Medical Physiology, 3rd Edition

Energy Liberation (Catabolism)

The general principle in energy catabolism is that the body first breaks down a complex storage polymer (e.g., glycogen or TAGs) to simpler compounds (e.g., glucose, FAs, lactate) that the cells can then metabolize to provide energy, mostly in the form of ATP, for cellular function.

The first step in energy catabolism is to break down glycogen or TAGs to simpler compounds

Skeletal Muscle

Glycogenolysis in skeletal muscle is catalyzed by glycogen phosphorylase (GP; Fig. 58-9A). In three ways, muscle contraction activates GP. First, epinephrine binds to a β-adrenergic receptor and thereby promotes the formation of cAMP (see p. 53). The cAMP activates PKA, which in turn phosphorylates glycogen phosphorylase kinase (PK). In parallel, PKA inactivates phosphoprotein phosphatase 1 (see Fig. 3-7), thereby stabilizing the phosphorylated state of PK. The now active form of PK converts the inactive glycogen phosphorylase b (GPb) to the active GPa. Second, muscle activity increases the turnover of ATP, thereby raising [AMP]i; the binding of AMP allosterically activates GPb. Conversely, at rest, ATP competes with AMP for this nucleotide-binding site on GPb and thus inhibits GPb. Third, intense activity of skeletal muscle causes [Ca2+]i to rise; the binding of Ca2+ to PK allosterically activates this enzyme, imageN58-13 and converts GPb to the active GPa. Skeletal muscle converts the product of glycogenolysis, glucose-1-phosphate (G1P), to G6P, which enters the glycolytic pathway within the muscle cell.

image

FIGURE 58-9 Mobilization of energy stores by epinephrine and glucagon. A, In muscle, epinephrine promotes glycogenolysis and glycolysis, thereby producing ATP (for muscle contraction) and lactate. See Figure 3-7 for a representation of how PKA inactivates phosphoprotein phosphatase 1 (PP1). B, In liver, primarily glucagon and also epinephrine trigger glucose production in the short term via glycogenolysis, and over the long term via gluconeogenesis. Because they have G6Pase, hepatocytes can generate glucose and export it to the blood. C, In adipocytes, epinephrine triggers production of FAs and glycerol, which leave the adipocytes and enter the blood. GS, glycogen synthase; HSL, hormone-sensitive triacylglycerol lipase; UDP, uridine diphosphate. Yellow halos surround the active forms of enzymes.

N58-13

Glycogen Phosphorylase Kinase

Contributed by Kitt Petersen, Gerald Shulman

PK has the subunit structure (αβγδ)4. The α and β subunits are substrates for PKA. The δ subunits are in fact calmodulin (CaM). In other words, PK is a Ca2+-CaM–dependent kinase (i.e., a CaM kinase). Thus, an increase in [Ca2+]i can activate PK allosterically and thereby promote the conversion of phosphorylase b to phosphorylase a, which favors glycogenolysis.

Liver

As is the case in muscle, GPa in hepatocytes converts glycogen to G1P. The signaling pathways that establish the GPa/GPb ratio are the same as in muscle, except that in the liver, it is mainly glucagon (see pp. 1050–1053) in the portal blood—and, to a lesser extent, epinephrine—that triggers the increase in [cAMP]i (see Fig. 58-9B). The liver then converts G1P to G6P, as in muscle. However, unlike muscle, liver contains the enzyme G6Pase; G6Pase cleaves the phosphate from G6P to yield glucose, which is free to enter the bloodstream. Thus, whereas glycogenolysis in skeletal muscle serves to meet local energy demands by releasing G6P, glycogenolysis in the liver serves to meet whole-body energy demands—mainly those of the central nervous system (CNS)—by releasing glucose to the blood.

Adipocytes

In adipocytes, HSL catalyzes lipolysis, the hydrolysis of the ester linkages of TAGS to release FAs and glycerol. Nearly all (95%) of the available energy of TAGS resides in the FA moieties. Two hormones can stimulate lipolysis: epinephrine, which the adrenal medulla secretes under conditions of low blood glucose or stress, and growth hormone (see pp. 990–995). As in skeletal muscle and hepatocytes, epinephrine acts through cAMP (see Fig. 58-9C). The result is activation of HSL and release of FAs, which exit the adipocytes into the bloodstream. There, the poorly soluble FAs bind to circulating albumin, which releases them at the sites of energy demand.

The second step in TAG catabolism is β-oxidation of FAs

In carbohydrate catabolism, after the breakdown of glycogen, the second step is glycolysis. In TAG metabolism, after the breakdown to FAs, the second step is the β-oxidation of FAs, which takes place in the mitochondrial matrix. (Recall that FA synthesis takes place in the cytoplasm; see p. 1178.) Before β-oxidation, the hepatocyte uses FA transport protein 5 (FATP5 or SLC27A5) to transport the FA from the extracellular fluid into the cytosol (Fig. 58-10), where acyl CoA synthase activates the FA to acyl CoA (i.e., the FA chain coupled to CoA). To deliver acyl CoA to the mitochondrial matrix, the cell uses carnitine acyltransferase I (CAT I) on the cytosolic side of the mitochondrial outer membrane to transfer the acyl group to carnitine. The resulting acylcarnitine moves through a porin (see p. 109) in the mitochondrial outer membrane to enter the intermembrane space. The carnitine/acylcarnitine transporter (CAC, SLC25A20) on the mitochondrial inner membrane moves acylcarnitine into the mitochondrial matrix. There, CAT II transfers the acyl group back to CoA to form acyl CoA and carnitine. The carnitine recycles to the cytosol via CAC and the porin, whereas acyl CoA undergoes β-oxidation.

image

FIGURE 58-10 FA transport into mitochondrion and β-oxidation. FAD/FADH2, oxidized and reduced forms of flavin adenine dinucleotide, respectively; NAD+/NADH, oxidized and reduced forms of nicotinamide adenine dinucleotide, respectively.

β-oxidation is a multistep process that removes a 2-carbon fragment from the end of an acyl CoA and releases the fragment as an acetyl CoA (see Fig. 58-10). The process also releases one reduced flavin adenine dinucleotide (FADH2), one NADH, and the remainder of the acyl chain (beginning with the original β carbon), which serves as the starting point for the next cycle. β-oxidation continues until it consumes the entire FA chain. For an FA chain containing n carbons, the number of cycles is ([n/2] − 1). The final cycle generates two acetyl CoA molecules. Unlike the breakdown of glucose, which can yield ATP even in the absence of O2 (via glycolysis), the catabolism of FAs to yield energy in the form of ATP can occur only in the presence of O2.

Malonyl CoA plays a central role in regulating the balance between FA synthesis and β-oxidation. When energy levels are high, the enzyme ACC generates malonyl CoA from acetyl CoA, as shown in Figure 58-7. In turn, malonyl CoA provides the 2-carbon building blocks for FA synthesis. In contrast, malonyl inhibits CAT I and thereby inhibits β-oxidation (see Fig. 58-10).

The final common steps in oxidizing carbohydrates, TAGs, and proteins to CO2 are the citric acid cycle and oxidative phosphorylation

Under aerobic conditions, cells containing mitochondria typically convert most of the pyruvate they generate from carbohydrate metabolism to acetyl CoA, rather than to lactate. Pyruvate—a 3-carbon piece of glucose's carbon skeleton—moves from the cytoplasm into the mitochondrial matrix (Fig. 58-11). There, pyruvate is oxidatively decarboxylated to acetyl CoA, which releases CO2 as well as an NADH. Acetyl CoA also forms in the mitochondria as the end product of the β-oxidation of FAs as well as amino-acid breakdown. The metabolism of the acetate moiety of acetyl CoA, as well as the oxidation of NADH and FADH2, is the final common pathway of aerobic catabolism, which releases CO2, H2O, ATP, and heat.

image

FIGURE 58-11 Citric acid cycle. FAD/FADH2, oxidized and reduced forms of flavin adenine dinucleotide, respectively; NAD+/NADH oxidized and reduced forms of nicotinamide adenine dinucleotide, respectively.

Citric Acid Cycle

A 2-carbon fragment—acetyl CoA—derived from glucose, FA, or amino-acid metabolism enters the phase of catabolism known as the citric acid cycle, also called internal respiration (see p. 590) or oxidative decarboxylation because the 2-carbon fragment ends up as two CO2 molecules (see Fig. 58-11). The citric acid cycle conserves the liberated energy as GTP and the reduced electron carriers NADH and FADH2.

Cells tightly control the citric acid cycle by three mechanisms that limit substrate flux through the citric acid cycle: substrate availability, product accumulation, and feedback inhibition of key enzymes. Cells regulate the citric acid cycle at four sites. The two entry-point enzymes, pyruvate dehydrogenase and citrate synthase, are the most important. Pyruvate dehydrogenase (PDH)imageN58-14 is inhibited allosterically by two of its immediate products (NADH and acetyl CoA) and one downstream product (ATP). The second primary site of regulation in the citric acid cycle is citrate synthase, which is under feedback inhibition of several immediate and downstream products (citrate, NADH, succinyl CoA, ATP). The citrate produced in this reaction feeds back negatively on PFK (see p. 1178), which effectively couples glycolysis and oxidative metabolism (see Fig. 58-6). Two other enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, may also be rate limiting for the citric acid cycle under the right conditions.

N58-14

Regulation of Pyruvate Dehydrogenase

Contributed by Ethan Nadel

The PDH enzyme complex is a massive assembly of three different enzymes (a total of 132 monomers) that catalyzes the conversion of pyruvate to acetyl CoA. It is inhibited allosterically by two of its immediate products (NADH and acetyl CoA) and one downstream product (ATP). The PDH reaction is also regulated by phosphorylation/dephosphorylation reactions that are catalyzed by kinases and phosphatases that are additional components of the enzyme complex. When [ATP]i is high, a protein kinase phosphorylates one of the proteins in the complex, thereby inactivating it and stopping the reaction. Conversely, when [ATP]i is low, a phosphatase hydrolyzes a phosphate group from this same enzyme, activating it and allowing the PDH enzyme complex reaction to proceed.

Oxidative Phosphorylation

The process by which mitochondria retrieve energy from FADH2 and NADH is oxidative phosphorylation. These reduced nucleotides are products of glycolysis (see Fig. 58-6), β-oxidation (see Fig. 58-10) conversion of pyruvate to acetyl CoA (see Fig. 58-11), and the citric acid cycle. Oxidative phosphorylation involves the transfer of electrons along a chain of molecules, and it ultimately traps the releasing energy in the formation of 1.5 ATP molecules per FADH2 and 2.5 ATP molecules per NADH (see Fig. 5-9B). imageN58-15

N58-15

Shuttle Systems for Moving Reducing Equivalents

Contributed by Emile Boulpaep, Walter Boron

As outlined in Figure 5-9B in the text, the electron transport chain of the mitochondrion uses the energy stored in the reducing equivalents NADH (and FADH2)—which themselves must be within the mitochondrial matrix—to pump H+ from the mitochondrial matrix to the mitochondrial intermembrane space (i.e., the space between the inner and outer mitochondrial membranes). Later, these H+ flow back into the mitochondrial matrix “downhill” through ATP synthase, which uses the recovered energy to convert ADP and Pi to ATP.

As outlined in Figure 58-11 and Table 58-4, most of these reducing equivalents (i.e., NADH and FADH2) originate in the mitochondrial matrix. In other words, they are created on the same side of the mitochondrial inner membrane on which they will be used. However, the two molecules of NADH that arise from the glycolysis of one glucose molecule (see Fig. 58-6A) are created in the cytoplasm. Thus, the cell must somehow move these reducing equivalents across the mitochondrial inner membrane and into the mitochondrial matrix before the mitochondrion can use them to generate ATP. However, the cell cannot move the reducing equivalents directly across the mitochondrial inner membrane. Instead, it must use one of two complicated shuttle systems to do the job. One is called the malate-aspartate shuttle, and the other is called the glycerol-3-phosphate shuttle. The efficiency of the two shuttle systems is quite different. The malate-aspartate shuttle is the more efficient of the two, ultimately yielding 2.5 ATPs per NADH. The glycerol-3-phosphate shuttle yields only 1.5 ATPs per NADH.

Malate-Aspartate Shuttle

Most cells—particularly heart, kidney, and liver—use the malate-aspartate shuttle system. The first step is catalyzed by malate dehydrogenase in the intermembrane space (IS):

image (NE 58-1)

Next, the malate–α-ketoglutarate exchanger in the mitochondrial inner membrane moves the malate into the mitochondrial matrix (MM) in exchange for α-ketoglutarate (αKG):

image (NE 58-2)

Next, malate dehydrogenase in the mitochondrial matrix regenerates the NADH:

image (NE 58-3)

This NADH can now enter the electron-transport chain. However, several more steps are required to complete the shuttle. Next, aspartate aminotransferase in the mitochondrial matrix consumes the oxaloacetate generated in Equation NE 58-3:

image (NE 58-4)

This is the αKG that exchanges for malate in Equation NE 58-2. In the meantime, the glutamate-aspartate exchanger in the mitochondrial inner membrane moves the newly formed aspartate out of the mitochondrion in exchange for glutamate:

image (NE 58-5)

This imported glutamate is the substrate in Equation NE 58-4. Finally, the enzyme aspartate aminotransferase in the intermembrane space catalyzes the following reaction:

image (NE 58-6)

Equation NE 58-6 (which takes place in the intermembrane space) is the inverse of Equation NE 58-4 (which takes place in the mitochondrial matrix).

Summing these six reactions, you will see that the net effect is to transfer NADH from the intermembrane space to the mitochondrial matrix. Because the NADH enters the electron transport chain at complex I (see Fig. 5-9B), the electron transport chain can use the energy stored in the NADH molecule to pump 10 H+ ions out of the mitochondrion across the mitochondrial inner membrane. Because the net stoichiometry of ATP synthesis is one ATP for every four H+, the NADH that passes through the glycerol-3-phosphate shuttle generates 10/4 or 2.5 ATPs.

Glycerol-3-Phosphate Shuttle

The brain and skeletal muscle use a shuttle system that—in terms of the number of ATPs generated per NADH—is only 60% as efficient as the malate-aspartate shuttle. In the first step, the cytosolic enzyme glycerol-3-phosphate dehydrogenase simultaneously converts NADH + H+ to NAD+ and converts dihydroxyacetone phosphate (DHAP, an intermediate of glycolysis; see Fig. 58-6B) to glycerol-3-phosphate:

image (NE 58-7)

Next, another glycerol-3-phosphate dehydrogenase—present on the outer surface of the mitochondrial inner membrane—converts the glycerol-3-phosphate back to DHAP while simultaneously transferring two H to an FAD to form FADH2:

image (NE 58-8)

This FADH2 can then transfer a pair of electrons that eventually pass through complexes III and IV (see Fig. 5-9B), which can pump six H+ ions out of the mitochondrion across the mitochondrial inner membrane. Because the net stoichiometry of ATP synthesis is one ATP for every four H+, the NADH that passes through the glycerol-3-phosphate shuttle generates only 6/4 or 1.5 ATPs.

Reference

Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 3rd ed. Worth Publishers: New York; 2000.

Ketogenesis

Conditions such as prolonged fasting, consumption of a low-carbohydrate diet, or untreated diabetes mellitus lead to the production of three water-soluble byproducts of incomplete FA oxidation, substances collectively known as ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone. What the conditions have in common is the accelerated β-oxidation of FAs, which produces acetyl CoA faster than the citric acid cycle can consume it. In addition, accelerated gluconeogenesis steals the OA (see Fig. 58-6B) that would be the entry point for acetyl CoA to enter the citric acid cycle (see Fig. 58-11). As a result, excess acetyl CoA spills over into the production of the three ketone bodies, primarily by liver mitochondria. As indicated by the downward orange arrows in Figure 58-12, the first three reactions in ketogenesis have the net effect of condensing two molecules of acetyl CoA and one H2O into one molecule of acetoacetate, two molecules of HS CoA, and one H+. Thus, uncontrolled ketogenesis, which occurs in the absence of insulin (e.g., diabetic ketoacidosis), causes a metabolic acidosis (see p. 635). The second and third reactions are essentially irreversible. Next, the liver can either reduce acetoacetate to D-β-hydroxybutyrate or decarboxylate the acetoacetate to acetone. The body rids itself of the volatile acetone and CO2 molecules in the expired air. The acetone gives the breath a fruity odor that can be useful in the physical diagnosis of diabetic ketoacidosis.

image

FIGURE 58-12 Ketogenesis and metabolism of ketone bodies. NAD+/NADH oxidized and reduced forms of nicotinamide adenine dinucleotide, respectively.

Extrahepatic tissues—especially the CNS and striated muscle—can consume either one D-β-hydroxybutyrate or one acetoacetate molecule, as shown by the upward red arrows in Figure 58-12, to produce two acetyl CoA molecules that can then enter the citric acid cycle (see Fig. 58-11). The key reaction—catalyzed by β-ketoacyl CoA transferase—bypasses the two irreversible ketogenic reactions by transferring a CoA from succinyl CoA to acetoacetate. This reaction is itself essentially irreversible. The reason that ketone bodies flow from the liver to extrahepatic tissues is that β-ketoacyl CoA transferase is not present in hepatocytes.

Oxidizing different fuels yields similar amounts of energy per unit O2 consumed

The source with the greatest energy density (kcal/g) is saturated FAs (see Table 58-1), which have a high density of carbon and hydrogen that can be oxidized to CO2 and H2O. Accordingly, the complete combustion of 1 g of an energy-rich fuel (i.e., lipid) requires more O2. However, the energy yield per O2 is similar among fuels, because about the same amount of O2 is needed to oxidize each carbon and hydrogen. The energy yield per unit of O2 is only slightly greater for carbohydrate (5.0 kcal/L of O2) than for lipid (4.7 kcal/L O2). Carbohydrate, with its greater energy yield per O2, is the body's preferred fuel for combustion during maximal exercise when O2 availability is limited. However, fat is the preferred fuel during prolonged activity, when O2 is available and fuel sources are abundant.

The metabolism of glucose by aerobic glycolysis and the citric acid cycle is far more efficient in providing energy in the form of ATP than anaerobic glycolysis in that complete oxidation of 1 molecule of glucose to CO2 and H2O via oxidative metabolism provides 30 to 32 molecules of ATP, whereas metabolism of 1 molecule of glucose to lactate by anaerobic glycolysis yields only ~2 molecules of ATP (Table 58-4). However, anaerobic glycolysis has the major advantage of being able to supply much more ATP per unit time than oxidative metabolism of glucose or fat. In this way, energy from glycogen supplies energy for muscle contraction during intense activities, such as sprinting. Responsible for this intense work are fast-twitch type 2 fibers, which have a much greater glycolytic capability but much lower mitochondrial density than slow-twitch type 1 fibers. Unfortunately, muscle can support this type of activity only for several minutes before lactate accumulates and the muscle cramps, or the muscle exhausts its stored glycogen. In contrast, oxidative metabolism of FAs is the major mechanism for supporting exercising muscle during prolonged activity, such as running a marathon, which is mostly the work of the slow-twitch type 1 muscle fibers (see pp. 1209–1212).

TABLE 58-4

Generation of ATP from the Complete Oxidation of Glucose*

Glycolysis image

2 ATP

Glycolysis → 2 × 1 NADH image

3 or 5 ATP

2 × (Pyruvate → acetyl CoA) → 2 NADH image

5 ATP

Citric acid cycle → 2 × 1 GTP image

2 ATP

Citric acid cycle → 2 × 3 = 6 NADH image

15 ATP

Citric acid cycle → 2 × 1 = 2 FADH2 image

3 ATP

Total

30 or 32 ATP per glucose

*The calculations in this table assume that the oxidative phosphorylation of each NADH molecule produced in the cytosol (row 2) will yield 1.5 or 2.5 ATP molecules, depending on which shuttle system the cell uses to transfer the reducing equivalents from the cytosol into the mitochondria. The calculations also assume that the oxidative phosphorylation of each NADH produced in the mitochondria (rows 3 and 5) yields 2.5 ATPs, and the oxidative phosphorylation of each FADH2 produced in the mitochondria (row 6) yields 1.5 ATPs. imageN58-15

ox. phos., oxidative phosphorylation.

In the case of FA metabolism (see Fig. 58-10), each cycle of β-oxidation yields a total of 14 ATP molecules (Table 58-5). The total number of ATP molecules generated from the FA depends on the number of carbon atoms in the FA chain. For example, palmitic acid, a 16-carbon FA, needs seven β-oxidation cycles to form eight acetyl CoA molecules. The seven cycles generate 7 × 14 = 98 ATPs. The leftover acetyl CoA represents an additional 10 ATPs, for a total of 108 ATPs. Because the initial activation of the palmitate to palmitoyl CoA involves converting an ATP to AMP plus pyrophosphate—and regenerating ATP from AMP requires forming two high-energy phosphate bonds—the net ATP production is 108 − 2 = 106 ATPs per palmitate oxidized.

TABLE 58-5

Generation of ATP in One Cycle of β-Oxidation of a Fatty Acid*

*The above calculations assume that oxidative phosphorylation generates 2.5 ATP per NADH, and 1.5 ATP per FADH2. imageN58-15

imageox. phos., oxidative phosphorylation.

For each primary fuel source, Table 58-6 provides the respiratory quotient (RQ; see p. 681) or ratio of moles of CO2 produced per mole of O2 consumed at the tissue level. The RQ reflects the density of oxygen atoms in the fuel source. For example, with carbohydrates, the cell needs to supply only enough external O2 to oxidize the carbon atoms to CO2. The H2O is already built into the carbohydrate molecule, which has an H-to-O ratio of 2 : 1.

TABLE 58-6

Respiratory Quotients of the Major Foodstuffs

 

ENERGY DENSITY (kcal/g)

RQ

Carbohydrate

4.1

1.00

Protein

4.3

0.80–0.85

Lipid

9.4

0.70

For carbohydrate oxidation,

image

(58-4)

Because lipids contain so many fewer oxygen atoms, lipid oxidation requires more external O2.

For lipid oxidation,

image

(58-5)

The RQ for protein oxidation is 0.80 to 0.85.

Protein oxidation constitutes a very minor fraction of the total fuel oxidation in any tissue. The brain and anaerobic tissues normally use carbohydrate nearly exclusively and have an RQ of ~1.0. Most tissues oxidize both carbohydrate and fat, and the RQ reflects this mixture. The whole-body RQ following an overnight fast is ~0.8 for people eating a typical Western diet; individuals with lower lipid intake have a higher RQ (i.e., approaching the value of 1 for carbohydrates).