Topic 12 Acetyl-CoA and Lipid Metabolism Notes

Acetyl-CoA and Lipid Metabolism Overview

Introduction

  • Acetyl-CoA is a crucial metabolite not only derived from carbohydrates but also from fatty acids.

  • Fatty acids can be derived from stored triacylglycerols or from dietary sources.

Sources of Acetyl-CoA

  • Fatty Acid Breakdown: When energy is required, triacylglycerol in adipocytes is degraded by lipases into glycerol and three fatty acids.

    • Released fatty acids enter the bloodstream, bind to albumin (a transport protein), and serve as fuel primarily for skeletal muscle, heart, and liver (not usable by the brain).

    • Released glycerol serves as a substrate for gluconeogenesis.

Degradation of Fatty Acids

  • Fatty acids entering the cell are transported into the cytosol via specific transporters.

  • Activation of Fatty Acids: Fatty acids are activated with CoA through:

    • A reaction that converts ATP to AMP and inorganic pyrophosphate (PPi).

    • Results in the formation of acyl-CoA, which is then transported into the mitochondrial matrix.

  • In the mitochondrial matrix, acyl-CoA undergoes β-oxidation, which consists of four chemical reactions that sequentially remove two carbon units from the fatty acyl chain:

    • Products of β-oxidation include: for a fatty acid chain with 2N carbons

    • N molecule of Acetyl-CoA

    • N-1 molecule of QH2 (reduced coenzyme Q)

    • N-1 molecule of NADH

    • A shorter fatty acyl-CoA chain (2 carbons less than the original).

Net Reaction for Palmitoyl-CoA β-oxidation

  • For a saturated fatty acid like palmitoyl-CoA (with 16 carbons):

    • Overall Reaction:
      from acetyl-CoA present in the matrix is: Palmitoyl-CoA + 7 Q + 7 NAD+ + 7 CoA → 8 Acetyl-CoA + 7 QH2 + 7 NADH

    • Note: Water and protons are omitted in this equation for simplicity.

  • The produced Acetyl-CoA and NADH can be directly used in the citric acid cycle and electron transport chain respectively, while QH2 enters the electron transport chain via Complex III.

  • Energy Yield: Each molecule of NADH yields approximately 2.5 ATP and each QH2 yields approximately 1.5 ATP, similar to contributions from the citric acid cycle.

Special Cases in β-oxidation

  • Unsaturated Fatty Acids: The oxidation of unsaturated fatty acids yields fewer QH2 molecules and may require NADPH consumption, depending on the positions of the double bonds.

  • Odd-Chain Fatty Acids: End-oxidation results in the production of propionyl-CoA, which can be converted into succinyl-CoA and utilized in the citric acid cycle.

Related Learning Objectives

  • Metabolic Fates of Acetyl-CoA: Identify and relate different pathways where Acetyl-CoA is utilized.

  • Mitochondrial Role: Describe mitochondria’s significant functions in energy metabolism.

  • Triacylglycerol Breakdown Products: Explain how products of triacylglycerol breakdown are utilized in the body.

  • Fatty Acid Metabolism Summary: Explain how fatty acids are synthesized and degraded, including reactants and products without detailing stoichiometry.

  • Ketone Bodies: Discuss the metabolic role and production of ketone bodies, especially under energy-deficient conditions.

  • Pathway Regulation: Predict the favored metabolic pathways under specific conditions without the need to memorize individual regulators.

Ketone Bodies and Their Significance

  • The liver can convert Acetyl-CoA to ketone bodies

  • Main Ketone Bodies:

    • Acetone

    • Acetoacetate

    • β-hydroxybutyrate

  • These compounds are exported to tissues like the brain and muscle, where they can be converted back to Acetyl-CoA for ATP generation.

  • Importance for the Brain: Since fatty acids cannot cross the blood-brain barrier, ketone bodies enable the brain to utilize energy stored in fatty acids.

Fatty Acid Biosynthesis

  • Occurs when energy supply is high in the form of Acetyl-CoA, converting it into fatty acids for membrane lipids or stored as triacylglycerols.

  • Transport of Acetyl-CoA: Acetyl-CoA must be moved from the mitochondrial matrix to the cytosol for fatty acid biosynthesis, facilitated by the citrate transport system which consumes NADH and ATP but produces NADPH.

  • Fatty acid synthesis is performed by fatty acid synthase, a multifunctional enzyme that catalyzes multiple reactions.

    • Net Reaction for Palmitate Synthesis:

  • Although mainly palmitate (16 carbon fatty acid) is synthesized, enzymes exist to modify fatty acids in terms of length and degree of saturation, but humans lack the ability to introduce double bonds beyond carbons 9-10 from the carboxylic end.

  • Dietary Requirement: Humans must obtain omega-6 and omega-3 fatty acids from the diet due to these limitations in fatty acid elongation and desaturation.

Regulation of Fatty Acid Metabolism

  • Similar regulatory mechanisms are involved in both fatty acid oxidation and biosynthesis:

    • Covalent Modification: Based on hormone signals affecting enzyme activity.

    • Allosteric Regulation: Enzyme activity is also modulated this way.

    • Enzyme Availability: Additionally, varying amounts of enzymes regulate metabolic pathways.

  • Hormonal Influence:

    • Insulin: Inhibits lipase activity in adipocytes, preventing fatty acid release from triacylglycerols, promotes fatty acid uptake, and stimulates synthesis of triacylglycerols. Also enhances fatty acid biosynthesis in the liver.

    • Glucagon: Activates lipase, leading to the release of fatty acids from triacylglycerols, promoting mobilization of energy stores.