Fatty Acid Catabolism

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Chapter 23

• Fatty Acid Catabolism
• Biochemistry
• by
• Reginald Garrett and Charles Grisham

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Outline

1. How Are Fats Mobilized from Dietary Intake and
   Adipose Tissue?
2. How Are Fatty Acids Broken Down?
3. How Are Odd-Carbon Fatty Acids Oxidized?
4. How Are Unsaturated Fatty Acids Oxidized?
5. Are There Other Ways to Oxidize Fatty Acids?
6. What Are Ketone Bodies, and What Role Do They
   Play in Metabolism?

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23.1 How Are Fats Mobilized from Dietary
Intake and Adipose Tissue?

• Triacylglycerols represent the major energy input
  in the modern American diet
• Provides 30-60 of calories

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• Triacylglycerols represent the major energy input
  in the modern American diet
• Provides 30-60 of calories
• Triacylglycerols are also the major form of
  stored energy in the body (80)

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Why Fatty Acids?

• (For energy storage?)
• Two reasons
• The carbon in fatty acids (mostly -CH2- groups)
  is reduced (so its oxidation yields the most
  energy possible).
• Fatty acids are not hydrated (as mono- and
  polysaccharides are), so they can pack more
  closely in storage tissues

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• Triacylglycerols represent the major energy input
  in the modern American diet
• Provides 30-60 of calories
• Triacylglycerols are also the major form of
  stored energy in the body
• Hormones (glucagon, epinephrine, ACTH) trigger
  the release of fatty acids from adipose tissue
  (Figure 23.2 chapter 32)

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Triacylglycerol lipase Hormone-sensitive lipase
Figure 23.2 Liberation of fatty acids from
triacylglycerols in adipose tissue is
hormone-dependent.
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• Triacylglycerols represent the major energy input
  in the modern American diet
• Provides 30-60 of calories
• Triacylglycerols are also the major form of
  stored energy in the body
• Hormones (glucagon, epinephrine, ACTH) trigger
  the release of fatty acids from adipose tissue
  (Figure 23.2 chapter 32)
• Degradation of dietary fatty acids occurs
  primarily in the duodenum (Figure 23.3 chapter
  24)

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Figure 23.3(a) The pancreatic duct secretes
digestive fluids into the duodenum, the first
portion of the small intestine. (b) Hydrolysis of
triacylglycerols by pancreatic and intestinal
lipases. Pancreatic lipases cleave fatty acids at
the C-1 and C-3 positions. Resulting
monoacylglycerols with fatty acids at C-2 are
hydrolyzed by intestinal lipases. Fatty acids and
monoacylglycerols are absorbed through the
intestinal wall and assembled into lipoprotein
aggregates termed chylomicrons (discussed in
Chapter 24).
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Figure 23.4 In the small intestine, fatty acids
combine with bile salts in mixed micelles, which
deliver fatty acids to epithelial cells that
cover the intestinal villi. Triacylglycerols are
formed within the epithelial cells.
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23.2 How Are Fatty Acids Broken Down?

• Franz Knoop showed that fatty acids must be
  degraded by removal of 2-C units (acetate)
• Albert Lehninger showed that this occurred in the
  mitochondria
• F. Lynen and E. Reichart showed that the 2-C unit
  released is acetyl-CoA, not free acetate
• The process begins with oxidation of the carbon
  that is "b" to the carboxyl carbon, so the
  process is called"b-oxidation"

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Figure 23.5 Fatty acids are degraded by repeated
cycles of oxidation at the ß-carbon and cleavage
of the Ca-Cß bond to yield acetate units, in the
form of acetyl-CoA.
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Coenzyme A activates Fatty Acids for degradation

• The process of b-oxidation begins with the
  formation of a thiol ester bond between the FA
  and the thiol group of CoA
• Acyl-CoA synthetase condenses fatty acids with
  CoA, with simultaneous hydrolysis of ATP to AMP
  and PPi (acyl-CoA ligase or fatty acid
  thiokinase)

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This reaction normally occurs at the outer
mitochondrial membrane or at the surface of the
endoplasmic reticulum
Figure 23.7 The mechanism of the acyl-CoA
synthetase reaction involves fatty acid
carboxylate attack on ATP to form an
acyl-adenylate intermediate. The fatty acyl CoA
thioester product is formed by CoA attack on this
intermediate.
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Carnitine as a Carrier

• Carnitine carries fatty acyl groups across the
  inner mitochondrial membrane
• Short chain fatty acids are transported directly
  into the mitochondrial matrix
• Long-chain fatty acids cannot be directly
  transported into the matrix
• Long-chain FAs are converted to acyl-carnitines
  and are then transported in the cell
• Acyl-CoA are formed inside the inner membrane

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Figure 23.8 The formation of acylcarnitines and
their transport across the inner mitochondrial
membrane. The process involves the coordinated
actions of carnitine acyltransferases on both
sides of the membrane and of a translocase that
shuttles O-acylcarnitines across the membrane.
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?-Oxidation of Fatty Acids

• A Repeated Sequence of 4 Reactions
• First 3 reactions is to create a carbonyl group
  on the ?-C
• Fourth cleaves the "?-keto ester" in a reverse
  Claisen condensation
• Products an acetyl-CoA and a fatty acid (two
  carbons shorter)
• The first three reactions are crucial and classic
  - we will see them in other pathways (TCA cycle)

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Figure 23.9The b-oxidation of saturated fatty
acids involves a cycle of four enzyme-catalyzed
reactions.
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Figure 19.2The tricarboxylic acid cycle.
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1. Acyl-CoA Dehydrogenase

• Oxidation of the C?-C? bond
• A family of membrane-bound (VLCAD) and three
  soluble matrix enzymes (LCAD, MCAD, and SCAD)
• Mechanism involves proton abstraction, followed
  by double bond formation and hydride removal by
  FAD
• Electrons are passed to an electron transfer
  flavoprotein, and then to the electron transport
  chain (chapter20)

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14C and longer
Figure 23.10 Very long-chain fatty acids proceed
through several cycles of ß-oxidation (left) via
membrane-bound enzymes in mitochondria, before
becoming substrates for the separate soluble
enzymes of ß-oxidation (right).
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ETF electron transfer flavoprotein
Figure 23.12 The acyl-CoA dehydrogenase
reaction. The two electrons removed in this
oxidation reaction are delivered to the electron
transport chain in the form of reduced coenzyme Q
(UQH2).
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Figure 20.4
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2. Enoyl-CoA Hydratase

• Adds water across the double bond
• The reaction is catalyzed by enoyl-CoA hydratase
• Also called crotonases
• Normal reaction converts trans-enoyl-CoA to
  L-?-hydroxyacyl-CoA

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3. L-Hydroxyacyl-CoA Dehydrogenase

• Oxidizes the ?-Hydroxyl Group
• This enzyme is completely specific for
  L-hydroxyacyl-CoA
• NADH produced in this reaction represents
  metabolic energy

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4. Thiolase (?-ketothiolase)

• Cleavage of the b-ketoacyl-CoA
• Cysteine thiolate on enzyme attacks the
  ?-carbonyl group of acyl-CoA
• Thiol group of a new CoA attacks the shortened
  chain, forming a new, shorter acyl-CoA
• Formation of a new thioester, this reaction is
  favorable and drives other three previous
  reactions of the b-oxidation

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Figure 23.15 The mechanism of the thiolase
reaction.
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Summary of ?-Oxidation

• Repetition of the cycle yields a succession of
  acetate units
• Thus, palmitic acid yields eight acetyl-CoAs
• Complete ?-oxidation of one palmitic acid yields
  106 molecules of ATP
• Large energy yield is consequence of the highly
  reduced state of the carbon in fatty acids
• This makes fatty acids the fuel of choice for
  migratory birds and many other animals (70)

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• Migratory Birds Travel Long Distances on Energy
  From Fatty Acid Oxidation
• Because they represent the most highly
  concentrated form of stored biological energy,
  fatty acids are the metabolic fuel of choice for
  sustaining the long flights of migratory birds
• These prodigious feats are accomplished by
  storing large amounts of triacylglycerols prior
  to flight
• These birds are often 70 fat by weight when
  migration begins (compared with values of 30 and
  less for nonmigratory birds)

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American golden plovers fly from Alaska to Hawaii
nonstop 3300 km in 35 hours more than 250,000
wing beats
The ruby-throated hummingbird flies nonstop
across the Gulf of Mexico
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• Fatty Acid Oxidation is an Important Source of
  Metabolic Water for Some Animals
• Large amounts of metabolic water are generated by
  b-oxidation
• For certain animals, the oxidation of stored
  fatty acids can be a significant source of
  dietary water
• Desert animals (such as gerbils)
• Killer whales (which do not drink seawater)
• Camels (whose hump is a large fat deposit)
• Metabolism of fatty acids from such stores
  provides needed water, as well as metabolic
  energy, during periods when drinking water is not
  available

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23.3 How Are Odd-Carbon Fatty Acids Oxidized?

• Oxidation yields propionyl-CoA
• Odd-carbon fatty acids are metabolized normally,
  until the last three-C fragment (propionyl-CoA)
  is reached
• Three reactions convert propionyl-CoA to
  succinyl-CoA (Figure 23.18)
• Propionyl-CoA carboxylase (biotin)
• Methylmalonyl-CoA epimerase
• Methylmalonyl-CoA mutase (B12)
• Succinyl-CoA can enter the TCA cycle

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Figure 23.19 The methylmalonyl-CoA epimerase
mechanism involves a resonance-stabilized
carbanion at the a-position.
Figure 23.18 The conversion of propionyl-CoA
(formed from b-oxidation of odd-carbon fatty
acids) to succinyl-CoA is carried out by a trio
of enzymes as shown. Succinyl-CoA can enter the
TCA cycle.
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• Conversion of inactive vitamin B12 to active
  5'-deoxyadenosylcobalamin involves three steps
• Two flavoprotein reductases convert Co3 to Co2
  and then to Co
• Co is a powerful nucleophile, which can attack
  the C-5 of ATP to form 5-deoxyadenosylcobalamin

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23.4 How Are Unsaturated Fatty Acids Oxidized?

• Monounsaturated fatty acids
• Oleic acid, palmitoleic acid
• Normal ?-oxidation for three cycles
• cis-?3-acyl-CoA cannot be utilized by acyl-CoA
  dehydrogenase
• Enoyl-CoA isomerase converts this to trans-
  ?2-acyl-CoA
• b-oxidation continues from this point

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Figure 23.21 b-Oxidation of unsaturated fatty
acids. In the case of oleoyl-CoA, three
b-oxidation cycles produce three molecules of
acetyl-CoA and leave cis-D3-dodecenoyl-CoA.
Rearrangement of enoyl-CoA isomerase gives the
trans-D2 species, which then proceeds normally
through the b-oxidation pathway.
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Polyunsaturated Fatty Acids

• Slightly more complicated
• Same as for oleic acid, but only up to a point
• 3 cycles of ?-oxidation
• enoyl-CoA isomerase
• 1 more round of ?-oxidation
• trans-?2, cis-?4 structure is a problem
• 2,4-Dienoyl-CoA reductase to the rescue

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Figure 23.22 The oxidation pathway for
polyunsaturated fatty acids
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23.5 Are There Other Ways to Oxidize Fatty
Acids?

1. Peroxisomal Glyoxysomal ?-oxidation
2. Branched-chain ?-oxidation
3. ?-oxidation (dicarboxylic acids)

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Peroxisomal ?-Oxidation requires FAD-dependent
acyl-CoA oxidase

• Peroxisomes - organelles that carry out
  flavin-dependent oxidations, regenerating
  oxidized flavins by reaction with O2 to produce
  H2O2
• Similar to mitochondrial ?-oxidation, but initial
  double bond formation is by acyl-CoA oxidase
• Electrons go to O2 rather than e- transport
• Fewer ATPs result
• Similar b-oxidation enzymes are also found in
  glyoxysomes in plant

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Figure 23.23 The acyl-CoA oxidase reaction in
peroxisomes. Electrons captured as FADH2 are
used to produce the hydrogen peroxide required
for degradative processes in peroxisomes and thus
are not available for eventual generation of ATP.
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Branched-Chain Fatty Acids

• An alternative to ?-oxidation is required
• Branched chain FAs with branches at odd-number
  carbons are not good substrates for ?-oxidation
• ?-oxidation is an alternative
• Phytanic acid ?-oxidase decarboxylates with
  oxidation at the alpha position
• ?-oxidation occurs past the branch

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Figure 23.24Branched-chain fatty acids are
oxidized by a-oxidation, as shown for phytanic
acid. The product of the phytanic acid oxidase,
pristanic acid, is a suitable substrate for
normal b-oxidation. Isobutyryl-CoA and
propionyl-CoA can both be converted to
succinyl-CoA, which can enter the TCA cycle.
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• ?-oxidation (dicarboxylic acids)
• In ER
• Cytochrome P-450
• O2

Figure 23.25 Dicarboxylic acids can be formed by
oxidation of the methyl group of fatty acids in a
cytochrome P-450-dependent reaction.
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23.6 What Are Ketone Bodies, and What Role Do
They Play in Metabolism?

• A special source of fuel and energy for certain
  tissues
• Some of the acetyl-CoA produced by fatty acid
  oxidation in liver mitochondria is converted to
  acetone, acetoacetate and ?-hydroxybutyrate
• These are called "ketone bodies"
• Source of fuel for brain, heart and muscle
• Major energy source for brain during starvation
• They are transportable forms of fatty acids

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Ketone Body synthesis

• In liver mitochondrial matrix
• First reaction is reverse thiolase
• Second reaction makes HMG-CoA
• Third reaction converts HMG-CoA to acetoacetate
  and acetyl-CoA by HMG-CoA lyase
• Acetoacetate is reduced to b-hydroxybutyrate by
  b-hydroxybutyrate dehydrogenase

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• Acetoacetate and b-hydroxybutyrate are
  transported through the blood from liver to
  target organ
• Acetoacetate and b-hydroxybutyrate are converted
  to acetyl-CoA

Figure 23.27 Reconversion of ketone bodies to
acetyl-CoA in the mitochondria of many tissues
(other than liver) provides significant metabolic
energy.