Tissues throughout the body 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, triacylglycerols are degraded to fatty acids and glycerol, which are released from the adipose tissue and transported to the energy-requiring tissues. 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.

Consider someone who has just awakened from a night’s sleep and begins a bout of exercise. Glycogen stores will be low, but lipids are readily available. How are these lipid stores mobilized?

Before fats can be used as fuels, the triacylglycerol storage form must be hydrolyzed to yield isolated fatty acids. This reaction is catalyzed by a hormonally controlled lipase. Under the physiological conditions facing an early-morning runner, glucagon and epinephrine will be present. In adipose tissue, these hormones trigger 7 TM receptors that activate adenylate cyclase (Section 14.1). 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 22.6). The phosphorylation of perilipin has two crucial effects. First, it restructures the fat droplet so that the triacylglycerols are more accessible to the mobilization. Second, the phosphorylation of perilipin triggers the release of a coactivator for adipose triglyceride lipase (ATGL). Once bound to the coactivator, 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. 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.

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If the coactivator required by ATGL is missing or defective, a rare condition (incidence unknown) 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, muscle weakness, and mild cognitive disability.

Fatty acids are not soluble in aqueous solutions. In order to reach tissues that require fatty acids, t he 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 22.7). The reverse process can take place by the reduction of dihydroxyacetone phosphate to glycerol 3-phosphate. Hydrolysis by a phosphatase then gives glycerol. Thus, glycerol and glycolytic intermediates are readily interconvertible.

Fatty acids separate from the albumin in the blood stream 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 occurs in the mitochondrial matrix, but in order to enter the mitochondria, the fatty acids are first activated through the formation of a thioester linkage to coenzyme A. Adenosine triphosphate (ATP) drives the formation of the thioester linkage between the carboxyl group of a fatty acid and the sulfhydryl group of coenzyme A. This activation reaction takes place on the outer mitochondrial membrane, where it is catalyzed by acyl CoA synthetase (also called fatty acid thiokinase).

Acyl CoA synthetase accomplishes the activation of a fatty acid in two steps. First, the fatty acid reacts with ATP to form an acyl adenylate. In this mixed anhydride, the carboxyl group of a fatty acid is bonded to the phosphoryl group of AMP. The other two phosphoryl groups of the ATP substrate are released as pyrophosphate. In the second step, the sulfhydryl group of coenzyme A attacks the acyl adenylate, which is tightly bound to the enzyme, 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. One high-transfer-potential compound is cleaved (between PP_i and AMP) and one high-transfer-potential compound is formed (the thioester acyl CoA). How is the overall reaction driven forward? The answer is that pyrophosphate is rapidly hydrolyzed by a pyrophosphatase. The complete reaction is

This reaction is quite favorable because the equivalent of two molecules of ATP is hydrolyzed, whereas only one high-transfer-potential compound is formed. We see here another example of a recurring theme in biochemistry: Many biosynthetic reactions are made irreversible by the hydrolysis of inorganic pyrophosphate.

Another motif recurs in this activation reaction. The enzyme-bound acyl adenylate intermediate is not unique to the synthesis of acyl CoA. Acyl adenylates are frequently formed when carboxyl groups are activated in biochemical reactions. Amino acids are activated for protein synthesis by a similar mechanism (Section 30.2), although the enzymes that catalyze this process are not homologous to acyl CoA synthetase. Thus, activation by adenylation recurs in part because of convergent evolution.

Fatty acids are activated on the outer mitochondrial membrane, whereas they are oxidized in the mitochondrial matrix. A special transport mechanism is needed to carry activated long-chain fatty acids across the inner mitochondrial membrane. These fatty acids must be conjugated to carnitine, an alcohol with both a positive and a negative charge (a zwitterion). The acyl group is transferred from the sulfur atom of coenzyme A 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.

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Acyl carnitine is then shuttled across the inner mitochondrial membrane by a translocase (Figure 22.8). The acyl group is transferred back to coenzyme A on the matrix side of the membrane. This reaction, which is catalyzed by carnitine acyltransferase II (carnitine palmitoyl transferase II), is simply the reverse of the reaction that takes place in the cytoplasm. The reaction is thermodynamically feasible because of the zwitterionic nature of carnitine. The O-acyl link in carnitine has a high group-transfer potential, apparently because, being zwitterions, carnitine and its esters are solvated differently from most other alcohols and their esters. Finally, the translocase returns carnitine to the cytoplasmic side in exchange for an incoming acyl carnitine.

A number of diseases have been traced to a deficiency of carnitine, the transferase, or the translocase. Inability to synthesize carnitine may be a contributing factor to the development of autism in males. The symptoms of carnitine deficiency range from mild muscle cramping to severe weakness and even death. 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. Medium-chain (C₈–C₁₀) fatty acids are oxidized normally in these patients because these fatty acids can enter the mitochondria, to some degree, in the absence of carnitine. These diseases illustrate that the impaired flow of a metabolite from one compartment of a cell to another can lead to a pathological condition.

A saturated acyl CoA is degraded by a recurring sequence of four reactions: oxidation by flavin adenine dinucleotide (FAD), hydration, oxidation by NAD⁺, and thiolysis by coenzyme A (Figure 22.9). 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.

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.

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 from the FADH₂ prosthetic group of the reduced acyl CoA dehydrogenase are transferred to a second flavoprotein called electron-transferring flavoprotein (ETF). In turn, ETF donates electrons to ETF: ubiquinone reductase, an iron–sulfur protein. Ubiquinone is thereby reduced to ubiquinol, which delivers its high-potential electrons to the second proton-pumping site of the respiratory chain (Section 18.3). Consequently, 1.5 molecules of ATP are generated per molecule of FADH₂ formed in this dehydrogenation step, as in the oxidation of succinate to fumarate.

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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 enzyme also hydrates a cis-Δ² double bond, but the product then is the d isomer. We shall return to this point shortly in considering how unsaturated fatty acids are oxidized.

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, which is specific for the l isomer of the hydroxyacyl substrate.

The preceding reactions have oxidized the methylene group 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.

Table 22.1 summarizes the reactions in fatty acid oxidation.

The shortened acyl CoA then undergoes another cycle of oxidation, starting with the reaction catalyzed by acyl CoA dehydrogenase (Figure 22.10). Fatty acid chains containing from 12 to 18 carbon atoms are oxidized by the long-chain acyl CoA dehydrogenase. The medium-chain acyl CoA dehydrogenase oxidizes fatty acid chains having from 14 to 4 carbons, whereas the short-chain acyl CoA dehydrogenase acts only on 4- and 6-carbon fatty acid chains. In contrast, β-ketothiolase, hydroxyacyl dehydrogenase, and enoyl CoA hydratase act on fatty acid molecules of almost any length.

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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 are formed.

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

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 seven FADH₂, 17.5 from the seven NADH, and 80 from the eight acetyl CoA molecules, 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 2 molecules of orthophosphate. Thus, the complete oxidation of a molecule of palmitate yields 106 molecules of ATP.