Chapter 22_ Oxidation of Fatty Acids_ Ketogenesis

Chapter 22: Oxidation of Fatty Acids: Ketogenesis

Objectives

  • Fatty Acid Transport and Activation

    • Describe the processes for transporting fatty acids in the blood.

    • Explain activation and transport of fatty acids into mitochondria.

  • β-Oxidation Pathway

    • Outline the β-oxidation pathway leading to acetyl-CoA production.

    • Explain ATP yield from fatty acid metabolism.

  • Ketone Bodies

    • Identify the three ketone bodies: acetoacetate, D-3-hydroxybutyrate, acetone.

    • Describe formation reactions in liver mitochondria.

    • Recognize conditions favoring ketone body synthesis.

  • Regulation of Ketogenesis

    • Indicate three stages of fatty acid metabolism regulation.

    • Understand risks of overproduction leading to ketosis and ketoacidosis.

    • Identify diseases linked to impaired fatty acid oxidation.

Biomedical Importance

  • Fatty acids are oxidized in mitochondria to acetyl-CoA, generating high energy.

  • Ketone Bodies and Their Role

    • Produced during high rates of fatty acid oxidation (acetoacetate, D-3-hydroxybutyrate, acetone).

    • Ketosis occurs during starvation and diabetes, leading to potential ketoacidosis due to the acidic nature of ketone bodies.

  • Impaired fatty acid oxidation can result in hypoglycemia, particularly in conditions of carnitine deficiency or enzyme deficiencies.

Fatty Acid Oxidation in Mitochondria

  • Separation of Processes

    • Breakdown of fatty acids is distinct from fatty acid synthesis, allowing individual control.

  • Fatty acids are transported as Free Fatty Acids (FFA).

    • Long-chain FFA bind to albumin in plasma, while shorter chains are more soluble.

Activation of Fatty Acids

  • Fatty acids must convert to acyl-CoA for catabolism (requires ATP).

    • Catalyzed by acyl-CoA synthetase, producing AMP and PPi.

    • PPi is hydrolyzed by pyrophosphatase, driving the reaction forward.

  • Role of Carnitine in Transport

    • Acyl-CoA cannot cross the inner mitochondrial membrane; converted to acylcarnitine.

    • Carnitine palmitoyltransferase-I transfers acyl groups to carnitine.

    • Acylcarnitine is transported into the matrix and converted back into acyl-CoA by carnitine palmitoyltransferase-II.

β-Oxidation of Fatty Acids

  • Mechanics of β-Oxidation

    • Two-carbon units are cleaved from acyl-CoA sequentially, forming acetyl-CoA.

    • Each cycle produces FADH2 and NADH, which generate ATP through oxidative phosphorylation.

  • The cycle requires oxygen and is catalyzed by specific enzymes at each step.

ATP Yield from Fatty Acid Oxidation

  • Complete oxidation of palmitate (C16) generates significant ATP (106 mol net).

    • Breakdown involves seven cycles producing 8 mol of acetyl-CoA.

    • FADH2 and NADH contribute to ATP synthesis.

Modified β-Oxidation in Peroxisomes

  • Shortening Very-Long-Chain Fatty Acids

    • Modified β-oxidation exists in peroxisomes for very long-chain fatty acids.

    • Does not generate ATP directly but forms acetyl-CoA and H2O2.

Ketonogenesis Under High Fatty Acid Oxidation Rates

  • Ketone Bodies Formation

    • Under high fatty acid oxidation, acetoacetate and D-3-hydroxybutyrate are formed in the liver.

    • Extrahepatic tissues utilize these as energy substrates.

    • Elevated levels can lead to ketosis, especially under fasting conditions.

Regulation of Ketogenesis

  1. FFA Mobilization from Adipose Tissue

    • Involves lipolysis in response to energy needs.

  2. Transport into Mitochondria

    • Carnitine palmitoyltransferase-I mediates the entry of fatty acids.

  3. Partition of Acetyl-CoA

    • Determines flow between ketogenesis and the citric acid cycle.

    • Regulation impacts energy yield per oxidation.

Clinical Aspects

  • Diseases Related to Fatty Acid Oxidation

    • Carnitine deficiencies lead to hypoglycemia.

    • Inherited deficiencies such as CPT-I or CPT-II affect energy metabolism.

    • Jamaican vomiting sickness and dicarboxylic aciduria highlight pathological cases.

  • Ketoacidosis

    • Prolonged elevation of ketone bodies leads to acidosis, especially in diabetes.

Summary

  • Fatty acid oxidation yields ATP through the β-oxidation and citric acid cycle processes.

  • Ketogenesis serves a critical role in energy metabolism, particularly when glucose is low.

  • Diseases associated with disturbed fatty acid oxidation demonstrate the pathway's clinical significance.