S1B4 - Fatty Acid Oxidation

Question 1

What are the possible fates for the acetyl-CoA produced in fatty acid oxidation?

Answer 1

Acetyl-CoA can enter the citric acid cycle or be converted to ketone bodies in the liver.

Question 2

What is the first step in the transport of fatty acyl-CoA across the mitochondrial membrane for fatty acid oxidation?

Answer 2

Fatty acyl-CoA reacts with carnitine to form fatty acyl-carnitine in a reaction catalyzed by carnitine acyltransferase 1 (CAT1 or CPT1) on the outer mitochondrial membrane.

Fatty acyl-CoA + carnitine → Fatty acyl carnitine + CoA

Question 3

How do the products of odd chain fatty acid metabolism differ from even chain fatty acid metabolism?

Answer 3

Odd chain fatty acids are degraded similarly to even chain fatty acids, except at the last round of β-oxidation, 1 propionyl-CoA is produced. Because propionyl-CoA can be converted to succinyl-CoA, odd chain fatty acid metabolism is gluconeogenic.

Question 4

What are long chain fatty acids converted to in the cytosol before they undergo oxidation? What enzyme catalyzes this reaction?

Answer 4

Before fatty acid oxidation can occur in the mitochondria, fatty acids longer than 14 carbons must be converted to fatty acyl-CoA by the enzyme fatty acyl-CoA synthase within the cytosol.

Question 5

What is the rate limiting enzyme of fatty acid oxidation?

Answer 5

The reaction catalyzed by carnitine acyltransferase 1 (CAT1) (or CPT1) is the rate limiting step of fatty acid oxidation.

Question 6

How is fatty acid oxidation negatively regulated?

Answer 6

Malonyl-CoA, a product of lipogenesis, allosterically inhibits carnitine acyltransferase 1 (CAT1 or CPT1) to prevent fatty acid oxidation from occurring at the same time as lipogenesis.

Question 7

What enzyme catalyzes the rate limiting step in ketogenesis? Where is the enzyme located in the cell?

Answer 7

The rate limiting enzyme in ketogenesis is HMG-CoA synthase. It is found within the mitochondria.

Question 8

How does long chain fatty acid oxidation differ from medium and short chain fatty acid oxidation?

Answer 8

Medium and short chain fatty acids (less than 14 carbons long) do not need to be activated to form fatty acyl-CoA or pass through the carnitine shuttle system to enter the mitochondria.

Question 9

Which of the following inhibits the carnitine shuttle system used to transport acyl-CoA into mitochondria?

A) beta-hytroxybutyrate

B) Citrate

C) Acetyl-CoA

D) Malonyl-CoA

E) Acyl-CoA

Answer 9

D) Malonyl-CoA

The carnitine shuttle system is inhibited during fatty acid biosynthesis. This prevents newly synthesized fatty acids from being shuttled back into the mitochondria, which would represent a futile cycle. Malonyl-CoA accomplishes this function.

Question 10

What is the inheritance of medium chain acyl-CoA dehydrogenase (MCAD) deficiency? What does it lead to an accumulation of?

Answer 10

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is an autosomal recessive disorder that leads to an accumulation of 8-10 carbon fatty acyl carnitines in the blood.

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD): MCAD is a enzyme required for metabolism of medium length fatty acids. MCAD deficiency results in an inability to oxidize medium length fatty acids (6-12 carbons).

• In a typical presentation a previously healthy infant/child presents with hypoketotic hypoglycemia, vomiting, and lethargy triggered by a common illness (such as the flu).
• In prolonged fasting or stressed with an infection the body depletes glucose stores and cannot metabolize medium-chain fatty acid stores to compensate.

The treatment is avoid prolonged fasting, increase carbohydrate and protein intake, and decrease fat intake.

Question 11

What are the 3 conditions under which ketogenesis occurs? What gluconeogenic molecule is depleted in all 3 conditions?

Answer 11

Starvation and diabetic ketoacidosis lead to increased gluconeogenesis which depletes oxaloacetate. Depletion of oxaloacetate leads to the inhibition of the citric acid cycle and a build up of acetyl-CoA, which is now shunted toward ketogenesis. Alcoholism leads to a buildup of NADH from ethanol metabolism. Excess NADH shunts oxaloacetate to malate via a reversible reaction catalyzed by malate dehydrogenase, and depletes oxaloacetate for the citric acid cycle.

Oxaloacetate + NADH ↔ Malate + NAD+

All 3 states lead to a buildup of acetyl-CoA and an increase in the production of ketone bodies.

Question 12

What are the symptoms of carnitine deficiency? How is it treated?

Answer 12

The symptoms of carnitine deficiency include:

• Hypoketotic hypoglycemia
• Weakness
• Hypotonia

​The treatment is carnitine supplementation

Question 13

How many rounds of oxidation does a 16 carbon fatty acid undergo? What is the yield?

Answer 13

Within the mitochondria, even chain fatty acids are degraded to acetyl-CoA by acyl-CoA dehydrogenases. One round of fatty acid oxidation yields 1 acetyl CoA, 1 NADH, 1 FADH2, and 1 fatty acyl-CoA that is 2 carbons shorter.

A 16 carbon fatty acid will undergo 7 rounds of oxidation to yield 8 acetyl-CoA, 7 NADH, and 7 FADH2.

Question 14

What is the typical presentation of medium-chain acyl-CoA dehydrogenase deficiency? How is it treated?

Answer 14

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD): MCAD is a enzyme required for metabolism of medium length fatty acids. MCAD deficiency results in an inability to oxidize medium length fatty acids (6-12 carbons).

• In a typical presentation a previously healthy infant/child presents with hypoketotic hypoglycemia, vomiting, and lethargy triggered by a common illness (such as the flu).
• In prolonged fasting or stressed with an infection the body depletes glucose stores and cannot metabolize medium-chain fatty acid stores to compensate.

The treatment is avoid prolonged fasting, increase carbohydrate and protein intake, and decrease fat intake.

Question 15

What are the 3 ketones produced in ketogenesis? What are their possible fates?

Answer 15

The 3 ketones produced are β-hydroxybutyrate, acetoacetate, and acetone. Acetoacetate rapidly breaks down to acetone. β-hydroxybutyrate and acetoacetate can be converted to acetyl-CoA in muscle and brain. Acetone is a volatile ketone that is exhaled by the lungs.

Question 16

Once fatty acyl carnitine enters the mitochondrial matrix, what is the next step of the carnitine shuttle? What enzyme catalyzes this step?

Answer 16

Within the mitochondria, fatty acyl-CoA is regenerated by carnitine acyltransferase 2 (CAT2 or CPT2) in a reaction that also yields free carnitine.

Fatty acyl carnitine + CoA → Fatty acyl-CoA + carnitine

Question 17

How is fatty acyl carnitine transported into the mitochondria?

Answer 17

Fatty acyl carnitine is transported into the mitochondria via a fatty acyl carnitine/carnitine antiporter, which regenerates carnitine on the cytoplasmic side for further transport of fatty acids.

Question 18

What process is inhibited by carnitine deficiency?

Answer 18

Carnitine deficiency is an inherited disorder that prevents the metabolism of long chain fatty acids from occurring.

Question 19

What does each beta-oxidation cycle produce?

Answer 19

Each cycle produces one acetyl-CoA, one FADH2, one NADH and an acylCoA reduced in length by two carbons.

Question 20

Where in the cell do the beta-oxidation reactions occur?

Answer 20

Since these reactions occur inside the mitochondria, the reduced cofactors are substrates for oxidative phosphorylation, and the acetyl-CoA is a substrate for the TCA cycle.

Question 21

What is the energy yield of palmitate (C16) after going through beta-oxidation?

Answer 21

Energy yield using palmitate (C16) as an example

• 7 cycles of β-oxidation would be required
• Therefore producing 7 FADH2 and 7 NADH
  
  • 7•2 ATP per FADH2 = 14 ATP
  • 7•3 ATP per NADH = 21 ATP
• 8 acetyl-CoA molecules for TCA cycle
  
  • 8•12 ATP per TCA cycle = 96 ATP
• Equivalent of 2 ATP were used to “activate” palmitate to palmitoyl-CoA
• Total = 129 mol ATP per mol palmitate

Question 22

What are the important variants of acyl-CoA dehydrogenases and what do they prefer to catalyze?

Answer 22

Each of the 4 reactions is catalyzed by a number of enzymes which have various specificities for chain length. The most important variants of these enzymes are the different acyl-CoA dehydrogenases.

• Very long-chain acyl-CoA dehydrogenase (VLCAD)
  
  • Prefers C12 to C24
• Long-chain acyl-CoA dehydrogenase (LCAD)
  
  • Prefers C12 to C16
• Medium-chain acyl-CoA dehydrogenase (MCAD)
  
  • Broad specificity but most active on C6 and C8
• Short-chain acyl-CoA dehydrogenase (SCAD)
  
  • Order of activity is C4>C6>C8

Question 23

What happens to beta-oxidation when insulin predominates?

Answer 23

When insulin predominates

1. Acetyl-CoA carboxylase is stimulated
2. The malonyl-CoA is a substrate for fatty acid synthesis
3. The malonyl-CoA is an inhibitor of CPT I

Question 24

What happense to beta-oxidation when glucagon predominates?

Answer 24

When glucagon predominates

1. Acetyl-CoA carboxylase is inhibited
2. Malonyl-CoA concentration decreases
3. Inhibition of CPT I is relieved

Question 25

What do you need to know about oxidation of unsaturated fatty acids?

Answer 25

Oxidation of unsaturated fatty acids

• These have to be handled differently because the double bonds are in the cis configuration, and may involve an odd-numbered carbon.
• Unsaturated fatty acids are oxidized according to the following scheme, which includes two extra enzymes:
  
  • An enoyl-CoA isomerase
  • A 2,4-dienoyl-CoA reductase

Question 26

What do you need to know about oxidation of very long chain fatty acids?

Answer 26

Oxidation of very long chain fatty acids (C24-C26)

• This process occurs in peroxisomes
• The process is a “modified” β-oxidation
  
  • The products are acetyl-CoA, and a fatty acyl-CoA reduced by 2 carbons
  • The acyl-CoA oxidation is not linked to cofactor reduction (no energy provided by this step)
  • The acyl-CoA dehydrogenase reduces O2 to H2O2
    
    • The H2O2 is reduced to H2O by catalase.
• The peroxisomal oxidation proceeds until the acyl-CoA is about 8 carbons
• The octanoyl-CoA and the acetyl-CoA produced by peroxisomal oxidation are transported to mitochondria for further oxidation.

Question 27

What do you need to know about oxidation of long-chain branched-chain fatty acids?

Answer 27

Long-chain branched-chain fatty acids are derived from plants.

• Animals do not produce branched-chain fatty acids
• The most common long-chain branched-chain fatty acid (phytanic acid) is a degradation product of chlorophyll.

These are oxidized in the peroxisomes.

The process involves both α- and β-oxidation.

The products are:

• CO2
• Propionyl-CoA
• Acetyl-CoA

Question 28

What do you need to know about ω-oxidation of fatty acids?

Answer 28

ω-oxidation of fatty acids

• The ω-carbon is the terminal methyl group of the fatty acid
• This process occurs in the endoplasmic reticulum and requires cytochrome P₄₅₀, O₂, and NADPH.
• The ω-CH₃ group is oxidized first to an alcohol, then to a carboxylic acid.
• This produces a dicarboxylic acid
• This pathway is thought to function to produce more water-soluble compounds from water insoluble fatty acids
  
  • The products may be oxidized as medium-chain fatty acids or
  • Excreted.

Question 29

Where in the body are the ketone bodies synthesized?

Answer 29

The ketone bodies (acetoacetate and β-hydroxybutyrate) are synthesized in liver mitochondria.

Question 30

Where in the body are ketone bodies used for energy?

Answer 30

Ketone bodies are utilized by peripheral tissues for energy

• Primarily muscle and brain.
• Enzymes are mitochondrial.

Question 31

What peripheral tissue enzyme is specific for utilization of ketone bodies?

Answer 31

Peripheral tissues (muscle and brain) express the enzyme specific for utilization of ketone bodies

• Acetoacetate: succinyl-CoA transferase

Question 32

What do you need to know about the control of ketone body formation?

Answer 32

Control of ketone body formation

• Ketone bodies are formed in the liver only when the liver is supplied with high concentrations of fatty acids.
  
  • The fatty acids come from lipolysis of adipose tissue.
  • Usually seen during prolonged starvation, and in uncontrolled type 1 diabetes mellitus.
• After an over night fast, the concentration of ketone bodies in the blood is on the order of 0.05 mM.
  
  • After a 2 day fast the concentration can rise to 2 mM
  • After 40 days starvation, up to 7 mM
• Acetone is derived from the spontaneous, non-enzymatic decarboxylation of acetoacetate.
• The amount of fatty acids provided to liver is under control of insulin and glucagon.

Question 33

What do you need to know about Refsum disease?

Answer 33

Refsum disease

• Rare genetic deficiency of a component required for peroxisomal oxidation of branched-chain fatty acids.
• Serious neurological issues:
• Retinitis pigmentosa
• Peripheral neuropathy
• Cerebellar ataxia
• Nerve deafness
• Treated by reducing phytanic acid from diet (restrict dairy and meat from ruminants)

Question 34

What do you need to know about Jamaican vomiting sickness?

Answer 34

Jamaican vomiting sickness

• Due to eating the unripe fruit of the akee tree, which contains hypoglycin.
• A potent inhibitor of acyl-CoA dehydrogenase.
• Symptoms are non-ketotic hypoglycemia. (one case reported a blood glucose concentration of 3 mg/dl).

Question 35

What is β-hydroxybutyrate converted to?

Answer 35

If the ketone is β-hydroxybutyrate, it is converted back to acetoacetate, then → → 2 acetyl-CoA

Question 36

What is the rate-limiting enzyme in the production of ketones?

Answer 36

The rate limiting enzyme in the formation of ketones is HMG-CoA synthase. (Not to be confused with HMG-CoA reductase, which is the rate limiting step in cholesterol synthesis.) HMG-CoA lyase acts upon HMG-CoA to form acetoacetate, but is not the rate-limiting step

Question 37

In the consumption of β-hydroxybutyrate as metabolic fuel, what is produced?

A) 3 oxaloacetate

B) 4 acyl-CoA

C) 1 succinate

D) 2 glucose-6-phosphate

E) 2 acetyl-CoA

Answer 37

E) 2 acetyl-CoA

Ketone utilization: β-hydroxybutyrate → acetoacetate → → 2 acetyl-CoA molecules

Question 38

Does fatty acid oxidation occur in a fasted or fed state?

Answer 38

In a fasted, hypoglycemic state, fatty acids are sent to the liver for oxidation.

Question 39

What causes the fruity odor detected on breath in ketoacidosis?

Answer 39

Ketones are excreted in urine. Acetone, from spontaneous decarboxylation of acetoacetate, causes the “fruity odor” detected on breath during ketoacidosis.

Question 40

What are the 3 types of ketones produced in ketogenesis?

Answer 40

Acetoacetate, β-hydroxybutyrate, and acetone are the ketones produced during ketogenesis.