects The effects of prostaglandins on smooth muscles could have therapeutic uses: induction of labor, control of blood pressure, … The prostaglandins and thromboxanes synthesized from EPA (an ω-3 fatty acid ) inhibit the release of arachidonate from membranes and also the synthesis of series-1 prostaglandins and thromboxanes  members of series-1 are more inflammatory → inflammation is potentially reduced  PGI 3 is as potent anti-platelet aggregator as PGI 2 but TXA 3 is a weaker aggregator than TXA 2 → longer clotting time o Part of the reason why ω-3 fatty acids have been receiving much attention

77. 77 Prostaglandin and thromboxane synthesis can be inhibited at two levels:  phospholipase A 2 – inhibited by steroidal anti- inflammatory drugs – arachidonate cannot be released  cyclooxygenase – the site of action of aspirin and other non-steroidal anti-inflammatory drugs COX has two main isozymes:  COX 1 - is constitutive and the thromboxane A 2 and prostacyclin it synthesizes are involved in hemostasis; prostacyclin is also involved in the reduction of proton secretion and increase of bicarbonate secretion in the stomach  COX 2 - is thought to be induced in response to inflammatory stimuli

78. 78 Aspirin inhibits both isozymes – anti-inflammation; prevention of blood clotting and, gastric ulcer New drugs were developed that could selectively inhibit COX 2 thereby preventing the adverse effects on the stomach  celebrex and vioxx But COX 2 can be constitutive in the kidneys where PGE 2 products lead to the dilation of arterioles and the excretion of sodium and water Vioxx was banned due to cases of hypertension associated with its use  aspirin acetylates a serine residue in the active site of COX and ibuprofen blocks the hydrophobic channel by which arachidonate enters the COX active site  the anti-clotting effect of aspirin is prominent because platelets lack a nucleus and do not make new COX (and TX A 2 ) while blood vessels can resynthesize COX (and PGI 2 )

79. 79 2.The lipooxygenase pathway 5-, 12- and 15-lipoxygenases synthesize three types of hydroperoxyeicosatetraenoate (HPETE) Different types of leukotrienes, lipoxins and HETE (hydroxyeicosatetraenoate) are synthesized from the HPETE These are molecules without ring structures; leukotrienes and lipoxins have three conjugated double bonds Glutathione is used in the synthesis of LTC 4 from LTA 4 Slow reacting substance of anaphylaxis (SRS-A) is a mixture of LTC 4, LTD 4 and LTE 4 - causes bronchoconstriction SRS-A, along with LTB 4, increase vascular permeability and the secretion of chemical mediators of immunity LXA 4 and LXB 4 are anti-inflammatory 3.The cytochrome P 450 pathway – monooxygenases act on arachidonate to give HETEs, diHETEs and epoxides

80. 80

81. THE BIOSYNTHESIS OF COMPLEX LIPIDS Complex lipids are used for energy storage, membrane building and the synthesis of important biological molecules fatty acids in the body are mainly found covalently bound to a backbone structure Other groups can also be found attached to the backbone structure The main classes of complex lipids are:  glycerolipids –backbone is glycerol  sphingolipids –backbone is sphingosine  derivatives of cholesterol–backbone is cholesterol Phospholipids are a class consisting of members of both glycerolipids and sphingolipids Most of the reactions involved in lipid biosynthesis are carried out in the smooth endoplasmic reticulum 81

82. 82

83. Glycerolipids Three classes; triacylglycerols, glycerophospholipids and ether glycerolipids The first two classes are synthesized from a common intermediate – phosphatidate  glycerol-3-phosphate from glycerol (in the liver) or from dihydroxyacetone phosphate (in the liver and adipose) undergoes successive esterifications by two fatty acyl CoA molecules  this is a different mechanism from the one used in the intestine to resynthesize dietary triacylglycerols  in the case of triacylglycerols, the phosphate group is removed from phosphatidate to give diacylglycerol (DAG); the third fatty acid is then added 83

84. 84 The synthesis of TAG One option for the synthesis of glycerophospholipids is changing phosphatidate to DAG and then esterifying it with an activated alcohol (head group) Or the phosphatidate can first be activated and then the alcohol group could be added Activation is through reaction with cytidine triphosphate (CTP) The main alcohol components of glycerophospolipids include choline, ethanolamine, serine, inositol and glycerol-3- phosphate

85. 85

86. 86 The linkage formed by the phosphate group with the glycerol backbone and the head group is known as a phosphodiester bond Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are produced using the first strategy Phosphatidylserine (PS) is synthesized by exchanging serine for the ethanolamine in PE PS can be decarboxylated to PE PE can be methylated to give PC o the methyl donor is S-adenosyl methionine (SAM) Phosphatidycholine is the dominant membrane lipid; most of the choline is obtained from the diet Dipalmitoylphosphatidylcholine is the major component of lung surfactant

87. 87

88. 88 Lipids synthesized using the second strategy include phosphatidylinositol (PI), phosphatidylglycerol (PG) and cardiolipin The addition of glycerol-3-phosphate to CDP-DAG gives PG PG can then be added to CDP-DAG to give cardiolipin Cardiolipin is abundant in the inner mitochondrial membrane PI can be phosphorylated to give phosphatidylinositiol 4,5- bisphosphate (PIP 2 ) o PIP 2 breaks down to the important signaling molecules inositol-1,4,5- triphosphate (IP 3 ) and DAG phosphatidyl inositol is unique among phospholipids in having a nearly fixed composition of fatty acids: stearic acid at C-1 and arachidonic acid at C-2

89. 89

90. 90 The synthesis of ether glycerolipids DHAP is used as a backbone without being changed to glycerol-3-phosphate C-1 of DHAP is esterified with a fatty acid and then the fatty acid is exchanged for a fatty alcohol which will be attached with DHAP by an ether linkage  the alcohol is synthesized by reducing the corresponding acyl-CoA with the help of NADPH The keto group at C-2 is then reduced and acylated Head groups are added to the 1-alkyl-2-acylglycero-3- phosphate in a manner similar to that of PC and PE Desaturation of the akyl group produces plasmalogens  ethanolamine plasmalogen is found in the myelin and choline plasmalogen in cardiac muscle

91. 91

92. 92 Platelet-activating factor (PAF) has a choline head group on C-3, acetyl on C-2 and a saturated (alkyl) group on C-1  The very short chain at C-2 makes PAF more water soluble than other glycerolipids  PAF is released by phagocytic cells and causes platelet aggregation, edema and hypotension  PAF synthesis takes place in the peroxisomes;

93. 93 Degradation of glycerophospholipids Phospholipases located on the cell membrane or in lysosomes carry out the degradation Phospholipase A 1 removes fatty acid from C-1 and A 2 from C-2; the fatty acid at C-2 is usually arachidonate Phospholipasse C cleaves the phosphate-glycerol bond; it is activated by hormones and is responsible for the synthesis of IP 3 and DAG from PIP 2 Phospholipase D separates the head group from phosphate

94. 94 Sphingolipids The precursors for the synthesis of the backbone are serine and palmitoyl-CoA The final product is ceramide, which is the derivative of the amino alcohol sphingosine Various head groups can be added to ceramide to give two types of lipids:  Phospholipids – sphingomyelin is synthesized by the transfer of choline from PC to ceramide Sphingomyelin is a major component of the myelin sheath  Glycolipids – a sugar molecule or a chain of sugar molecules is attached to the hydroxyl at C-1 of ceramide Cerebrosides – a single sugar (usually glucose or galactose) is added from a UDP-activated form

95. 95

96. 96 Galactocerebrosides are characteristic of the cell membranes of neural tissue while glucocerebrosides are found in non-neural tissue 3’-phosphoadenosine - 5’-phosphosulfate (PAPS) donates sulfate to galactocerebroside to form sulfatides Globosides– are formed when additional sugars (including amino sugars) are added to ceramides Gangliosides – globosides that contain a sialic acid N- acetylneuraminic acid (NANA) in addition to sugars; acidic NANA N-acetyl-galactosamine (GalNac)

97. 97

98. 98 Gangliosides on cell surfaces may play an important role as sites of recognition  the oligosaccharide moieties of glycoproteins and glycolipids are informational molecules  lectins are proteins that recognize specific oligosaccharides on cell surface and facilitate communication and adhesion between cells e.g. bacterial and viral infection, half- life of erythrocytes and hormones, … the ABO blood group antigens are determined by the variation of the sugar residues present in glycolipids/glycoproteins on the surface of RBC and endothelial cells

99. 99

100. 100 Sphingolipid degradation and sphingolipidoses Lysosomal enzymes remove members of the head group one by one in order to finally give ceramide Deficiency in any one of the enzymes would lead to the accumulation of partial breakdown products in the tissues leading to serious disease mainly affecting neural tissue Some examples:  Tay-Sachs disease – a common deficiency in hexos- aminidase which removes GalNac; retardation in development, paralysis, blindness and death before age 4  Niemann-Pick disease – a rare deficiency in sphingomyelinase which cleaves phosphocholine; mental retardation and early death  Gaucher disease – deficiency in glucosidase; accumulation of glucosyl ceramide in the liver and spleen

101. 101

102. 102 Cholesterol metabolism Cholesterol is as essential component of cell membranes and it serves as a precursor for bile salts, steroid hormones and vitamin D Cholesterol is synthesized in all cells of the body but the most active tissues are the liver, adrenal cortex, the intestine and the gonads The entire cholesterol structure is derived from acetate In the first stage of cholesterol synthesis, three molecules of acetyl-CoA are changed in the SER to the six carbon molecule mevalonate The HMG-CoA synthase used here is a cytosolic isozyme the conversion of HMG-CoA to mevalonate by HMG-CoA reductase is the rate –limiting and committed step of the whole process of cholesterol synthesis

103. 103 HMG-CoA reductase is regulated in 3 ways: 1.Phosphorylation by cAMP- dependent kinases – inactivates the enzyme 2.Degradation – HMG-CoA reductase has a half-life of 3 hrs. which could be shortened further if cholesterol levels are high 3. Repression – high levels of cholesterol reduce the synthesis of the mRNA for the enzyme Stage 1

104. 104 The regulation of HMG-CoA reductase

105. 105 In the second stage of cholesterol synthesis, mevalonate will be changed to two types of activated 5-carbon intermediates (isoprenes): isopentenyl pyrophosphate and dimethylallyl pyrophosphate In the third stage, the 30 carbon molecule squalene is produced from 6 units of activated isoprene In the fourth and final stage, squalene is changed to lanosterol, a molecule with the characteristic steroid ring  Lanosterol will be changed to cholesterol after 20 more reactions Some of the cholesterol synthesized in the liver is used for the synthesis of hepatocyte membranes Most of the cholesterol is secreted in the form of biliary cholesterol, cholesterol esters or bile salts  cholesterol esters are synthesized by acyl-CoA-acyl cholesterol transferase (ACAT)

106. 106 Stage 2

107. 107 Stage 3

108. 108 Stage 4  4 rings (steroid nucleus)  one tail  27 carbons  a hydroxyl group  NADPH & ATP needed

109. 109 Lipoproteins and the transport of cholesterol (and other lipids) The liver exports the cholesterol, cholesterol esters and TAG that it has synthesized in the form of very low density lipoproteins (VLDL )  phospholipids and apoproteins are present in VLDL, just like the case of chylomicrons The main difference with regards to apoproteins is that VLDL assembly begins with Apo B-100 – a non-truncated form unlike Apo B-48 of chylomicrons Apo C II and E play the same role that they had in chylomicrons  The TAG content of VLDL is depleted by the tissues of the adipose, muscle and (lactating) mammary gland  About half of the VLDL remnants known as intermediate density lipoproteins (IDL) are taken up by the liver

110. 110 The remaining half of the IDL undergo further TAG depletion, gain cholesterol esters and become low density lipoproteins (LDL) LDL is endocytosed based on recognition of apo B-100  80 % of the LDL are internalized by the liver  20 % of the LDL travel to tissues that need more cholesterol than they can synthesize e.g. the adrenal cortex, gonads, tissues undergoing repair  if more LDL are present in the blood than that can be taken up by the liver, adrenal cortex and gonads, non- specific uptake by macrophages near endothelial cells would increase  this would lead to atherosclerosis

111. 111 The secretion of VLDL

112. 112 The fate of VLDL

113. 113 The lipoproteins with the highest content of proteins is called high density lipoproteins (HDL) HDL are synthesized as nascent HDL by the liver and the intestine  disk-shaped with a layer of phospholipids, cholesterol and many apoproteins; the interior contains little TAG and cholesterol esters HDL can also be assembled on apoproteins (especially A I) that bud-off or are shed by other lipoproteins Mature HDL (HDL 3 ) are formed when nascent HDL acquires cholesterol and phospholipids from tissues (especially endothelial cells); HDL become globular The transport of cholesterol to HDL is facilitated by ATP- biniding cassette protein 1 (ABC 1)  ABC 1 brings cholesterol to the outer leaflet of the lipid bilayer

114. 114 HDL take cholesterol from the surface; but cholesterol could still move back to the cell This is avoided through the action of lecithin-acyl cholesterol transferase (LCAT) which HDL acquire from the circulation  LCAT transfers a fatty acid from lecithin to cholesterol trapping it inside HDL in the form of cholesterol ester HDL can lose their cholesterol ester in two major ways: 1.Scavenger receptor (SR-B1) –mediated transfer to the liver and steroidogenic tissues  The transfer to the liver is known as reverse cholesterol transport  The process is not endocytosis – cholesterol esters are delivered to the tissues and what is left of the HDL reenters the circulation and keeps on picking up more cholesterol

115. 115 HDL metabolism

116. 116 2.Exchange of cholesterol ester for TAG with IDL  The exchange is carried out through cholesterol ester transfer protein (CETP)  HDL depleted of cholesterol ester and rich in TAG become HDL 2 ; HDL 2 can be endocytosed by the liver The exchange of cholesterol esters and TAG

117. 117 The relative diameters of lipoproteins

118. 118 Receptor-mediated endocytosis of lipoproteins The internalization of LDL is a representative example LDL receptors are found on specialized areas of the cell membrane that are called coated pits (because they are covered with the protein clathrin) The receptor binds with apo B-100 The pits are changed to vesicles that contain the LDL and enter the cell; the vesicles are known as early endosomes After the clathrin and LDL receptors are returned back to the membrane late endosomes are formed Lysosomes that contain hydrolases are released by the Golgi complex and fuse with the late endosomes The main products of the actions of the enzymes are cholesterol, fatty acids and amino acids (from apo B-100) Cholesterol is quickly reesterified (with fatty acids different from the original ones) by ACAT

119. 119

120. 120 Excess levels of cholesterol in the cell inhibit the transcription of LDL receptor mRNA and activate the action of ACAT  excess levels of cholesterol in the cell are reflections of both synthesis and delivery by LDL The non-specific scavenger receptors (SR-A1/A2) on macrophages can bind different types of molecules These receptors are non-saturable, i.e, they will continue engulfing LDL even when the levels of cholesterol in the macrophages are high what is the link between LDL, HDL and atherosclerosis? Atherosclerosis is a constriction and blockage of arteries of the brain, heart and other organs by atheromas The initial step in the formation of atheromas is physical damage to the endothelium caused by different risk factors  hypertension, free radicals from cigarette smoking, elevated LDL- especially modified LDL, etc

121. 121 Modification of LDL LDL Apo B-100 Derivatization: Aldehydes Glucosylation eg. diabetes Oxidation: Degradation of B-100 by reactive oxygen species Derivatized LDL Oxidized LDL Increased residence time of LDL in the plasma may increase the chance for modification The polyunsaturated fatty acids inside LDL are oxidized

122. 122 The damage to the vessel elicits an inflammatory response  adhesiveness and permeability of the endothelium for plateletes and leukocytes increases The monocytes cross the endothelium and mature into macrophages inside the subintimal space The scavenger receptors on macrophages take in modified LDL and become foam cells The macrophages secrete reactive oxygen species and this leads to the production of more oxidized LDL and cytokines secreted by vascular cells in response to oxidized LDL attract monocytes to the subintimal space and induce the proliferation of smooth muscle cells Through time a fatty streak of necrotic tissue and free lipid surrounded by a fibrous cap of smooth muscle cells would form

123. 123 The fatty streak enlarges over time, deforms the overlying endothelium and protrudes into the lumen, blocking blood flow the eventual thinning of the fibrous cap leads to the rupture of the streak  the release of the contents of the plaque into the blood stream would lead to thrombosis; the clot would worsen the occlusion of the vessels HDL may be protective by picking up accumulating cholesterol from the endothelium before the advanced lesion forms HDL also has an enzyme that degrades oxLDL  LDL - “bad cholesterol”; HDL - “good cholesterol”  the LDL:HDL ratio is thought to be a good measure of the probability of developing atherosclerosis

124. 124

125. 125 Genetic defects in lipoprotein metabolism Rare disorders  Abetalipoproteinemia - inability to associate apo B with lipids; chylomicrons, VLDL and LDL cannot be synthesized accumulation of triacyglycerols in the liver; malabsorption of lipids and lipid soluble vitamins in the intestine  Tangier disease - cholesterol cannot be transported from cells to HDL due to deficiency of ABC-1 transporters cholesterol-poor HDL is rapidly removed from the circulation and degraded Familial hypercholesterolemia (FH) - is a common condition characterized by the deficiency or non-functionality of LDL receptors LDL cholesterol levels in the blood increase significantly

126. 126 Heterozygotes have around half the activity of LDL receptors while homozygotes have nearly no LDL receptor activity Homozygotes usually die of coronary artery disease in childhood In addition to the blood vessels, FH leads to the deposition of lipids in the skin, eye and tendons – xanthomas Because LDL cholesterol is unable to adequately enter into cells, endogenous cholesterol synthesis will continue uninhibited The likely treatment for homozygotes is a liver transplant The situation in heterozygotes can be improved by stimulating cells to produce additional LDL receptors; this can be achieved through different ways:  statins – competitively inhibit HMG-CoA reductase; the decrease in intracellular cholesterol levels stimulates the synthesis of LDL receptors

127. 127  Resins – bind and facilitate the excretion of bile salts as opposed to recirculation; the liver has to use more of the intracellular cholesterol pool for bile salt synthesis; LDL receptor synthesis activated  Ezetimibe - blocks the intestinal absorption of cholesterol; less cholesterol is sent to the liver through chylomicrons fibers may also serve the same function Dietary and life style changes may supplement and/or even precede pharmacotherapy in the reduction of blood cholesterol  fibers – remove bile acids through the feces  nicotinic acid – moderates the release of fatty acids from the adipose (which then can travel to the liver and be secreted in VLDL); vitamin E prevents oxidation of LDL  reduce the consumption of saturated fatty acids, sucrose and fructose  regular exercise, controlling weight, …

128. 128 The synthesis of important derivatives of cholesterol BILE ACIDS Present mainly as Na + and K + salts The strategy of synthesis is to add more hydroxyl groups in order to increase solubility and conjugation to increase ionization (and solubility) In the liver cholesterol is first changed to 7 α- hydroxy- cholesterol by a mixed-function oxidase ; rate limiting step After the introduction of additional hydroxyl groups, reduction of the ring double bond and shortening of the side chain, two types of primary bile acids are produced – cholate and chenocholate The carboxyl group of cholate and chenocolate are conjugated with glycine or taurine Intestinal bacterial remove glycine, taurine and the hydroxyl at C-7 producing secondary bile acids - solubility decreases

129. 129 The portion of the primary bile acids that has not been modified by the bacteria along with the relatively soluble type 2 0 bile acid- deoxycholic acid -are reabsorbed into the liver The least soluble of the secondary bile acids - lithocholic acid - which has a single hydroxyl group is mainly excreted Coprostanol, which is produced by bacterial hydrogenation of biliary cholesterol secreted into the intestine, is also excreted

130. 130

131. 131

132. 132

133. 133 STEROID HORMONES Five types of hormones are produced by the oxidation and the removal of the side chain of cholesterol: glucocorticoids, mineralcorticoids, androgens, estrogens and progestins Cytochrome P-450 enzymes that require NADPH are characteristic of the synthesis pathways The synthesis of steroid hormones begins with the formation of pregnenolone through the action of desmolase – a mitochondrial enzyme Desmolase removes 6 carbons from cholesterol (27 C) Pregnenolone moves from the mitochondria to the SER where oxidation of hydroxyl and change in position of the double bond yields progesterone (21 C) The other steroid hormones are all derived from progesterone Glucocorticoids have 21 C, androgens 19 C and estrogens 18 C

134. 134

135. 135

136. 136 VITAMIN D Cholecalciferol (vitamin D 3 ) is synthesized from 7- dehydrocholesterol present in the skin

137. 137 Cholecalciferol is then changed to the active form of vitamin D - 1,25–dihydrocholecalciferol (calcitriol)

138. 138 o isopentenyl pyrophosphate (an intermediate in cholesterol biosynthesis) is a precursor to many important molecules known collectively as isoprenoids