this chapter was a very long look at fatty acid and ketone synthesis and metabolism, and also looked at their preferential usage as fuels during the fasting state.
intro
the first section was an introduction of sorts to fatty acids. fatty acids are generally long, alipathic carbon chains with a carboxylic acid at one end. in the body, they are mostly stored as triacylglycerols in adipose tissue- recall the formation of triacylglycerols from dietary lipids from chapter 32. fatty acids are differentiated by the properties of the carbon chain- specifically, chain length and degree of saturation. fatty acids with 2-4 carbons are considered small, 6-10 considered medium, 12-26 considered large, and beyond that, "very large". the fatty acids from animal products are generally unsaturated, meaning the carbon chain is made entirely of single bonds, or monounsaturated, meaning the carbon chain contains one double bond. the most common fatty acids from animal sources are thus palmitate (C16- "16 carbons"), stearate (C18), and oleate (C18:1- "18 carbons, one double bond"). plant sources have longer chain fatty acids, branched fatty acids, and polyunsaturated fatty acids.
transport and activation
when fatty acids need to be used as fuels, they are released from triacylglycerol stores in adipose tissue and transported to tissues for oxidation. longer chain fatty acids are hydrophobic (because carbon chains are non polar), and thus they need to be transported bound to the hydrophobic pocket of serum albumin, which allows it to be transported in the blood and also traverse through cellular membranes without disrupting membrane proteins. when fatty acids reach the cellular membranes, they are taken in either by diffusion or facilitated transport, and head to the mitochondria. before entering the mitochondrial matrix, they are "activated" by attachment of a CoA unit. this takes place via an enzyme on the outer mitochondrial membrane, fatty acyl synthetase. this reaction involves using ATP to form a high energy fatty acyl-AMP intermediate, which then exchanges the AMP for a CoA, forming a fatty acyl CoA.
this fatty acyl CoA then diffuses into the intermembrane space. in order to traverse through the inner mitochondrial membrane, they are activated by an enzyme on the outer membrane called CPT1, which exchanges the fatty acyl CoA's CoA for a carnitine molecule. this fatty acyl carnitine is then translocated through the inner mitochondrial membrane into the matrix, and another enzyme on the inner membrane, CPT2, transfers the carnitine back for a CoA, reforming the fatty acyl CoA. the activated fatty acid can now undergo beta oxidation for energy production.
normal long chain ß-oxidation
in the mitochondrial matrix, activated fatty acids are successively cleaved in a "ß-oxidation spirals", producing 2 carbon fragments of acetyl CoA and 1 molecule of NADH and FADH2 each per spiral. the ß oxidation involves 4 steps:
1. formation of a double bond between alpha and beta carbons: oxidation of fatty acyl CoA to trans-∆2 fatty enoyl CoA via fatty acyl CoA dehydrogenase. one molecule of FADH2 is formed.
2. addition of a water to the beta carbon, forming an alcohol: trans-∆2 enoyl CoA is converted to L-ß-hydroxy acyl CoA via enoyl hydratase.
3. oxidation of the alcohol into a ketone: L-ß hydroxy acyl CoA oxidized to ß-keto acyl CoA via ß-keto dehydrogenase. NADH is formed.
4. cleavage of acetyl CoA and reformation of activated fatty acid: ß-keto acyl CoA is cleaved into acetyl CoA and fatty acyl CoA is reformed.
this spiral is repeated until the fatty acid chain is completely split into molecules of acetyl CoA, each time producing one molecule of FADH2 and one molecule of NADH. thus, a normal long chain 16 carbon fatty acid molecule (palmitoyl CoA), cleaved 7 times, produces 8 molecules of acetyl CoA, 7 molecules of FADH2, and 7 molecules of NADH. thus ß-oxidation itself yields 28 molecules of ATP just from the NADH and FADH2 it produces, not including the potential energy output from acetyl CoA.
ß oxidation of unusual fatty acids
unsaturated fatty acids have one or more double bonds that often need to be modified in order to undergo ß oxidation. the double bonds in unsaturated fatty acids are generally in the cis formation, and ß oxidation requires them to be in the trans-formation. thus double bonds in unsaturated fatty acids are either isomerized to the trans configuration or simply reduced to single bonds during ß oxidation. odd chain length fatty acids are oxidized in a similar way to even chain length fatty acids, until the last cleavage of a 5 carbon chain produces a 2 carbon acetyl CoA and a 3 carbon propionyl CoA. propionyl CoA can then be converted to succinyl CoA and used an intermediate in the TCA cycle, or even converted to malate and used in the gluconeogenic pathway. medium chain length fatty acids are hydrophilic enough that they can go directly from the intestines through the blood to the liver, where they go directly to the mitochondrial matrix of hepatic cells and are activated and ß oxidized by mitochondrial enzymes.
branched chain fatty acids, formed mainly from degradation of chlorophyl from plant sources, require a special oxidation procedure because the branching interferes with normal ß oxidation enzymatic activity. in this procedure the alpha carbon is hydroxylated and then oxidized, which releases the original carboxylic acid as CO2 and forms a new carboxylic acid on the alpha carbon. this has the net effect of shortening the fatty acid chain by one carbon, and the branch point no longer interferes with normal beta oxidation.
very long chain fatty acids (VLCFA), with chains longer than 26 carbons long, need to be shortened first in the peroxisomes, which are organelles in which multiple reactions occur which produce H2O2. the VLCFA's are activated and oxidized/shortened by a set of enzymes that is basically analogous to mitochondrial oxidation of regular long chain fatty acids, with a few exceptions. first, activated VLCFA's do not need carnitine to be transported into the peroxisome. secondly, whereas the first step of ß oxidation in the mitochondria produces FADH2, the first step in the peroxisomes produces H2O2. thus the net production from one spiral of oxidation in the peroxisome is just acetyl CoA and NADH. when the VLCFA's are shortened to short to medium length fatty acids, they are activated to carnitine intermediates by peroxisomal enzymes and transported to the mitochondria for further ß oxidation, along with the NADH that was produced.
fatty acid oxidation regulation
the regulation of fatty acid oxidation occurs in several different avenues. first of all, it is dependent on the relative energy usage and thus the ATP/AMP and NADH/NAD+ ratios. if energy demands are low, then the products of fatty acid oxidation, acetyl CoA, NADH, and FADH2, will build up and will not be oxidized further in the TCA cycle and electron transport chain, ultimately inhibiting fatty acid oxidation. secondly, fatty acid oxidation can be regulated by the amount of CoA present in the mitochondrial matrix, which is required to reform the fatty acyl CoA from fatty acyl carnitine in preparation for ß oxidation. thirdly, fatty acid oxidation is regulated by the CPT1 enzyme, which converts fatty acyl CoA into fatty acyl carnitine so it can be moved into the matrix for ß oxidation. CPT1 is inhibited by the compound malonyl CoA, which is formed from acetyl CoA via the enzyme acetyl CoA carboxylase.
acetyl CoA carboxylase is further regulated by two things: the relative glucagon/insulin ratio and ATP/AMP ratio. if insulin levels are high, indicating the fed, high glucose level state, acetyl carboxylase is activated, which produces malonyl CoA, which inhibits CPT1, ultimately inhibiting fatty acid oxidation. if energy demand is high, AMP levels are high, and this activates a protein kinase which inhibits acetyl CoA carboxylase- this prevents the formation of malonyl CoA, which allows fatty acid oxidation to occur. in other words: when blood sugar is high, fatty acids are not oxidized. during high energy usage, fatty acids are oxidized.
ketone body synthesis and oxidation
ketone bodies, mainly the compounds acetoacetate and ß-hydroxybutyrate, are an alternative source of fuel which are produced during the fasting state from acetyl CoA molecules in the liver when the energy demands of the liver have been met by ß-oxidation of fatty acids. the synthesis pathway is as follows:
1. two molecules of acetyl CoA are combined to form acetoacetyl CoA.
2. the enzyme HMG CoA synthase then adds another CoA to acetoacetyl CoA, forming HMG CoA.
3. HMG CoA is cleaved into acetoacetate and acetyl CoA.
4. acetoacetate can either by spontaneously decarboxylated to form acetone and CO2, or can be reduced by ß-hydroxybutyrate dehydrogenase to form ß-hydroxybutyrate.
during ketone oxidation, ß-hydroxybutyrate is oxidized back to acetoacetate by the same enzyme that catalyzed the synthesis reaction, ß-hydroxybutyrate. acetoacetate is then activated with a CoA from succinyl CoA and cleaved into 2 acetyl CoA molecules, which can then be oxidized.
fuel homeostasis
the last section looked at when fatty acids and ketones are used preferentially as fuels. fatty acids in the blood are generally released 3-4 hours after a meal, and continue to increase in concentration up until 2-3 days of fasting. after 2-3 days of fasting, ketone bodies become used predominantly, especially in the brain, where it can contribute up to 2/3 of the brain's energy needs. ketone body synthesis in the liver only when there is sufficient buildup of acetyl coA, which occurs when the energy requirements of the liver have been met by fatty acid oxidation.
when fatty acids and ketones are in the blood, they are used in preference to glucose. this regulation occurs at the enzymes that regulate the glycolytic processes, PFK-1 and pyruvate dehydrogenase. PFK-1 is the major regulatory enzyme from glycolysis, and is inhibited by ATP and citrate, both of which are produced by fatty acid oxidation (recall that citrate is an intermediate of the TCA cycle). pyruvate dehydrogenase is the enzyme that oxidizes the pyruvate produced from glycolysis into acetyl CoA. it is inhibited by NADH and acetyl CoA, both byproducts of fatty acid oxidation as well. thus, fatty acid oxidation inhibits the glycolytic pathway.
questions
review of fatty acids...
1. the fatty acids oxidized as fuels are principally...
2. where do the adipose stores of triacylglycerol come from?
3. what percentage of the american diet does fat constitute?
4. what are the most common dietary fatty acids?
5. what is the difference between the dietary fatty acids found in animals vs. vegetables?
6. medium chain length fatty acids are present principally in...
7. how are short/medim/long chain fatty acids classified?
8. describe fatty acid synthesis in the liver.
transport and activation of fatty acids...
9. why are fatty acids bound to proteins when they are transported in the blood and cells?
10. describe the release of long chain fatty acids from adipose tissue.
11. describe the two ways in which fatty acids pass through a cell's plasma membrane.
12. what happens after the fatty acid enters the cell?
13. describe the activation of long chain fatty acids.
14. what is the enzyme that catalyzes the activation of long chain fatty acids? where is it found in the cell?
15. where are the enzymes for the activation of very long, and medium chain length fatty acids found?
16. what are the different metabolic pathways that are represented by the three locations in which long chain fatty acid synthases are located?
17. what happens to fatty acids that are not being used for energy generation?
18. describe the transport of activated long chain fatty acyl groups into the mitochondria.
19. where does carnitine come from and where is it stored?
ß-oxidation of long chain fatty acids...
20. what are the products of beta-oxidation of long chain fatty acids?
21. describe the four steps of beta oxidation of long chain fatty acids.
22. describe the transfer of electrons from FADH2 created from ß oxidation of fatty acid to the electron transport chain.
23. what is the energy yield from oxidation of one molecule of palmitoyl CoA?
24. describe how fatty acid chain length is related to the enzymes involved in beta oxidation of fatty acids.
unsaturated, odd chain length, and medium chain length fatty acid oxidation...
25. describe the most common unsaturated fatty acids.
26. what must happen to unsaturated fatty acids in order for them to undergo ß oxidation?
27. how is an odd chain length fatty acid oxidized?
28. what cofactor is required for odd chain length fatty acid oxidation?
29. compare the fate of the products of odd chain length fatty acid oxidation with even chain length fatty acid oxidation.
30. compare the oxidation of long chain length fatty acids with medium chain length fatty acids.
regulation of fatty acid oxidation
31. how is fatty acid oxidation regulated by the cell's energy requirements?
32. describe the regulation of fatty acid oxidation via CoASH.
33. describe the regulation of fatty acid oxidation via CPT1.
34. describe how the enzyme acetyl CoA carboxylase is regulated.
oxidation of unusual fatty acids
35. where do branched chain fatty acids come from?
36. where do very long chain fatty acids come from?
37. describe the transport of very long chain fatty acids into the peroxisome.
38. how does the first step of peroxisomal fatty acid oxidation differ from that of mitochondrial ß-oxidation?
39. describe the ultimate fate of very long chain fatty acyl CoA's in the peroxisome.
40. what occurs in peroxisomes that is related to VLCFA catabolism?
41. what are the two most common branched chain fatty acids?
42. describe the oxidation of branched chain fatty acids.
43. describe w-oxidation of fatty acids.
ketone synthesis and metabolism
44. how are ketone bodies formed?
45. what happens to acetoacetate? what does the formation of its products depend on?
46. describe the oxidation of ketone bodies.
47. where does the mitochondria get the CoA in the formation of acetoacetyl CoA?
48. describe the energy yield of oxidation of 1 mole of ß-hydroxybutyrate.
49. what are ketogenic amino acids?
fuel homeostasis
50. when are fatty acids used as fuels as opposed to glucose?
51. when do fatty acid levels in the blood start to rise after a meal?
52. when are ketones used as fuels?
53. what does usage of ketones prevent breakdown of?
54. describe how fatty acid oxidation inhibits the glycolytic pathways.
55. what are some tissues, besides the brain, that use ketone bodies preferentially during fasting?
56. what is ketone body synthesis stimulated by?
answers
1. long chain fatty acids released from adipose cell triacylglycerol stores.
2. dietary fat and triacylglycerol synthesized in the liver.
3. 38%
4. 2 saturated fatty acids: palmitate (C16), stearate (C18), the monounsaturated fatty acid oleate (C18:1), and the polyunsaturated essential fatty acid linoleate (C18:2).
5. fatty acids from animals is generally saturated or monounsaturated (palmitate, stearate, oleate), whereas fatty acids from vegetables contains linoleate and some longer chain and polyunsaturated fatty acids.
6. dairy fat, maternal milk, vegetable oils.
7. short chain: 2-4 carbons. medium chain: 6-10 carbons. long chain: 12-26 carbons.
8. fatty acid synthesis occurs in the liver in the presence of an excess of glucose. the fatty acid palmitate is generated, which can be elongated to form stearate or unsaturated to form oleate.
9. because of their hydrophobicity/insolubility in water, and potential to disrupt the hydrophobic bonds in membrane proteins.
10. fatty acids are released from triacylglyceride stores in adipose tissue (between meals, during overnight fasting, or during exercise) via lipases and are transported in the blood bound in the hydrophobic pocket of albumin.
11. through diffusion or through fatty acid binding proteins
12. it is then bound to intracellular fatty acid binding proteins and transported to the inner mitochondria.
13. the fatty acid is combined with ATP to form a high energy fatty acyl-AMP intermediate, releasing the two phosphates (pyrophosphate) from ATP in the process. the AMP is then switched for a CoA and the pyrophosphate is cleaved.
14. fatty acyl CoA synthetase, present in the rough ER, outer mitochondrial membrane, and peroxisomal membranes.
15. only in the peroxisomal membrane for very long, and only in the mitochondrial matrix of liver and kidney cells for medium length fatty acids.
16. triacylglycerol and phospholipid synthesis in rough ER, oxidation and plasmalogen synthesis in peroxisome, beta-oxidation in mitochondria.
17. they are re-esterified into triacylglycerols in the liver and some other tissues.
18. activated long chain fatty acyls are formed on the outside of the outer mitochondrial membrane. they cross through into the inter membrane space (by diffusion?) and undergo a reaction catalyzed by CPT I (carnitine palmitoyltransferase I) in which their CoA group is exchanged for a carnitine molecule, forming a fatty acylcarnitine intermediate. this compound is transported into the mitochondrial matrix via a translocase, and undergoes a second reaction catalyzed by CPT II in which a CoA group is exchanged for carnitine (which then flows back into the intermembrane space) and fatty acyl CoA is reformed.
19. either from diet, or from a synthesis from a side chain of lysine. the pathway of the latter begins in the skeletal muscle and is finished in the liver. stored in skeletal muscle.
20. acetyl CoA units, FADH2, and NADH.
21. 1) the bond between the alpha and beta carbon in the fatty acyl CoA is oxidized to a double bond, forming trans ∆2-fatty enoyl CoA. FADH2 is formed
2) a water molecule is added to the trans ∆2-fatty enoyl CoA beta carbon, forming L-ß-hydroxy acyl CoA
3) the L-ß hydroxy acyl CoA is oxidized to a ketone, ß-keto acyl CoA. NADH is formed.
4) ß-keto acyl CoA is cleaved between the ß and α carbons and combined with another CoASH, releasing an Acetyl CoA molecule and forming another fatty acyl molecule.
22. FADH2 is bound to the acyl CoA dehydrogenase enzyme, and its electrons are transferred to flavoproteins ETF and ETF-QO in the matrix, which then transfer the electrons to CoQ of the electron transport chain.
23. palmitoyl CoA is a 16 carbon molecule which is cleaved 7 times into 8 molecules of Acetyl CoA. in the process is produces 7 molecules of FADH2 and 7 molecules of NADH. 7 FADH2 yields 10.5 moles of ATP (1.5 ATP per FADH2) and 7 NADH yields 17.5 moles of ATP (2.5 ATP per NADH), thus totalling 28 moles ATP (this does not account for the eventual oxidation of the acetyl coA molecules produced).
24. the enzymes in ß oxidation of fatty acids have specificities to different chain length fatty acids. as a fatty acid is broken down in the ß oxidation spiral into shorter chain fatty acyl CoA's, it is acted on by enzymes with shorter chain length specificity.
25. oleate (C18:1, ∆9) and linoleate (18:2, ∆9,12). both have cis-double bonds.
26. their cis double bonds must be converted to trans between the ß and alpha carbons, or reduced.
27. an odd chain length fatty acid is ß oxidized until the last three carbons remain, which forms propionyl CoA. propionyl CoA is converted into methylmalonyl CoA, which is ultimately converted into succinyl CoA.
28. vitamin B12.
29. in both, acetyl CoA is produced, which is oxidized in the TCA cycle to produce ATP. however, odd chain oxidation yields succinyl CoA by way of propionyl CoA. this reaction is both an anaplerotic reaction, supplying an intermediate of the TCA cycle, and can also be used in the gluconeogenic pathway, by conversion of succinyl CoA to malate, to pyruvate, to glucose.
30. medium chain length fatty acids are more water soluble and thus aren't stored in adipocyte triglycerides. after a meal they are transported to the liver and there they are transported into the mitochondrial matrix, where they undergo activation to fatty acyl CoA's and ß oxidation.
31. fatty acid oxidation produces Acetyl CoA, FADH2 and NADH, all of which ultimately supply electrons to the electron transport chain for generation of ATP. if energy requirements are low, then there will be a buildup of un-oxidized acetyl CoA, FADH2, and NADH, and thus fatty acid oxidation will be inhibited.
32. the formation of fatty acyl CoA in the mitochondrial matrix, the activated fatty acid which undergoes ß-oxidation, requires mitochondrial stores of CoASH (see question 18). thus CoASH must be replenished by the TCA cycle or other metabolic pathways.
33. CPT1 can be inhibited by malonyl CoA, which is synthesized from acetyl CoA via the enzyme acetyl CoA carboxylase.
34. acetyl CoA carboxylase is inhibited by a AMP-dependent protein kinase and stimulated by insulin-dependent mechanisms. thus during periods of high energy demand (and thus high AMP levels), malonyl CoA is inhibited, allowing for ß-oxidation of fatty acids. conversely, during the fed state, insulin stimulates the conversion of acetyl CoA to malonyl CoA, which then inhibits fatty acid oxidation.
35. chlorophyll degradation.
36. synthesis in the body, especially in the brain and nervous system.
37. after the very long chain fatty acids are activated to very long chain fatty acyl coA's by the "very long acyl coA synthetase" enzyme on the peroxisomal membrane, they enter the peroxisome directly, without the need for carnitine or the CPT enzymes.
38. the first step involves transfer of electrons to oxygen, producing hydrogen peroxide-- compare this with the FADH2 produced by the first step of mitochondrial oxidation, which can produce 1.5 ATP's.
39. they are ß-oxidized to acetyl CoA until a medium to short length fatty acids, which are then converted to carnitine derivatives and transported to the mitochondrial matrix where they are activated to acyl CoA's and ß oxidized.
40. the hydrogen peroxide that is produced by the first step of VLCFA catabolism is converted to water and oxygen via catalase.
41. phytanic and pristanic acid.
42. branched chain fatty acids are oxidized in the peroxisomes via alpha and ß oxidation, releasing propionyl and acetyl coA alternately until a medium length branched fatty acid is reached. this medium length fatty acid is then converted to a carnitine derivative and transported to the mitochondria for further ß oxidation.
43. fatty acids can also be w-oxidized in microsomes when ß-oxidation in mitochondria is inhibited for some reason. this involves oxidation of the w-carbon to an alcohol, and then a carboxylic acid. these dicarboxylic acids can be further ß-oxidized, enter into blood, or secreted in urine.
44. ketone bodies are formed from Acetyl CoA from the ß oxidation of fatty acids. 2 acetyl coA combine via thiolase into acetoacetyl CoA (a reverse of the last reaction of ß-oxidation). acetoacetyl CoA is then combined with another acetyl CoA molecule to form HMG CoA via HMG CoA synthase. HMG CoA lyase then catalyses the splitting of HMG CoA into acetyl CoA and acetoacetate. acetoacetate
45. acetoacetate can spontaneously decarboxylate into acetone and CO2. it can also be reduced to ß-hydroxybutyrate via ß-hydroxybutyrate dehydrogenase. both ß-hydroxybutyrate and acetoacetate can enter the blood and the interconversion between the two ketone bodies depends on cellular NAD/NADH levels.
46. once in the mitochondrial matrix of tissues, ß-hydroxybutyrate and acetoacetate can be oxidized to generate ATP. ß-hydroxybutyrate is oxidized back to acetoacetate by the same enzyme involved in ketone synthesis, ß-hydroxybutyrate dehydrogenase, releasing an NADH. a CoA is then added to acetoacetate, releasing 2 molecules of acetyl coA which can be used in the TCA cycle.
47. it transfers it from the TCA intermediate succinyl CoA via the enzyme succinylCoA:acetoacetate CoA transferase.
48. 1 mole of ß-hydroxybutyrate produces 2 moles of acetyl CoA which ultimately yields 20 moles ATP through the TCA cycle. subtracted from this is 1 ATP which is lost from the use of succinyl CoA to activate acetoacetate (whereas normally it would produce 1 GTP through the TCA cycle). added to this is the energy yield from the NADH produced in ß-hydroxybutyrate's oxidation, 2.5 ATP. this yields a total of 21.5 moles of ATP.
49. amino acids such as leucine, isoleucine, lysine, tryptophan, phenylalanine, tyrosine, which can be converted into ketone bodies.
50. between meals, during overnight fasting, or during long term mild exercise.
51. 3-4 hours after a meal, progressively increasing during fasting up until 2-3 days.
52. after 2-3 days, ketone levels in the blood enable them to be used for fuel in the brain, supplying up to 2/3 of the brain's energy requirements.
53. prevents breakdown of skeletal muscle protein, which is a major source for glycolytic pathways.
54. when fatty acids are present in the blood, they are used preferentially to glucose. the products of fatty acid oxidation are: acetyl CoA, NADH, FADH, and an increased ATP/AMP ratio. these products inhibit the enzymes that activate the glycolytic pathway: 1) PFK-1 is the main regulatory enzyme of glycolysis and is inhibited by ATP and citrate- from the TCA cycle. 2) pyruvate dehydrogenase is the enzyme that converts pyruvate from glycolysis to acetyl CoA for use in the TCA cycle; it is inhibited by the products of fatty acid oxidation, NADH and acetyl CoA.
55. intestinal mucosal cells and adipocyte cells.
56. during times of fasting, when the ß-oxidation of fatty acids produces enough NADH and FADH2 to satisfy the energy requirements of the liver, acetyl CoA is then used for ketone body synthesis.
actual questions
1. why can't VLCFA's be oxidized in the mitochondria like regular long chain fatty acids?
2. why do activated fatty acids need carnitine to be transported into the mitochondrial matrix?
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