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Metabolism
Metabolism encompasses all chemical reactions in the body's cells, necessary to sustain life. These chemical processes allow the body to grow, reproduce, maintain structure, and respond to changes in its environment. These processes can be anabolic, in which energy is used in the formation of substances such as proteins or nucleic acids, or catabolic, in which organic substances are broken down during cellular respiration to harvest energy to support cellular processes such as biosynthesis, transport of molecules or ions across cell membranes, or locomotion. A healthy, sedentary young man weighing 70 kg requires 2100 kcal (~30 kcal/kg body weight) 
N58-1 to sustain resting metabolism for 1 day, an amount known as the resting metabolic rate (RMR). The number of calories increases with increased activity, illness, or other stress. For example, the metabolic rate can rise 2- to 3-fold with exposure to a cold environment, or up to 10-fold during heavy exercise. The basal metabolic rate (BMR) is a clinical term for metabolism that is measured under standardized conditions in which the subject (1) has had a full night of restful sleep, (2) has been fasting for 12 hours, (3) is in a neutral thermal environment (see p. 1196 ), (4) has been resting physically for 1 hour, and (5) is free of psychic and physical stimuli. In adults the BMR ( units: kilocalories per hour and per square meter of body surface area) is ~5% higher for males than for females and falls with age. The BMR is less than the RMR.
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Kleiber's Law
One gram of tissue from a small mammal has a higher resting metabolic rate (RMR; see [CR] ) than the same mass of tissue from a larger mammal (e.g., a human). By plotting heat production versus body weight for animals over a range of five orders of magnitude, Max Kleiber showed that metabolic rate (in kcal/day) is not proportional to body weight (in kg) but to body weight raised to the power 
.
Allometry is the study of how body size relates to various biological parameters (e.g., metabolic rate, anatomic parameters).
Regulation of energy metabolism in humans involves a complex interplay among ingested nutrients, hormones, and interorgan exchanges of substrates to maintain a constant and adequate supply of fuel for all organs of the body. Because energy acquisition by the body is intermittent, whereas energy expenditure is continuous, the body needs to store and then parcel out energy in a carefully coordinated fashion. Insulin (see pp. 1035–1050 ) is the key hormone that orchestrates this exchange and distribution of substrates between tissues under fed and fasting conditions. Glucagon (see pp. 1050–1053 ), catecholamines (see pp. 1030–1031 ), cortisol (see pp. 1018–1026 ), and growth hormone (see pp. 990–995 ) play major roles in energy regulation at times of acute energy needs, which occur during exercise, under conditions of stress, or in response to hypoglycemia. The major organs involved in fuel homeostasis are (1) the liver, which is normally the major producer of glucose; N58-2 (2) the brain, which in the fasted state is the major utilizer of glucose; and (3) the muscle and adipose tissue, which respond to insulin and store energy in the form of glycogen and fat, respectively.
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Role of the Kidneys in Fuel Homeostasis
Although the kidneys can also produce glucose by gluconeogenesis, their net contribution to whole-body glucose production is typically <10% except under conditions of prolonged fasting, when they can contribute up to 30% to 40%.
The purpose of this chapter is to review how humans utilize energy and the means by which the body manages its energy stores during times of feeding, fasting, and exercise.
Forms of Energy
Virtually all energy that sustains humans derives, directly or indirectly, from breaking and releasing the energy stored in carbon-carbon bonds that were created in plants during photosynthesis. Cellulose, the principal form of this stored energy in the biosphere, consists of polymers of glucose joined by β-1,4 linkages that humans cannot digest (see pp. 914–915 ). However, ruminants can degrade cellulose to glucose because they have cellulase-producing bacteria in their digestive tracts. Humans obtain their energy from food in three forms: (1) carbohydrates, (2) proteins, and (3) lipids. Moreover, each form consists of building blocks: monosaccharides (glucose, fructose, and galactose) for carbohydrates, amino acids for proteins, and fatty acids for lipids.
Carbohydrates, which exist in the body mainly in the form of glucose, contain 4.1 kcal/g of energy. The major storage form is glycogen, a polymer of glucose (10 6 to 10 8 Da) that consists of glucose molecules linked together by α-1,4 linkages in the straight portions of the polymer ( Fig. 58-1 ) and by α-1,6 linkages at the frequent branch points (see Fig. 45-3 ). Virtually all cells of the body store glycogen; the highest concentrations occur in liver and muscle. Cells store glycogen in cytoplasmic granules that also contain the enzymes needed for glycogen synthesis and degradation. Glycogen is highly hydrophilic, containing 1 to 2 g of water per gram of glycogen, and thus provides a handy storage depot for glucose without affecting the osmotic pressure of the intracellular space. However, this packaging of glycogen with water makes glycogen a relatively inefficient means of storing energy because it yields only 1 to 2 kcal for each gram of hydrated glycogen instead of the theoretical 4.1 kcal/g of dry carbohydrate. In contrast to the other potential stored forms of energy (lipid and protein), the liver can quickly break down glycogen by glycogenolysis to provide glucose for the brain during hypoglycemia. Similarly, muscle can quickly break down glycogen to glucose-6-phosphate (G6P) to provide the energy necessary to run a high-intensity anaerobic sprint.
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The Pentose Phosphate Pathway (or Hexose Monophosphate Shunt)
Figure 58-1 mentions that glucose-6-phosphate can have three major fates. The anabolic series of reactions summarized in this figure convert G6P to glycogen. The glycolytic pathway summarized in Figure 58-6 A is a catabolic pathway that converts G6P to pyruvate. The third fate— the pentose phosphate pathway —is another catabolic series of reactions that converts G6P to ribose-5-phosphate.
The pentose phosphate pathway has two major products, NADPH and ribose-5-phosphate. The cell can use the reducing equivalents in NADPH (i.e., energy “currency”) to reduce double bonds in the energy-consuming synthesis of FAs and steroids. These reactions are particularly important in such tissues as liver, adipose tissue, mammary gland, and adrenal cortex. Note that the cell cannot use NADH to create NADPH. Thus, the pentose phosphate pathway is critical. The second product of the pathway, ribose-5-phosphate, is important for the synthesis of ribonucleotides, which is particularly important in growing and regenerating tissues. The pentose phosphate pathway involves four reactions, the first and third of which involve the conversion of NADP + to NADPH and H + .
If the cell does not use the ribose-5-phosphate to generate ribonucleotides, the cell can use a complex series of reactions to convert the ribose-5-phosphate to fructose-6-phosphate. This sequence of reactions (i.e., from G6P to ribose-5-phosphate to fructose-6-phosphate) bypasses or “shunts” the conversion of G6P to fructose-6-phosphate, which would otherwise be catalyzed by phosphoglucose isomerase (see Fig. 58-6 A ). For this reason, the pentose phosphate pathway is also called the hexose monophosphate shunt. *
* Note that the term shunt is a bit of a misnomer, inasmuch as the “shunt” is not a shortcut from glucose-6-phosphate to fructose-6-phosphate (normally catalyzed in one step by phosphoglucose isomerase), but rather a lengthy detour!
However, the reader should be reassured that the shunt does not permit the cell to generate two NADPH molecules for free. Three G6P molecules (3 × 6 = 18 carbons) must traverse the hexose monophosphate shunt to generate six NADPH molecules (3 × 2 = 6) plus two fructose-6-phosphate (2 × 6 = 12 carbons) molecules, a single glyceraldehyde-3-phosphate (1 × 3 = 3 carbons), and three CO 2 molecules that arise from a decarboxylation reaction in the pentose phosphate pathway (3 × 1 = 3 carbons). If those three glucose molecules had gone through the classical glycolytic pathway, they would have generated 3 × 2 = 6 net ATPs and 6 NADHs (see Table 58-3 ). However, if those same three glucose molecules all go through the pentose phosphate pathway, the net result is only five ATPs, only five NADHs, but six NADPHs. Thus, the cell gives up only one ATP and one NADH for the sake of generating six NADPHs—not a bad deal for the cell!
The liver normally contains 75 to 100 g of glycogen but can store up to 120 g (8% of its weight) as glycogen. Muscle stores glycogen at much lower concentrations (1% to 2% of its weight). However, because of its larger mass, skeletal muscle has the largest store of glycogen in the body (300 to 400 g). N58-3 A typical 70-kg human has up to ~700 g of glycogen (~1% of body weight). Thus, the total energy stored in the body in the form of glycogen can be nearly 3000 kcal ( Table 58-1 ); this is still only a tiny fraction of that stored in the form of lipid, enough to supply resting metabolism for less than a day and a half, assuming 100% efficiency. Nonetheless, carbohydrate stores are essential because certain tissues, particularly the brain, rely heavily on carbohydrates for their fuel. Whereas muscle contains the largest store of glycogen in the body, this pool of glycogen cannot contribute directly to blood glucose in response to hypoglycemia because muscles lack glucose-6-phosphatase (G6Pase), which is necessary to convert G6P derived from glycogenolysis to glucose. Instead, the primary role of muscle glycogen is to supply energy locally for muscle contraction during intense exercise.
TABLE 58-1
Energy of Body Stores
| CHEMICAL | WEIGHT (kg) |
ENERGY DENSITY (kcal/g) |
ENERGY (kcal) |
|---|---|---|---|
| Glycogen | 0.7 | 1.5 * | 1,050 |
| Protein | 9.8/2 = 4.9 † | 4.3 | 21,000 |
| Lipid | 14 | 9.4 | 131,600 |
* Approximate energy density of hydrated glycogen.
† Because only half of this protein can be mobilized as a fuel source, the total yield is only ~21,000 kcal.
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Glycogen Supercompensation
Exercise induces an increase in GLUT4 expression in skeletal muscle, which leads to an increase in insulin-stimulated glucose uptake and glycogen accumulation. The effect wears off ~40 hours after exercise. Thus, after glycogen has been depleted by exercise, carbohydrate feeding results in enhanced glycogen accumulation, known as supercompensation. Under conditions of supercompensation (i.e., high-carbohydrate feeding following intense exercise), athletes can increase muscle glycogen content to up to 3% of body weight (rather than the usual 1% stated in the text).
If, after intense exercise, supercompensation of rats is prevented by feeding a carbohydrate-poor diet, the increase in GLUT4 expression is substantially prolonged (i.e., 66 versus only 40 hours following exercise).
References
-
Garcia-Roves PM, Han D-H, Song Z, et. al.: Prevention of glycogen supercompensation prolongs the increase in muscle GLUT4 after exercise. Am J Physiol Endocrinol Metab 2003; 285: pp. E729-E736.
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Ren J-M, Semenkovich CF, Gulve EA, et. al.: Exercise induces rapid increases in GLUT4 expression, glucose transport capacity, and insulin-stimulated glycogen storage in muscle. J Biol Chem 1994; 269: pp. 14396-14401.
Proteins are linear polymers of l -amino acids ( Fig. 58-2 ), which have the general molecular structure + H 3 N–HC(R)–COO − . Different functional R groups distinguish the 20 amino acids incorporated into nascent proteins during mRNA translation. In addition, four other amino acids are present in mature proteins: γ-carboxyglutamic acid, hydroxylysine, 4-hydroxyproline, and 3-hydroxyproline. However, these amino acids result from post-translational modification of amino acids that are already in the polypeptide chain. In α-amino acids, the amino group ( 
), the carboxyl group (–COO − ), and R all attach to the central or α-carbon atom. In proteins, the amino acids are linked together by peptide bonds that join the α-amino group of one amino acid with the α-carboxyl group of another. Nine of the amino acids are termed essential amino acids ( Table 58-2 ) because the body cannot synthesize them at rates sufficient to sustain growth and normal functions. Thus, we must obtain these amino acids in our diet.
TABLE 58-2
Essential and Nonessential α-Amino Acids
| ESSENTIAL | NONESSENTIAL |
|---|---|
|
|
Proteins contain 4.3 kcal/g, about the same as carbohydrates. A typical 70-kg human with 14% protein (9.8 kg)—only about half of which is available as a fuel source—can thus store ~21,000 kcal (see Table 58-1 ) in the form of available protein—which could potentially provide ~10 days' worth of energy. Unlike carbohydrate, protein is not a primary energy reserve in humans. Instead, proteins serve other important structural and functional roles. Structural proteins make up skin, collagen, ligaments, and tendons. Functional proteins include enzymes that catalyze reactions, muscle filaments such as myosin and actin, and various hormones. The body constantly breaks down proteins to amino acids, and vice versa, which allows cells to change their protein makeup as demands change. Thus, it is not surprising that protein catabolism makes only a small contribution—much less than 5%—to normal resting energy requirements. In contrast, during starvation, when carbohydrate reserves are exhausted, protein catabolism can contribute as much as 15% of the energy necessary to sustain the resting metabolic requirements by acting as major substrates for gluconeogenesis (see p. 1176 ).
In the healthy human adult who is eating a weight-maintaining diet, amino acids derived from ingested protein replenish those proteins that have been oxidized in normal daily protein turnover. Once these protein requirements have been met, the body first oxidizes excess protein to CO 2 and then converts the remainder to glycogen or triacylglycerols (TAGs).
Lipids are the most concentrated form of energy storage because they represent, on average, 9.4 kcal/g. Lipids are dietary substances that are soluble in organic solvents but not in water and typically occur in the form of TAGs ( Fig. 58-3 A ). The gastrointestinal (GI) tract breaks down ingested TAGs (see pp. 925–927 ) into fatty acids (FAs) and sn 2-monoacylglycerols (see Fig. 58-3 B ). FAs are composed of long carbon chains (14 to 24 carbon atoms) with a carboxyl terminus, and can be either saturated with hydrogen atoms or unsaturated (i.e., double bonds may connect one or more pairs of carbon atoms). When fully saturated, FAs have the general form CH 3 –(CH 2 ) n –COOH (see Fig. 58-3 C ).
In contrast to glycogen and protein, fat is stored in a nonaqueous environment and therefore yields energy very close to its theoretical 9.4 kcal/g of TAGs. This greater efficiency of energy storage provided by fat is crucial for human existence in that it allows for greater mobility and promotes survival during famine. Therefore, although humans have two large storage depots of potential energy (protein and fat), fat serves as the major expendable fuel source. Most of the body's fat depots exist in the subcutaneous adipose tissue layers, although fat also exists to a small extent in muscle and in visceral (deeper) depots in obese individuals. A typical 70-kg human with 20% fat (14 kg) thus carries 131,600 kcal of energy stored in adipose tissue (see Table 58-1 ). Assuming an RMR of 2100 kcal/day and 100% efficiency of converting the fat to energy, mobilization and subsequent oxidation of this entire depot could theoretically sustain the body's entire resting metabolic requirement for several weeks.
Energy Balance
Energy input to the body is the sum of energy output and storage
The first law of thermodynamics states that energy can be neither created nor destroyed; in a closed system, total energy is constant. This concept is illustrated in Figure 58-4 . Humans acquire all of their energy from ingested food, store it in different forms, and expend it in different ways. In the steady state, energy intake must equal energy output.
The GI tract breaks down ingested carbohydrates (see pp. 914–915 ), proteins (see pp. 920–921 ), and lipids (see p. 925 ) into smaller components and then absorbs them into the bloodstream for transport to sites of metabolism. For example, the GI tract reduces ingested carbohydrates to simple sugars (e.g., glucose, fructose), which are then transported to liver and muscle cells and either oxidized to release energy or converted to glycogen and fat for storage. Oxidation of fuels generates not only free energy but also waste products and heat (thermal energy).
The body's energy inputs must balance the sum of its energy outputs and the energy stored. When the body takes in more energy than it expends, the person is in positive energy balance and gains weight. In the case of adults, this gain is mostly in the form of fat. In healthy children during growth periods, this gain is mostly in the form of muscle, organ, and bone growth. Conversely, when energy intake is less than expenditure, this negative energy balance leads to weight loss, mostly from fat and, to a lesser extent, from protein in muscle.
A person can gain or lose weight by manipulating energy intake or output. An optimal strategy to encourage weight loss involves both increasing energy output and reducing energy intake. In most people, a substantial decrease in energy intake alone leads to inadequate nutrient intake, which can compromise bodily function.
Nitrogen balance —the algebraic sum of whole-body protein degradation and protein synthesis —is an indication of the change in whole-body protein stores. It is estimated from dietary protein intake and urinary nitrogen (i.e., urea) excretion. Children eating a balanced diet are in positive nitrogen balance because they store amino acids as protein in the process of growth. Patients who have suffered burns or trauma are usually in negative nitrogen balance because of the loss of lean (mostly muscle) body mass.
The inefficiency of chemical reactions leads to loss of the energy available for metabolic processes
The second law of thermodynamics states that chemical transformations always result in a loss of the energy available to drive metabolic processes—the Gibbs free energy (G). The total internal energy (E) of the human body is the sum of the disposable or free energy (G) plus the unavailable or wasted energy, which ends up as heat (i.e., the product of absolute temperature, T, and entropy, S):
For example when you ingest glucose, the total internal energy increases by a small amount (ΔE). Some of this energy will be stored as glycogen (ΔG), and some will be wasted as heat (T ⋅ ΔS). According to Equation 58-1 , as long as the temperature is constant, the change in total internal energy will have two components:
Thus, some of the increased total energy (ΔE) will be stored as glycogen (ΔG). However, because of the inefficiencies of the chemical reactions that convert glucose to glycogen, some of the ΔE is wasted as heat (T ⋅ ΔS). Another way of stating the second law is that T ⋅ ΔS can never be zero or negative, and chemical reactions can never be 100% efficient.
If we add no energy to the body (i.e., ΔE is zero), the body's total free energy must decline (i.e., ΔG is negative). This decline in G matches the rise in T ⋅ S, reflecting inefficiencies inherent in chemical transformations. Consider, for example, what would happen if you took 1 mole of glucose (180 g), put it into a bomb calorimeter with O 2 , and completely burned the glucose to CO 2 and H 2 O. This combustion would yield 686 kcal in the form of heat but would conserve no usable energy. Now consider what happens if your body burns this same 1 mole of glucose. In contrast to the bomb calorimeter, your mitochondria not only would oxidize glucose to CO 2 and H 2 O but also would conserve part of the free energy in the form of ATP. Each of the many chemical conversion steps from glucose to CO 2 and H 2 O makes available a small amount of the total energy contained in glucose. Converting 1 mole of ADP and inorganic phosphate (P i ) to 1 mole of ATP under the conditions prevailing in a cell consumes ~11.5 kcal/mole. Therefore, if a particular step in glucose oxidation releases at least 11.5 kcal/mole, it can be coupled to ATP synthesis. The conversion of the lower-energy ADP to the higher-energy ATP traps energy in the system, thus conserving it for later use. The cellular oxidation of 1 mole of glucose conserves ~400 kcal of the potential 686 kcal/mole; the remaining 286 kcal/mole is liberated as heat.
Free energy, conserved as high-energy bonds in ATP, provides the energy for cellular functions
ATP consists of a nitrogenous ring (adenine), a 5-carbon sugar (ribose), and three phosphate groups ( Fig. 58-5 ). The last two phosphates are connected to the rest of the molecule by high-energy bonds. The same is true for a related nucleotide, GTP. If we compare the free energies of phosphate bonds of various molecules, we see that the high-energy phosphate bonds of ATP lie toward the middle of the free-energy scale. Thus, in the presence of P i , ADP can accept energy from compounds that are higher on the free-energy scale (e.g., phosphocreatine), whereas ATP can release energy in the formation of compounds that are lower on the free-energy scale (e.g., G6P). ATP can therefore store energy derived from energy-releasing reactions and release energy needed to drive other chemical reactions.
Examples of the chemical reactions fueled by converting ATP to ADP and P i include the formation of bridges between actin and myosin during muscle contraction and the pumping of Ca 2+ against its electrochemical gradient during muscle relaxation. N58-4
N58-4
Hydrolysis of ATP
Although the conversion of ATP to ADP and P i is often referred to as a hydrolysis reaction, because the traditional representation is ATP + H 2 O → ADP + P i + H + , the reaction actually occurs in two steps. The first typically involves transfer of a part of the phosphate group to an intermediate molecule, thus increasing the intermediate's free-energy content. The second step involves displacing the phosphate moiety, which releases P i and energy. In contrast, true hydrolysis reactions merely release heat, which cannot be trapped to drive chemical processes.
Energy Interconversion From Cycling between 6-Carbon and 3-Carbon Molecules
Some metabolic reactions are neither uniquely anabolic nor catabolic, but they serve to interconvert the carbon skeletons of the building blocks of the three major energy forms—carbohydrates, proteins, and lipids. In this subchapter, we focus on two major pathways of interconversion: glycolysis and gluconeogenesis.
Glycolysis converts the 6-carbon glucose molecule to two 3-carbon pyruvate molecules
The breakdown of glucose to pyruvate ( Fig. 58-6 A ) can occur in the presence of O 2 (aerobic glycolysis) or the absence of O 2 (anaerobic glycolysis). This process yields 47 kcal of free energy per mole of glucose. Of this energy, the cell can trap enough to yield directly 2 moles of ATP per mole of glucose ( Table 58-3 ), even under the relatively inefficient anaerobic conditions. Under aerobic conditions, the mitochondria can generate an additional three or five ATP molecules per glucose molecule from two reduced nicotinamide adenine dinucleotide ( NADH ) molecules. N58-5 Cells that contain few mitochondria (e.g., fast-twitch muscle fibers; see pp. 249–250 ) or no mitochondria (i.e., erythrocytes N58-6 ) rely exclusively on anaerobic glycolysis for energy.
TABLE 58-3
Generation of ATP from Glycolysis *
| REACTION | ATP CHANGE PER GLUCOSE |
|---|---|
| Glucose → G6P | −1 |
| Fructose-6-phosphate → Fructose-1,6-bisphosphate | −1 |
| 2 × (1,3-Bisphosphoglycerate) → 2 × (3-Phosphoglycerate) | +2 |
| 2 × Phosphoenolpyruvate → 2 × Pyruvate | +2 |
| Net +2 |
* Under anaerobic conditions, glycolysis—which takes place in the cytosol—yields two lactate molecules plus two ATP molecules per glucose molecule. Under aerobic conditions, the metabolism of glucose does not proceed to lactate and thus yields a net gain of two NADH molecules per glucose molecule. The oxidative phosphorylation of each NADH molecule 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. N58-15 Therefore, under aerobic conditions, glycolysis of one glucose molecule generates two ATP molecules directly plus three or five ATP molecules via the oxidative phosphorylation of the two NADH molecules, for a total of five or seven ATP molecules per glucose molecule. This yield is summarized in the first two rows of Table 58-4 .
N58-5
NADH/NAD + versus NADPH/NADP +
NADH and NAD + are, respectively, the reduced and oxidized forms of nicotinamide adenine dinucleotide (NAD) and their close analogs are NADPH and NADP + , the reduced and oxidized forms of nicotinamide adenine dinucleotide phosphate (NADP). The coenzymes NADH and NADPH each consist of two nucleotides joined at their phosphate groups by a phosphoanhydride bond. NADPH is structurally distinguishable from NADH by the additional phosphate group residing on the ribose ring of the nucleotide, which allows enzymes to preferentially interact with either molecule.
Total concentrations of NAD + /NADH (10 −5 M) are higher in the cell by ~10-fold compared to NADP + /NADPH (10 −6 M). Ratios of the oxidized and reduced forms of these coenzymes offer perspective into the metabolic activity of the cell. The high NAD + /NADH ratio favors the transfer of a hydride from a substrate to NAD + to form NADH, the reduced form of the molecule and oxidizing agent. Therefore, NAD + is highly prevalent within catabolic reaction pathways where reducing equivalents (carbohydrate, fats, and proteins) transfer protons and electrons to NAD + . NADH acts as an energy carrier, transferring electrons from one reaction to another. Conversely, the NADP + /NADPH ratio is low, favoring the transfer of a hydride to a substrate oxidizing NADPH to NADP + . Thus, NADPH is utilized as a reducing agent within anabolic reactions, particularly the biosynthesis of fatty acids.
References
-
Nelson DL, Cox MM: Lehninger Principles of Biochemistry.6th ed2012.WH FreemanNew York
-
Wikipedia. s.v. Nicotinamide adenine dinucleotide. Last modified May 8, 2015 http://en.wikipedia.org/wiki/Nicotinamide_adenine_dinucleotide Accessed May 15, 2015
N58-6
Red Blood Cells
As noted on pages 434–435 of the text, mature erythrocytes lack all organelles, including mitochondria. Of the glucose that they take up, 90% undergoes glycolysis to provide ATP. The remaining 10% follows the pentose phosphate pathway (see Fig. 58-1 )—also known as the hexose monophosphate shunt or the phosphogluconate pathway—to produce NADPH (i.e., reducing equivalents) as well as 5-carbon sugars.
About a century and a half ago, Pasteur recognized that glycolysis by yeast occurs faster in anaerobic conditions than in aerobic conditions. N58-7 This Pasteur effect reflects the cell's attempt to maintain a constant [ATP] i by controlling the rate at which glycolysis breaks down glucose to generate ATP. The key is the allosteric regulation of enzymes that catalyze the three reactions in the glycolytic pathway that are essentially irreversible: hexokinase (or glucokinase in liver and pancreas), phosphofructokinase (PFK), and pyruvate kinase (highlighted in green in Fig. 58-6 A ). In each case, either the direct reaction product (i.e., G6P in the case of hexokinase) or downstream metabolic products (e.g., ATP in the case of the other two) inhibits the enzyme. If glycolysis should temporarily outstrip the cell's need for ATP, the buildup of products slows glycolysis. Thus, introducing O 2 activates the citric acid cycle (see p. 1185 ), which raises [ATP] i , inhibits PFK and pyruvate kinase, and slows glycolysis.
N58-7
Allosteric Regulation of Glycolytic Enzymes by Metabolic Products (Pasteur Effect)
As pointed out on page 1176 , the three enzymes of the glycolytic pathway that catalyze reactions that are essentially irreversible are also all under all allosteric control.
Hexokinase in muscle (the comparable enzyme in liver is glucokinase) catalyzes the essentially irreversible conversion of free glucose to G6P (see Fig. 58-6 A ) and thereby traps the sugar inside the cell and enables it to enter the glycolytic pathway. Hexokinase is inhibited by increased concentrations of its reaction product, G6P. Thus, when the concentration of G6P increases excessively, hexokinase is temporarily inhibited (negative feedback), which reduces the rate of G6P production and enables it to match once again the rate of consumption in the steady state.
The enzyme phosphofructokinase (PFK) catalyzes the essentially irreversible conversion of fructose-6-phosphate to fructose-1,6-bisphosphate (see Fig. 58-6 A ). The activity of PFK is allosterically regulated by the cellular energy status, increasing with decreases in [ATP] i (or with increases in ATP breakdown products), and vice versa. Further, an increased concentration of citrate —which indicates that pyruvate is being provided to the citric acid cycle faster than it can be used—increases the inhibitory influence of ATP on PFK, attenuating the rate of glycolysis further and thus signaling that the cell's energy needs are being met.
Pyruvate kinase catalyzes the final step of glycolysis, the essentially irreversible transfer of the phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding ATP and pyruvate (see Fig. 58-6 A ). A high [ATP] i decreases the affinity of pyruvate kinase for PEP, reducing the rate of reaction at the [PEP] i normally prevailing. Pyruvate kinase is also inhibited by acetyl CoA, which is the entry molecule to the citric acid cycle, a source of high ATP production.
The allosteric inhibitors (indicated above in bold italics ) of the aforementioned three enzymes are responsible for the observation by Pasteur (the so-called Pasteur effect ) that glycolysis is slower under aerobic conditions, when these allosteric inhibitors are present at higher concentrations.
Under anaerobic conditions, cells convert pyruvate to lactate, which is accompanied by the accumulation of H + — lactic acidosis. This acidosis, in turn, can impede muscle contraction by decreasing muscle cell pH, which can result in muscle cramps and inhibition of key glycolytic enzymes needed for ATP synthesis. Thus, sustained skeletal muscle activity depends on the aerobic metabolism of pyruvate as well as FAs.
Gluconeogenesis converts nonhexose precursors to the 6-carbon glucose molecule
Gluconeogenesis is essential for life because the brain and anaerobic tissues —formed elements of blood (erythrocytes, leukocytes), bone marrow, and the renal medulla—normally depend on glucose as the primary fuel source. The daily glucose requirement of the brain in an adult is ~120 g, which accounts for most of the 180 g of glucose produced by the liver. The major site for gluconeogenesis is the liver, with the renal cortex making a much smaller contribution. During prolonged fasting (2 to 3 months), the kidney can account for up to 40% of total glucose production.
Although glycolysis converts glucose to pyruvate (see Fig. 58-6 A ) and gluconeogenesis converts pyruvate to glucose (see Fig. 58-6 B ), gluconeogenesis is not simply glycolysis in reverse. The thermodynamic equilibrium of glycolysis lies strongly on the side of pyruvate formation (i.e., the ΔG is very negative). Thus, in contrast to glycolysis, gluconeogenesis requires energy, consuming four ATP, two GTP, and two NADH molecules for every glucose molecule formed. Most of the ΔG decrease in glycolysis occurs in the three essentially irreversible steps indicated by single arrows in Figure 58-6 A . Gluconeogenesis bypasses these three irreversible, high-ΔG glycolytic reactions by using four enzymes: pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase (FBPase), and G6Pase (see Fig. 58-6 B ). The enzymes of gluconeogenesis and glycolysis are present in separate cellular compartments to minimize futile cycling of substrates between glycolysis and gluconeogenesis; the glycolytic enzymes reside in the cytosolic compartment, whereas the gluconeogenic enzymes are present in the mitochondria (pyruvate carboxylase) or in the lumen of the endoplasmic reticulum (G6P).
The liver accomplishes gluconeogenesis by taking up, and converting to glucose, several nonhexose precursors (see Fig. 58-6 B ). These include two breakdown products of glycolysis (lactate and pyruvate), all the intermediates of the citric acid cycle, 18 of the 20 amino acids, and glycerol. Regardless of the precursor—except for glycerol—all pathways go through oxaloacetate (OA). Thus, the liver can convert lactate to pyruvate, and then convert pyruvate to OA via pyruvate carboxylase, consuming one ATP. Similarly, the citric acid cycle can convert all its intermediates to OA. Finally, the liver can deaminate all amino acids—except leucine and lysine—to form pyruvate, OA, or three other intermediates of the citric acid cycle (α-ketoglutarate, succinyl coenzyme A [CoA], or fumarate). The major gluconeogenic amino acids are alanine and glutamine. N58-8 Leucine and lysine are not gluconeogenic because their deamination leads to acetyl CoA, which cannot generate net OA (see p. 1185 ). Similarly, FAs are not gluconeogenic because their breakdown products are almost exclusively acetyl CoA. In contrast, leucine and lysine are ketogenic because cells can convert acetyl CoA to FAs ( Fig. 58-7 ) or ketone bodies (see p. 1185 ).
Once the liver has converted the precursor to OA, the next step is the conversion of OA to phosphoenolpyruvate (PEP) by PEPCK, which consumes one GTP molecule (see Fig. 58-6 B ). The liver can convert PEP to fructose-1,6-bisphosphate (F-1,6-BP) by using the glycolytic enzymes in reverse. The gluconeogenic precursor glycerol enters the pathway at dihydroxyacetone phosphate. FBPase converts F-1,6-BP to fructose-6-phosphate, and G6Pase completes gluconeogenesis by converting G6P to glucose.
The major gluconeogenic precursors are (1) lactate, which is derived from glycolysis in muscle and anaerobic tissues; (2) alanine, which is mostly derived from glycolysis and transamination of pyruvate in skeletal muscle; and (3) glycerol, which is derived from lipolysis in adipocytes.
Reciprocal regulation of glycolysis and gluconeogenesis minimizes futile cycling
We already noted that key glycolytic and gluconeogenic enzymes are located in separate compartments. The liver also reciprocally and coordinately regulates these processes so that when one pathway is active the other pathway is relatively inactive. This regulation is important because both glycolysis and gluconeogenesis are highly exergonic, and therefore no thermodynamic barrier prevents futile cycling of substrates between these two pathways. Because glycolysis creates two ATP molecules and gluconeogenesis consumes four ATP and two GTP molecules, a full cycle from one glucose to two pyruvates and back again would have a net cost of two ATP and two GTP molecules. The liver regulates flux through these pathways in the short term mostly by allosteric regulation of enzyme activity, and in the long term by transcriptional regulation of gene expression.
Allosteric Regulation
PFK (glycolysis) is stimulated by AMP, whereas it is inhibited by citrate and ATP (see Fig. 58-6 A ). FBPase (gluconeogenesis) is inhibited by AMP and is activated by citrate (see Fig. 58-6 B ). In addition, fructose-2,6-bisphosphate ( F-2,6-BP ), N58-9 which is under the reciprocal control of glucagon and insulin, reciprocally regulates these two enzymes, stimulating PFK and inhibiting FBPase. In the fed state, when the glucagon level is low (see pp. 1050–1051 ) and the insulin level is high (see p. 1041 ), [F-2,6-BP] is high, which promotes consumption of glucose. Conversely, in the fasted state, [F-2,6-BP] is low, which promotes gluconeogenesis.
N58-9
Fructose-2,6-Bisphosphate
Although structurally very similar to fructose-1,6-bisphosphate, fructose-2,6-bisphosphate (F-2,6-BP) is neither a substrate nor a product in either glycolysis or gluconeogenesis. However, it is an allosteric regulator of both glycogenolysis and gluconeogenesis.
The enzyme phosphofructokinase 2 (PFK-2) phosphorylates fructose-6-phosphate to yield F-2,6-BP. Conversely, the enzyme fructose-2,6,-bisphosphatase 2 (FBPase-2) removes one phosphate from F-2,6-BP to yield fructose-6-phosphate. Interestingly, a single bifunctional protein mediates both the PFK-2 and FBPase-2 activities. These two enzymes are distinct from phosphofructokinase 1 (PFK in Fig. 58-6 A ) and fructose-1,6-bisphosphatase (FBPase-1; see Fig. 58-6 B ).
Fructose-6-phosphate is an important allosteric regulator of both glycolysis and gluconeogenesis. By modulating the two enzymes noted above, insulin raises but glucagon lowers cytosolic [F-2,6-BP]. Thus, in the fed state (high insulin/low glucagon), [F-2,6-BP] rises, thereby stimulating glycolysis but inhibiting gluconeogenesis.
Similarly, the liver reciprocally regulates pyruvate kinase (glycolysis) and pyruvate carboxylase/PEPCK (gluconeogenesis). High concentrations of ATP and alanine inhibit pyruvate kinase, whereas ADP inhibits pyruvate carboxylase. Furthermore, acetyl CoA inhibits pyruvate kinase but activates pyruvate carboxylase.
In this way, high concentrations of biosynthetic precursors and ATP favor gluconeogenesis and suppress glycolysis. Conversely, high concentrations of AMP, reflecting a low energy charge of the liver, suppress gluconeogenesis and favor glycolysis.
Transcriptional Regulation
More long-term regulation of gluconeogenesis and glycolysis occurs by hormonal regulation of gene expression. The major hormones involved in this process are insulin, glucagon, epinephrine, and cortisol. In contrast to allosteric regulation, which occurs in seconds to minutes, transcriptional regulation occurs over hours to days. Insulin, which increases following a meal (see p. 1041 ), stimulates the expression of the glycolytic enzymes PFK and pyruvate kinase, as well as the enzyme that makes F-2,6-BP. N58-9 In addition, as noted in Figure 51-8 , insulin suppresses the expression of the key gluconeogenic enzymes PEPCK, FBPase, and G6Pase. Insulin leads to the phosphorylation of the FOXO1 transcription factor, which prevents FOXO1 from entering the nucleus and activating transcription of genes that encode these enzymes.
Conversely, glucagon, the levels of which increase during starvation, inhibits the expression of the glycolytic enzymes PFK and pyruvate kinase, as well as the enzyme that makes F-2,6-BP. Epinephrine and norepinephrine, released under conditions of stress, have actions similar to those of glucagon. At the same time, these hormones stimulate the expression of the gluconeogenic enzymes PEPCK and G6Pase via cAMP and protein kinase A (PKA). Phosphorylation of the transcriptional factor CREB (cAMP response element–binding protein; see p. 89 ) directly promotes both increased transcription of gluconeogenic genes (e.g., PEPCK) and increased transcription of the transcriptional cofactor PGC-1α (peroxisome proliferator–activated receptor-γ coactivator-1α). PGC-1α then binds and activates the transcription factors HNF4 and FOXO1, which further promote the transcription of these key gluconeogenic enzymes.
Cells can convert glucose or amino acids into FAs
The body—principally the liver—can convert glucose to FAs. As shown in Figure 58-6 A , glycolysis converts glucose to pyruvate, which can enter the mitochondrion via the mitochondrial pyruvate carrier, or MPC (see Fig. 58-7 ). When ATP demand is low, high levels of ATP, acetyl CoA, and NADH inside the mitochondria inhibit pyruvate dehydrogenase, which converts pyruvate to acetyl CoA—the normal entry point into the citric acid cycle (see p. 1185 ). Conversely, high levels of ATP and acetyl CoA stimulate pyruvate carboxylase, which instead converts pyruvate to OA, the last element in the citric acid cycle. The mitochondrion then converts the OA and acetyl CoA to citrate, which it exports to the cytosol via an exchanger called the citrate carrier, or CIC (SLC25A1). A cytosolic enzyme called citrate lyase converts the citrate back to OA and acetyl CoA. The hepatocyte converts the cytosolic OA to malate or pyruvate, each of which can re-enter the mitochondrion. Thus, the net effect is to make acetyl CoA disappear from the mitochondrion and appear in the cytosol for FA synthesis.
As noted above, the breakdown of ketogenic amino acids N58-8 yields acetyl CoA (see p. 1176 ). This acetyl CoA can also contribute to FA synthesis. In addition, the breakdown of other amino acids yields pyruvate or intermediates of the citric acid cycle, which again can contribute to FA synthesis.
The synthesis of FAs from acetyl CoA takes place in the cytosol, whereas the oxidation of FAs to acetyl CoA occurs in the mitochondrion (see below, pp. 1183–1185 ). The first committed step—and the rate-limiting step—in FA synthesis is the ATP-dependent carboxylation of acetyl CoA (2 carbons) to form malonyl CoA (3 carbons), catalyzed by acetyl CoA carboxylase (ACC). N58-10 The next step is the sequential addition of 2-carbon units to a growing acyl chain, shown as –CO–(CH 2 ) n –CH 3 in Figure 58-7 , to produce an FA. With each round of elongation, a malonyl CoA molecule reacts with FA synthase, which then decarboxylates the malonyl moiety and condenses the growing acyl chain to the remaining 2-carbon malonyl fragment. The subsequent reduction, dehydration, and reduction steps—all catalyzed by the same multifunctional peptide—complete one round of elongation. Starting from a priming acetyl group (which comes from acetyl CoA), seven rounds of addition—in addition to a hydrolysis step to remove the acyl chain from the enzyme—are required to produce palmitate:
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Acetyl CoA Carboxylase Isoforms
The two isoforms of ACC, encoded by different genes, are the following:
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ACC1:
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265 kDa
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Cytosolic
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Highly expressed in lipogenic tissues: liver, adipocytes, lung, mammary gland
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Involved in FA synthesis
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ACC2:
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280 kDa
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Has a unique 144-amino-acid sequence at the amino terminus; amino acids 1 to 20 may be a leader sequence
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Membrane protein, associated with mitochondria, thought to face the cytosol
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Expressed in heart, skeletal muscle more than liver
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Regulates β-oxidation by producing malonyl CoA; carnitine acyltransferase I is the committed step
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where NADPH and NADP + are the reduced and oxidized forms of nicotinamide adenine dinucleotide phosphate, respectively. The cell esterifies the FAs to glycerol to make TAGs (see Fig. 58-3 A ).
N58-11 The liver can package TAGs as very-low-density lipoproteins ( VLDLs; see p. 968 ) for export to the blood.
The body permits only certain energy interconversions
The body has a hierarchy for energy interconversion. As discussed above, the body can convert amino acids to glucose (gluconeogenesis) and fat, glucose to fat, and glucose to certain amino acids. However, the body cannot convert fat to either glucose or amino acids. Fats can only be stored or oxidized. The reason is that cells oxidize FAs two carbons at a time to acetyl CoA (a 2-carbon molecule; see pp. 1183–1185 ), which they cannot convert into pyruvate (a 3-carbon molecule) or OA (a 4-carbon molecule). The only exceptions are the uncommon FAs that have an odd number of carbon atoms, and even with these, only the terminal 3-carbon unit escapes oxidation to acetyl CoA. Thus, almost all carbon atoms in FAs end up as acetyl CoA, which enters the citric acid cycle (see p. 1185 ). There, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase release the two carbon atoms of acetyl CoA as two CO 2 molecules, which thus yields no net production of OA or pyruvate. In contrast, plants have two additional enzymes (the glyoxylate cycle) that allow them to convert two molecules of acetyl CoA to OA and glucose.
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