Fatty Acid Metabolism -- Biochemistry -- NCBI Bookshelf

Chapter 22: Fatty Acid Metabolism

A3038

We turn now from the metabolism of carbohydrates to that of fatty acids. A fatty acid contains a long hydrocarbon chain and a terminal carboxylate group. Fatty acids have four major physiological roles. First, fatty acids are building blocks of phospholipids and glycolipids. These amphipathic molecules are important components of biological membranes, as we discussed in Chapter 12. Second, many proteins are modified by the covalent attachment of fatty acids, which targets them to membrane locations (Section 12.5.3). Third, fatty acids are fuel molecules. They are stored as triacylglycerols (also called neutral fats or triglycerides), which are uncharged esters of fatty acids with glycerol (Figure 22.1). Fatty acids mobilized from triacylglycerols are oxidized to meet the energy needs of a cell or organism. Fourth, fatty acid derivatives serve as hormones and intracellular messengers. In this chapter, we will focus on the oxidation and synthesis of fatty acids, processes that are reciprocally regulated in response to hormones.

22.0.1. An Overview of Fatty Acid Metabolism

Figure 22.2
Steps in Fatty Acid Degradation and Synthesis

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Figure 22.2

Steps in Fatty Acid Degradation and Synthesis

The two processes are in many ways mirror images of each other.

Fatty acid degradation and synthesis are relatively simple processes that are essentially the reverse of each other. The process of degradation converts an aliphatic compound into a set of activated acetyl units (acetyl CoA) that can be processed by the citric acid cycle (Figure 22.2

). An activated fatty acid is oxidized to introduce a double bond; the double bond is hydrated to introduce an oxygen; the alcohol is oxidized to a ketone; and, finally, the four carbon fragment is cleaved by coenzyme A to yield acetyl CoA and a fatty acid chain two carbons shorter. If the fatty acid has an even number of carbon atoms and is saturated, the process is simply repeated until the fatty acid is completely converted into acetyl CoA units.

Fatty acid synthesis is essentially the reverse of this process. Because the result is a polymer, the process starts with monomers—in this case with activated acyl group (most simply, an acetyl unit) and malonyl units (see Figure 22.2

). The malonyl unit is condensed with the acetyl unit to form a four-carbon fragment. To produce the required hydrocarbon chain, the carbonyl must be reduced. The fragment is reduced, dehydrated, and reduced again, exactly the opposite of degradation, to bring the carbonyl group to the level of a methylene group with the formation of butyryl CoA. Another activated malonyl group condenses with the butyryl unit and the process is repeated until a C₁₆ fatty acid is synthesized.

Contents

22.1 Triacylglycerols Are Highly Concentrated Energy Stores

22.2 The Utilization of Fatty Acids as Fuel Requires Three Stages of Processing

22.3 Certain Fatty Acids Require Additional Steps for Degradation

22.4 Fatty Acids Are Synthesized and Degraded by Different Pathways

22.5 Acetyl Coenzyme A Carboxylase Plays a Key Role in Controlling Fatty Acid Metabolism

22.6 Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory Enzyme Systems

Summary

Problems

Selected Readings