Unit 10
Lipid Metabolism

Overview

As are carbohydrates, lipids are important biochemical fuel molecules. Contrast the metabolism of a six carbon lipid (below) with the overall equation for the metabolism of glucose (Unit 6).

CH3 (CH2)4COOH + 8 O2 → 6 CO2 + 6H2O

Gram per gram, lipids provide almost twice as much ATP as do carbohydrates, and in some tissues—heart is a good example—lipids are the preferred metabolic fuel. Lipids are the major stored fuel in human beings.

In this unit, we will consider the conversion of fatty acids to acetyl-CoA—the molecule which enters the citric acid cycle in the middle stage of carbohydrate metabolism. We will also discuss the fate of excess acetyl-CoA, and consider fatty acid biosynthesis and cholesterol.

This unit is shorter than the one dealing with the metabolism of carbohydrates. In part, this is because many of the chemical strategies that have evolved to metabolize lipids are similar to those already discussed for carbohydrate. Furthermore, metabolism is convergent: all fuel molecules are directed either through the citric acid cycle or directly into the electron transport chain. Since we have already discussed the citric acid cycle and the electron transport chain, you already know half of lipid metabolism.

Unit 10 is divided into four lessons:

  1. Fatty Acid Metabolism (Lipids → Acetyl-CoA)
  2. Ketone Bodies—The Fate of Unused Acetyl-CoA
  3. Fatty Acid Biosynthesis
  4. Cholesterol

Objectives

After completing this unit, you should be able to

  1. explain how lipids are degraded to acetyl-CoA in vivo.
  2. describe how and why ketone bodies are produced.
  3. explain how fatty acid chains are synthesized in vivo.
  4. explain how cholesterol metabolism is regulated.
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Glossary

bile salt biological detergent; modified cholesterol
desaturase enzyme that can introduce a double bond into saturated fatty acids
endocytosis entry of exogenous molecules into cells by infolding of the cell membrane to form an internal vesicle
hypercholesterolemia elevated level of serum cholesterol; there is no “normal” cholesterol level as such; “elevated” is determined from mortality tables that relate serum cholesterol levels with incidence of heart disease
ketogenesis conversion of acetyl-CoA to acetoacetate and β-hydroxybutyrate in liver mitochondria
ketone body the ketone containing products of ketogenesis: acetoacetate, β-hydroxybutyrate, and acetone
lipolysis breakdown of triacylglycerols to glycerol + three fatty acids in vivo
malonyl-ACP three carbon starting material for lipid biosynthesis; ACP acyl carrier protein) forms a high-energy bond with the malonyl starting material
β-oxidation degradation of fatty acids to acetyl-CoA units
prostaglandin hormone-like compound that acts on cells in the same area in which it is synthesized (i.e., a local hormone)
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