Fat Digestion and Metabolism Notes

Fat Digestion and Metabolism

Overview

This lecture covers fat digestion, absorption, fatty acid oxidation, and ketone bodies.

Learning Objectives

• Explain the importance of bile in lipid digestion and absorption.
• Describe the digestion and absorption of triacylglycerol (TAG).
• Understand how triacylglycerol is transported in the bloodstream and made available to tissues.
• Describe how fatty acids are transported into mitochondria.
• Describe the process of fatty acid oxidation.
• Explain the importance of ketogenesis.
• Describe the pathways of ketogenesis and ketone body utilization.

Digestion and Absorption of TAG

1. Emulsification:

   • Bile salts in the small intestine emulsify TAG, increasing the surface area of hydrophobic lipid droplets.
   • This allows digestive enzymes to act effectively.
   • Prevention of emulsification leads to fatty stool (steatorrhea).

2. Digestion:

   • TAG is digested into free fatty acids (FFA) and 2-monoacylglycerol.
   • $TAG \rightarrow FFA + 2-monoacylglycerol + FFA$

3. Absorption of Lipids in Micelles:

   • Free fatty acids and 2-monoacylglycerol, along with bile salts, form micelles.
   • Micelles are disk-shaped clusters of amphipathic lipids (hydrophobic inside, hydrophilic outside), making them soluble in the aqueous environment of the intestine.

4. Re-synthesis of TAG:

   • Fatty acids are activated to fatty acyl CoA by fatty acyl CoA synthetase.

5. Formation of Chylomicrons and Exocytosis:

   • TAG, along with other lipids and proteins, are packaged into chylomicrons.
   • Chylomicrons are then secreted from the intestinal cells into the lymphatic system and eventually enter the bloodstream.

Use of Dietary Lipids by Tissues

• Lipoprotein Lipase (LPL):
  • TAG in chylomicrons is degraded to free fatty acids and glycerol by lipoprotein lipase (LPL).
  • LPL is synthesized primarily by adipocytes and muscle cells.
  • TAG breakdown occurs primarily in the capillaries of skeletal and cardiac muscle and adipose tissues.

Fate of Free Fatty Acids

1. Entry into Cells:
   • Free fatty acids can directly enter adjacent muscle cells or adipocytes.
2. Transport in Blood:
   • They can be transported in the blood in association with serum albumin until taken up by cells.

Most cells can oxidize fatty acids to produce energy. Adipocytes can also re-esterify free fatty acids to produce TAG molecules for storage.

Fate of Glycerol

Glycerol released from TAG is taken up from the blood and phosphorylated by hepatic glycerol kinase to produce glycerol 3-phosphate, which can either enter glycolysis or gluconeogenesis.

Fatty Acid Activation

• Fatty acids are activated by thiokinase to form fatty acyl CoA.

β-Oxidation of Fatty Acids

• The major pathway for catabolism of fatty acids is a mitochondrial pathway called β-oxidation.
• Two-carbon fragments are successively removed from the carboxyl end of fatty acyl CoA, producing acetyl CoA, NADH, and FADH2.

Carnitine Shuttle

• Purpose: Transports long-chain fatty acids into the mitochondrial matrix for β-oxidation.

1. CPT-I (Carnitine Palmitoyltransferase I):
   • Acyl group is transferred from CoA to carnitine by CPT-I, forming acylcarnitine.
   • This is the rate-limiting step.
2. CACT (Carnitine-Acylcarnitine Translocase):
   • Acylcarnitine enters the matrix in exchange for free carnitine.
3. CPT-II (Carnitine Palmitoyltransferase II):
   • Catalyzes the transfer of the acyl group from carnitine to CoA in the matrix, regenerating free carnitine.

Fatty Acid Oxidation

• Occurs in the matrix of the mitochondria and requires oxygen.
• Step-by-step removal of 2-carbon units as acetyl CoA.
• Produces energy in the form of reduced coenzymes, FADH2 and NADH.
• Acetyl CoA can be further oxidized in the matrix to produce more energy.

Regulation of Fatty Acid Oxidation

• Malonyl CoA inhibits CAT-I, turning off fatty acid oxidation in the well-fed state.
• Fatty acid oxidation is most important during fasting and starvation to avoid using limited glucose.
• Regulation is mainly due to control of adipose tissue, which stores fatty acids in the fed state and releases them during fasting (and exercise).

Steps in Fatty Acid β-Oxidation

1. Oxidation: Produces FADH2.
2. Hydration: Adds water across the double bond.
3. Oxidation: Produces NADH.
4. Thiolytic Cleavage: Releases a molecule of acetyl CoA.
   • The fatty acid chain is shortened by two carbons.

Energy Yield from Fatty Acid Oxidation

• Each acetyl CoA gives 10 ATP by the citric acid cycle (CAC).
• The oxidation of palmitoyl CoA to $CO<em>2$ and $H</em>2O$ produces 8 acetyl CoA, 7 NADH, and 7 FADH2.
• $n/2-1$ number of cycles are needed for an n-carbon fatty acid
• This generates 108 ATP, minus 2 ATP for activation, resulting in a net yield of 106 ATP from palmitate.

Ketone Bodies

• Water-soluble and transported across the inner mitochondrial membrane, blood-brain barrier, and cell membranes.
• Used as a fuel source by various tissues, including the CNS, and are preferred substrates for aerobic muscle and heart.

Types of Ketone Bodies

• Acetoacetate
• β-Hydroxybutyrate
• Acetone (a non-metabolized product released in breath)

Ketogenesis

• FA oxidation is high in the liver during starvation, diabetes, and severe muscular exercise.
• Hepatic acetyl CoA from FA degradation activates pyruvate carboxylase.
• The oxaloacetate produced is used by the liver for gluconeogenesis rather than the citric acid cycle.
• Therefore, acetyl CoA goes towards ketone body synthesis.

Ketone Body Production

• The liver produces ketone bodies in the fasting state as fatty acid oxidation increases and glycolysis switches to gluconeogenesis.
• During starvation, the brain can switch to using ketone bodies for ~50% of its energy needs.

Ketone Body Synthesis

1. Formation of Acetoacetyl CoA:
   • Occurs by reversal of the thiolase reaction ($2 Acetyl CoA \rightarrow Acetoacetyl CoA$).
2. Formation of HMG CoA:
   • Mitochondrial HMG CoA synthase combines a third molecule of acetyl CoA with acetoacetyl CoA to produce HMG CoA.
   • HMG CoA synthase is the rate-limiting step and is present in significant quantities only in the liver.
3. Cleavage of HMG CoA:
   • HMG CoA lyase cleaves HMG CoA to produce acetoacetate and acetyl CoA.
4. Formation of β-Hydroxybutyrate and Acetone:
   • Acetoacetate can be reduced to form β-hydroxybutyrate with NADH as the hydrogen donor.
   • Acetoacetate can also spontaneously decarboxylate to form acetone.

Ketone Body Catabolism

• Occurs in extra-hepatic mitochondria.

1. Oxidation of β-Hydroxybutyrate:
   • β-Hydroxybutyrate is oxidized to acetoacetate by β-hydroxybutyrate dehydrogenase, producing NADH.
2. Activation of Acetoacetate:
   • Acetoacetate is provided with a CoA molecule taken from succinyl CoA by succinyl CoA:acetoacetate CoA transferase (thiophorase).
3. Cleavage of Acetoacetyl CoA:
   • Acetoacetyl CoA is converted to two acetyl CoAs by Thiolase.

Ketone Body Catabolism (cont.)

• Extrahepatic tissues, including the brain but excluding cells lacking mitochondria (e.g., RBCs), efficiently oxidize acetoacetate and β-hydroxybutyrate.
• The Liver produce ketone bodies, but cannot utilize ketone bodies as fuel, because it lacks thiophorase.

Ketosis

• Increased production of ketone bodies in the liver.
• Ketogenesis > Ketolysis
• Ketonemia: Increased ketone bodies in the blood.
• Ketonurea: Appearance of ketone bodies in urine.
• Ketoacidosis: Decreased pH of blood due to acetoacetate & β-OH butyrate.

Causes of Ketosis

• Starvation
• Diabetes Mellitus Type I
• High fat and low carbohydrate diet

Ketoacidosis

• Due to increased levels of acetoacetate & β-OH butyric acids.
• These acids are buffered by physiological $NaHCO<em>3$ buffer to produce Na acetoacetate & Na OH butyrate and $H</em>2CO<em>3$, which decomposes to $H</em>2O$ and $CO_2$.
• Excess production of ketone bodies leads to depletion of all $NaHCO_3$, resulting in excess $H^+$ and decreased pH, leading to acidosis.

Lethal Effects of Ketoacidosis

• Acidosis: Affects brain centers and decreases Hb ability to bind to $O_2$.
• Ketonurea: Causes osmotic diuresis and dehydration, leading to loss of $Na^+$ in urine (as Na salts of KBs are excreted in urine), causing hyponatremia.

Review Questions

1. The compound that transports activated fatty acids to mitochondria is carnitine.
2. Ketone bodies are produced primarily in the mitochondria.
3. The condition where an excess amount of ketone bodies are present in the blood is known as ketonemia.
4. Ketone bodies are produced in liver cells.
5. The formation of acetoacetyl-CoA from 2 Acetyl CoA is catalyzed by Thiolase.
6. Acetone is the volatile ketone body.
7. Carnitine facilitates the transport of long-chain fatty acids from the cytosol to mitochondria.
8. The formation of HMG CoA catalyzed by HMG CoA synthase is the rate-limiting step of ketone body formation.
9. Carnitine Palmitoyl transferase I is the rate-limiting step of fatty acid oxidation and is inhibited by malonyl CoA.
10. Hepatocytes can't utilize ketone bodies due to deficiency of thiophorase.
11. The location of fatty acid oxidation is the mitochondrial matrix.

Summary

• TAG digestion begins with emulsification and partial hydrolysis, and products are absorbed and packaged into lipoproteins which circulate in the plasma.
• Adipocytes secrete enzymes to release the fatty acids and then store them as TAG in large lipid droplets for later release during fasting or exercise.
• After diffusion into cells, fatty acids are activated, and their entry into mitochondria for oxidation is regulated.
• FA oxidation involves the sequential removal of two carbon units, producing reduced coenzymes, all of these can be further oxidized to produce ATP.
• During fasting/starvation, acetyl CoA from fatty acids can be converted into ketone bodies by the liver and used as an alternative fuel to glucose by other tissues, including the brain.