27.1 Fatty Acids Are Processed in Three Stages

✓ 1 Identify the repeated steps of fatty acid degradation.

Figure 27.1: Lipid degradation. Lipids are hydrolyzed by lipases in three steps to yield fatty acids and glycerol. The fatty acids are taken up by cells and used as a fuel. Glycerol also enters the liver, where it can be metabolized by the glycolytic or gluconeogenic pathways.

In Chapter 14, we examined how dietary triacylglycerols are digested, absorbed, transported, and stored. Now, we will examine how the stored triacylglycerols are made biochemically accessible. Peripheral tissues, such as muscle, gain access to the lipid energy reserves stored in adipose tissue through three stages of processing. First, the lipids must be mobilized. In this process of lipolysis, triacylglycerols are degraded to fatty acids and glycerol, which are released from the adipose tissue and transported to the energy-requiring tissues (Figure 27.1). Second, at these tissues, the fatty acids must be activated and transported into mitochondria for degradation. Third, the fatty acids are broken down in a step-by-step fashion into acetyl CoA, which is then processed in the citric acid cycle.

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!clinic! CLINICAL INSIGHT: Triacylglycerols Are Hydrolyzed by Hormone–Stimulated Lipases

Consider someone who has just awakened from a night’s sleep and begins a bout of exercise. After the night’s fast, glycogen stores are low, but lipids are readily available. How are these lipid stores mobilized to provide fuel for muscles and other tissues?

Triacylglycerols are stored inside a fat cell (adipocyte) as a lipid droplet, an intracellular compartment surrounded by a single layer of phospholipids and the numerous proteins required for fatty acid metabolism (Figure 11.3). Originally believed to be inert-lipid deposits, lipid droplets are now understood to be dynamic organelles essential for the regulation of lipid metabolism. Triacylglycerol mobilization and deposition take place on the surface of the droplet. Before fats can be used as fuels, the triacylglycerol storage form must be hydrolyzed to yield isolated fatty acids. Under the physiological conditions facing an early-morning runner, glucagon and epinephrine will be present. In adipose tissue, these hormones trigger 7TM receptors that activate adenylate cyclase. The increased level of cyclic AMP then stimulates protein kinase A, which phosphorylates two key proteins: perilipin, a fat-droplet-associated protein, and hormone-sensitive lipase (Figure 27.2). The phosphorylation of perilipin has two crucial effects. First, it restructures the fat droplet so that the triacylglycerols are more accessible to degradation. Second, the phosphorylation of perilipin triggers the release of a coactivator for adipose triglyceride lipase (ATGL). Once bound to the cofactor, ATGL initiates the mobilization of triacylglycerols by releasing a fatty acid from triacylglycerol, forming diacylglycerol. Diacylglycerol is converted into a free fatty acid and monoacylglycerol by the hormone-sensitive lipase, which has been activated by phosphorylation. Finally, a monoacylglycerol lipase completes the mobilization of fatty acids with the production of a free fatty acid and glycerol. Thus, epinephrine and glucagon induce lipolysis. Although their role in muscle is not as firmly established, these hormones probably also regulate the use of triacylglycerol stores in that tissue.

Figure 27.2: Triacylglycerols in adipose tissue are converted into free fatty acids in response to hormonal signals. The phosphorylation of perilipin restructures the lipid droplet and releases the coactivator of ATGL. The activation of ATGL by binding with its coactivator initiates the mobilization. Hormone-sensitive lipase releases a fatty acid from diacylglycerol. Monoacylglycerol lipase completes the mobilization process. Abbreviations: 7TM, seven transmembrane; ATGL, adipose triglyceride lipase; CA, coactivator; HS lipase, hormone-sensitive lipase; MAG lipase, monoacylglycerol lipase; DAG, diacylglycerol; TAG, triacylglycerol.

If the coactivator required by ATGL is missing or defective, a rare condition called Chanarin-Dorfman syndrome results. Fats accumulate throughout the body because they cannot be released by ATGL. Other symptoms include dry skin (ichthyosis), enlarged liver and muscle, and mild cognitive disability.

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Free Fatty Acids and Glycerol Are Released Into the Blood

Fatty acids are not soluble in aqueous solutions. In order to reach tissues that require fatty acids, the released fatty acids bind to the blood protein albumin, which delivers them to tissues in need of fuel.

Glycerol formed by lipolysis is absorbed by the liver and phosphorylated. It is then oxidized to dihydroxyacetone phosphate, which is isomerized to glyceraldehyde 3-phosphate. This molecule is an intermediate in both the glycolytic and the gluconeogenic pathways.

Hence, glycerol can be converted into pyruvate or glucose in the liver, which contains the appropriate enzymes (Figure 27.3). The reverse reaction can take place just as readily. Thus, glycerol and glycolytic intermediates are interconvertible.

Figure 27.3: Lipolysis generates fatty acids and glycerol. The fatty acids are used as fuel by many tissues. The liver processes glycerol by either the glycolytic or the gluconeogenic pathway, depending on its metabolic circumstances. Abbreviation: CAC, citric acid cycle.

Fatty Acids Are Linked to Coenzyme A Before They Are Oxidized

Fatty acids separate from the albumin in the bloodstream and diffuse across the cell membrane with the assistance of transport proteins. In the cell, fatty acids are shuttled about in association with fatty-acid-binding proteins.

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Fatty acid oxidation takes place in mitochondria, so how do these fuels gain access to the site of degradation? First, fatty acids must be activated by reacting with coenzyme A to form acyl CoA. This activation reaction takes place on the outer mitochondrial membrane, where it is catalyzed by acyl CoA synthetase.

The activation takes place in two steps:

  1. The fatty acid reacts with ATP to form an acyl adenylate, and the other two phosphoryl groups of the ATP substrate are released as pyrophosphate:

  2. The sulfhydryl group of CoA then attacks the acyl adenylate to form acyl CoA and AMP:

    These partial reactions are freely reversible. In fact, the equilibrium constant for the sum of these reactions is close to 1, meaning that the energy levels of the reactants and products are about equal. The reaction is driven forward by the hydrolysis of pyrophosphate by pyrophosphatase:

    We see here another example of a recurring theme in biochemistry: many biosynthetic reactions are made irreversible by the hydrolysis of inorganic pyrophosphate. Thus, the complete reaction for fatty acid activation is

Activation is not the only step necessary to move fatty acids into the mitochondrial matrix. Activated fatty acids can cross the outer mitochondrial membrane through the voltage-dependent ion channels, also called porin channels. However, transport across the inner mitochondrial membrane requires that the fatty acids be linked to the alcohol carnitine. The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine to form acyl carnitine. This reaction is catalyzed by carnitine acyltransferase I (also called carnitine palmitoyl transferase I), which is bound to the outer mitochondrial membrane:

Figure 27.4: Acyl carnitine translocase. The entry of acyl carnitine into the mitochondrial matrix is mediated by a translocase. Carnitine returns to the cytoplasmic side of the inner mitochondrial membrane in exchange for acyl carnitine.

Acyl carnitine is then shuttled across the inner mitochondrial membrane by a translocase (Figure 27.4). The acyl group is transferred back to CoA by carnitine acyltransferase II (carnitine palmitoyl transferase II) on the matrix side of the membrane. Finally, the translocase returns carnitine to the cytoplasmic side in exchange for an incoming acyl carnitine, allowing the process to continue.

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!clinic! CLINICAL INSIGHT: Pathological Conditions Result if Fatty Acids Cannot Enter the Mitochondria

A number of diseases have been traced to a deficiency of carnitine, carnitine transferase, or translocase. The symptoms of carnitine deficiency range from mild muscle cramping to severe weakness and even death. Inability to synthesize carnitine may be a contributing factor to the development of autism in males. In general, muscle, kidney, and heart are the tissues primarily impaired. Muscle weakness during prolonged exercise is a symptom of a deficiency of carnitine acyltransferases because muscle relies on fatty acids as a long-term source of energy. These diseases illustrate that the impaired flow of a metabolite from one compartment of a cell to another can lead to a pathological condition. Carnitine is now popular as a dietary supplement, and its proponents claim that it increases endurance, enhances brain function, and promotes weight loss. The actual effectiveness of carnitine as a dietary supplement remains to be established.

Acetyl CoA, NADH, and FADH2 Are Generated by Fatty Acid Oxidation

After the activated fatty acid is in mitochondria, it is ready for metabolism. The goal of fatty acid degradation is to oxidize the fatty acid—two carbon atoms at a time—to acetyl CoA and to gather the released high-energy electrons to power oxidative phosphorylation. A saturated acyl CoA is degraded by a recurring sequence of four reactions: oxidation by flavin adenine dinucleotide (FAD), hydration, oxidation by nicotinamide adenine dinucleotide (NAD+), and thiolysis by coenzyme A (Figure 27.5). The fatty acid chain is shortened by two carbon atoms as a result of these reactions, and FADH2, NADH, and acetyl CoA are generated. Because oxidation takes place at the β-carbon atom, this series of reactions is called the β-oxidation pathway.

Figure 27.5: The reaction sequence for the degradation of fatty acids. Fatty acids are degraded by the repetition of a four-reaction sequence consisting of oxidation, hydration, oxidation, and thiolysis.

The first reaction in each round of degradation is the oxidation of acyl CoA by an acyl CoA dehydrogenase to give an enoyl CoA with a trans double bond between C-2 and C-3:

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DID YOU KNOW?

The symbol Δn (capital delta, superscript number) is used to denote the position of the first carbon atom participating in a double bond. Thus, Δ2 designates a double bond between carbon 2 and carbon 3.

As in the dehydrogenation of succinate in the citric acid cycle, FAD rather than NAD+ is the electron acceptor because the ΔG for this reaction is insufficient to drive the reduction of NAD+. Electrons picked up by FAD are transferred to the electron-transport chain and, ultimately, to ubiquinone, which is thereby reduced to ubiquinol. Then, ubiquinol delivers its high-potential electrons to the second proton-pumping site of the respiratory chain.

The next step is the hydration of the double bond between C-2 and C-3 by enoyl CoA hydratase:

The hydration of enoyl CoA is stereospecific. Only the l isomer of 3-hydroxyacyl CoA is formed when the trans-Δ2 double bond is hydrated.

The hydration of enoyl CoA is a prelude to the second oxidation reaction, which converts the hydroxyl group at C-3 into a keto group and generates NADH. This oxidation is catalyzed by l-3-hydroxyacyl CoA dehydrogenase:

The preceding reactions have oxidized the methylene group (—CH2—) at C-3 to a keto group. The final step is the cleavage of 3-ketoacyl CoA by the thiol group of a second molecule of coenzyme A, which yields acetyl CoA and an acyl CoA shortened by two carbon atoms. This thiolytic cleavage is catalyzed by β-ketothiolase:

Figure 27.6: The first three rounds in the degradation of palmitate. Two carbon units are sequentially removed from the carboxyl end of the fatty acid.

Table 27.1 summarizes the reactions in fatty acid degradation. The shortened acyl CoA then undergoes another cycle of oxidation, starting with the reaction catalyzed by acyl CoA dehydrogenase (Figure 27.6).

Table 27.1 Principal reactions required for fatty acid degradation

Recent evidence suggests that the enzymes of fatty acid oxidation are associated with one another to form a super complex. Moreover, this super complex is in turn associated with the inner mitochondrial membrane, the site of the electron-transport chain and ATP synthesis. This organization allows the rapid movement of substrates from enzyme to enzyme and gives the high-energy electrons generated by fatty acid oxidation immediate access to the electron-transport chain.

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The Complete Oxidation of Palmitate Yields 106 Molecules of ATP

We can now calculate the energy yield derived from the oxidation of a fatty acid. In each reaction cycle, an acyl CoA is shortened by two carbon atoms, and one molecule each of FADH2, NADH, and acetyl CoA is formed:

The degradation of palmitoyl CoA (C16-acyl CoA) requires seven reaction cycles. In the seventh cycle, the C4-ketoacyl CoA is cleaved by the thiol group of coenzyme A to two molecules of acetyl CoA. Hence, the stoichiometry of the oxidation of palmitoyl CoA is

!quickquiz! QUICK QUIZ 1

Describe the repetitive steps of β oxidation. Why is the process called β oxidation?

Approximately 2.5 molecules of ATP are generated when the respiratory chain oxidizes each of these NADH molecules, whereas 1.5 molecules of ATP are formed for each FADH2 because their electrons enter the chain at the level of ubiquinol. Recall that the oxidation of acetyl CoA by the citric acid cycle yields 10 molecules of ATP. Hence, the number of ATP molecules formed in the oxidation of palmitoyl CoA is 10.5 from the 7 molecules of FADH2, 17.5 from the 7 molecules of NADH, and 80 from the 8 molecules of acetyl CoA, which gives a total of 108. The equivalent of 2 molecules of ATP is consumed in the activation of palmitate, in which ATP is split into AMP and two molecules of orthophosphate. Thus, the complete oxidation of a molecule of palmitate yields 106 molecules of ATP.