Oxidation of Fatty Acids

• Stage 1 of fatty acid oxidation is beta oxidation

  • Fatty acids undergo oxidative removal of successive two carbon units in the form of acetyl-CoA
• Stage 2 in fatty acid oxidation is the oxidation of acetyl-CoA groups to CO2 in the citric acid cycle

  • Occurs in the mitochondrial matrix
  • Generates NADH, FADH2, and one GTP
• Stage three of fatty acid oxidation is electron transfer chain and oxidative phosphorylation

  • Generates ATP from NADH and FADH2
• An important theme in biochemistry is interaction among metabolic pathways. Which pathway would obviously be most affected by increased beta oxidation of fatty acids?

  • Glycolysis
  • Citric acid cycle
  • Glyoxylate pathway
  • Pentose phosphate pathway
  • Gluconeogenesis

  Citric acid cycle.
• In the second stage of fatty acid oxidation, the acetyl groups of acetyl-CoA are oxidized to CO2 in the citric acid cycle, which takes place in the mitochondrial matrix.
• Acyl-CoA dehydrogenase is a flavoprotein with tightly bound FAD that catalyzed the dehydrogenation of fatty acyl-CoA to yield trans-detla2-enol-CoA
• Acyl-CoA dehydrogenase isozymes are specific for fatty acyl chain lengths:

  • VLCAD (inner mitochondrial matrix): 12 to 18 carbons
  • MCAD (matrix): 4 to 14 carbons
  • SCAD (matrix): 4 to 8 carbons
• Electron transfer flavoprotein (ETF) is an electron carrier that accepts electrons from FADH2
• ETF:ubiquinone oxioreductase is a flavoprotein that accepts electrons from ETF

  • Passes electrons through ubiquinone into the mitochondrial respiratory chain
• Which sequence of electron carriers transfers electrons from a fatty acyl-CoA to the mitochondrial respiratory chain?

  • ETF --> ubiquinone --> ETF:ubiquinone oxidoreductase --> FADH2
  • FADH2--> ETF --> ETF:ubiquinone oxidoreductase --> ubiquinone
  • ETF --> FADH2 --> ETF:ubiquinone oxidoreductase --> ubiquinone
  • FADH2--> ETF:ubiquinone oxidoreductase --> ubiquinone --> ETF


  FADH2 --> ETF --> ETF:ubiquinone oxidoreductase --> ubiquinone
• Each molecule of FADH2 formed during the oxidation of the fatty acyl-CoA donates a pair of electrons to the electron transfer flavoprotein (ETF). Electrons move from ETF toa second flavoprotein, ETF:ubiquinone oxidoreductase, and through ubiquinone into the mitochondrial respiratory chain.
• The oxidation catalyzed by acyl-CoA dehydrogenase is analogous to succinate dehydrogenation. In both:

  • The enzyme is bound to the inner membrane
  • A double bond is introduced into a carboxylic acid between the alpha and beta carbons
  • FAD is an electron acceptor
  • Electrons from the reaction ultimately enter the respiratory chain and pass to oxygen.
• Enoyl-CoA hydratase catalyzes the addition of water to the double bond of the trans-delta 2- enoyl-CoA to form L-beta-hydroxyacyl-CoA (3-hydroxylacyl-CoA)

  • Formally analogous to the fumarase reaction in the citric acid cycle
• Beta-hydroxyacyl-CoA dehydrogenase catalyzes the dehydrogenation of L-beta-hydroxyacyl-CoA to form beta-ketoacyl-CoA

  • Enzyme is specific for the L stereoisomer
  • Closely analogous to the malate dehydrogenase reaction of the citric acid cycle
• NADH dehydrogenase (Complex I) is an electron carrier of the respiratory chain

  • Accepts electrons from the NADH formed in the beta-hydroxyacyl-CoA dehydrogenase reaction
• Acyl-CoAacetyl-transferase (thiolase) catalyzes the reaction of beta-ketoacyl-CoA with free coenzyme A to yield acetyl CoA and a fatty acyl-CoA shortened by two carbons

  • Reverse Claisen condensation
• The reactions of mitochondrial beta oxidation do not include

  • A hydratase
  • A thiolase
  • A dehydrogenase
  • An oxidase

  An oxidase
• The four enzymes of mitochondrial beta oxidation are:

  • Acyl-CoA dehydrogenase
  • Enoyl-CoA hydratase
  • Beta-hydroxyacyl-CoA dehydrogenase
  • Acyl-CoA acetyltransferase (thiolase)
• Trifunctional protein (TFP) is a multienzyme complex associated with the inner mitochondrial membrane that catalyzes steps 2 to 4 of the beta oxidation pathway for fatty acyl chains of 12 plus carbons. This allows for efficient substrate channeling.
• TFP is a heterooactamer of alpha4beta4 subunits:

  • alpha subunits contain enoyl-CoA hydratase and beta-hydroxyacyl-CoA dehydrogenase activity
  • beta subunits contain thiolase activity
• The first three reactions of the beta oxidation sequence convert the stable single bond between methylene groups to a much less stable C-C bond
• The ketone function on the beta carbon makes it a good target for nucleophilic attack
• The terminal -CH2-CO-S-CoA is a good leaving group, facilitating the breakage of the alpha-beta bond
• The shortened fatty acyl-CoA reenters the beta oxidation sequence for removal of another, and then another acetyl-CoA
• Each pass of beta oxidation removes:

  • One molecule of acetyl-CoA
  • Two pairs of electrons
  • Four protons (H+)
• The overall reaction, beginning with palmitoyl-CoA, is:
• The overall reaction, beginning with palmitoyl-CoA, is:

  • Palmitoyl-CoA + CoA + FAD + NAD+ + H2O --> myristoyl-CoA + acetyl-CoA + FADH2 + NADH + H+
• The overall reaction, beginning with palmitoyl-CoA, is:

  • Palmitoyl-CoA + 7 CoA + 7 FAD + 7 NAD+ + 7 H2O --> 8 acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+
• The overall reaction to convert palmitoyl-CoA to 8 acetyl-CoA:

  • Consumes 8 H2O
  • Produces either 7 NADH or 7 FADH2, depending on the organelle
  • Requires 8 rounds of beta oxidation
  • Produces 7 NADH and 7 FADH2


  Produces 7 NADH and 7 FADH2
• Palmitoyl-CoA, the coenzyme A derivative of palmitate (16:0), undergoes 7 rounds of beta oxidation, with each round producing 1 NADH via the acyl-CoA dehydrogenase reaction and 1 FADH2 via the beta-hydroxyacyl-CoA dehydrogenase reaction
• Each FADH2 donates a pair of electrons to ETF. This generates 1.5 molecules of ATP.
• Each NADH donates a pair of electrons to the mitochondrial NADH dehydrogenase. This generates 2.5 molecules of ATP.
• In total, 4 ATP are formed for each pass through beta oxidation.
• Each pair of electrons transferred from NADH or FADH2 to oxygen yields one H2O (metabolic water)
• The reduction of oxygen by NADH also consumes one H+ per NADH molecule

  • NADH + H+ + 1/2 O2 --> NAD+ + H2O
• Metabolic water is important in hibernating animals and camels.
• Which statement is false about fat oxidation in hibernating bears?

  • The energy of fat oxidation allows bears to maintain a body temperature close to normal
  • Amino groups released during fat oxidation can be used to make amino acids
  • Fat oxidation releases water, which replenishes water lost by breathing
  • Degradation to triacylglycerols provides a substrate for gluconeogenesis

  Amino groups released during fat oxidation can be used to make amino acids
• Fat metabolism provides bears with energy, water, and glucose precursors, but not amino groups. Urea formed during the breakdown of amino acids is reabsorbed in the kidneys and recycles, with the amino groups reused to make new animo acids for maintaining body proteins.
• The overall reaction for beta oxidation, including electron transfers and oxidative phosphorylation, is:

  • Palmitoyl-CoA + 7 COA + 7 O2 + 28 Pi+ 28 ADP --> 8 acetyl-CoA + 28 ATP + 7 H2O
• Acetyl-CoA produced from the oxidation of fatty acids can be oxidized to CO2 and H2O by the citric acid cycle.