Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 In adults, the rates at which organic molecules are continu- ously synthesized (anabolism) and broken down (catabo- lism) are approximately equal. Chemical Reactions I. The difference in the energy content of reactants and products is the amount of energy (measured in calories) that is released or added during a reaction. II. The energy released during a chemical reaction either is released as heat or is transferred to other molecules. III. The four factors that can alter the rate of a chemical reaction are listed in Table 4–2. IV. The activation energy required to initiate the breaking of chemical bonds in a reaction is usually acquired through collisions with other molecules. SECTION B SUMMARY V. Catalysts increase the rate of a reaction by lowering the activation energy. VI. The characteristics of reversible and irreversible reactions are listed in Table 4–3. VII. The net direction in which a reaction proceeds can be altered, according to the law of mass action, by increases or decreases in the concentrations of reactants or products. Enzymes I. Nearly all chemical reactions in the body are catalyzed by enzymes, the characteristics of which are summarized in Table 4–4. II. Some enzymes require small concentrations of cofactors for activity. a. The binding of trace metal cofactors maintains the conformation of the enzyme’s binding site so that it is able to bind substrate. b. Coenzymes, derived from vitamins, transfer small groups of atoms from one substrate to another. The coenzyme is regenerated in the course of these reactions and can be used over and over again. Regulation of Enzyme-Mediated Reactions The rates of enzyme-mediated reactions can be altered by changes in temperature, substrate concentration, enzyme concentration, and enzyme activity. Enzyme activity is altered by allosteric or covalent modulation. Multienzyme Metabolic Pathways I. The rate of product formation in a metabolic pathway can be controlled by allosteric or covalent modulation of the enzyme mediating the rate- limiting reaction in the pathway. The end product often acts as a modulator molecule, inhibiting the rate-limiting enzyme’s activity. II. An “irreversible” step in a metabolic pathway can be reversed by the use of two enzymes, one for the forward reaction and one for the reverse direction via another, energy-yielding reaction. ATP In all cells, energy from the catabolism of fuel molecules is transferred to ATP. The hydrolysis of ATP to ADP and P i then transfers this energy to cell functions. metabolism chemical equilibrium anabolism irreversible reaction catabolism law of mass action calorie enzyme kilocalorie substrate activation energy active site catalyst cofactor reversible reaction coenzyme SECTION B KEY TERMS 69 Protein Activity and Cellular Metabolism CHAPTER FOUR Energy-requiring cell functions ATP Force and movement Active transport across membranes Molecular synthesis Chemical energy 40% Heat energy 60% Fuel molecules CO 2 + H 2 O + NH 3 ADP + P i Catabolism FIGURE 4–17 Flow of chemical energy from fuel molecules to ATP and heat, and from ATP to energy-requiring cell functions. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 _ vitamin rate-limiting reaction NAD ϩ end-product inhibition FAD adenosine triphosphate enzyme activity (ATP) metabolic pathway 1. How do molecules acquire the activation energy required for a chemical reaction? 2. List the four factors that influence the rate of a chemical reaction and state whether increasing the factor will increase or decrease the rate of the reaction. 3. What characteristics of a chemical reaction make it reversible or irreversible? SECTION B REVIEW QUESTIONS 4. List five characteristics of enzymes. 5. What is the difference between a cofactor and a coenzyme? 6. From what class of nutrients are coenzymes derived? 7. Why are small concentrations of coenzymes sufficient to maintain enzyme activity? 8. List three ways in which the rate of an enzyme- mediated reaction can be altered. 9. How can an irreversible step in a metabolic pathway be reversed? 10. What is the function of ATP in metabolism? 11. Approximately how much of the energy released from the catabolism of fuel molecules is transferred to ATP? What happens to the rest? 70 PART ONE Basic Cell Functions METABOLIC PATHWAYS SECTION C Three distinct but linked metabolic pathways are used by cells to transfer the energy released from the break- down of fuel molecules of ATP. They are known as gly- colysis, the Krebs cycle, and oxidative phosphorylation (Figure 4–18). In the following section, we will describe the major characteristics of these three pathways in terms of the location of the pathway enzymes in a cell, the relative contribution of each pathway to ATP pro- duction, the sites of carbon dioxide formation and oxy- gen utilization, and the key molecules that enter and leave each pathway. In this last regard, several facts should be noted in Figure 4–18. First, glycolysis operates only on carbo- hydrates. Second, all the categories of nutrients— carbohydrates, fats, and proteins—contribute to ATP production via the Krebs cycle and oxidative phos- phorylation. Third, mitochondria are essential for the Krebs cycle and oxidative phosphorylation. Finally one important generalization to keep in mind is that glycolysis can occur in either the presence or absence of oxygen, whereas both the Krebs cycle and oxidative phosphorylation require oxygen as we shall see. Cellular Energy Transfer Glycolysis Glycolysis (from the Greek glycos, sugar, and lysis, breakdown) is a pathway that partially catabolizes car- bohydrates, primarily glucose. It consists of 10 enzy- matic reactions that convert a six-carbon molecule of glucose into two three-carbon molecules of pyruvate, the ionized form of pyruvic acid (Figure 4–19). The Glycolysis Carbohydrates Pyruvate Lactate CO 2 H 2 O Fats and proteins Energy ADP + P i AT P Krebs cycle Coenzyme—2H O 2 Fats Oxidative phosphorylation Cytosol Mitochondria Mitochondria FIGURE 4–18 Pathways linking the energy released from the catabolism of fuel molecules to the formation of ATP. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 71 Protein Activity and Cellular Metabolism CHAPTER FOUR CH 2 OH O HO OH H H 2 C P O O O – – O CH 2 AT P ADP O H OH H HO OH H H H P i OH H P O O O – O – CH 2 OH O OH H OH H HO OH H H H 8 4 6 Glucose Glucose 6-phosphate Dihydroxyacetone phosphate HO HH H 2 CP O O O – – O O OH H H Fructose 6-phosphate 3 CH 2 P O O O – O – OH CH 2 P O C O – OH CH 2 O P O O – O – CH O CO CH 2 OH H 2 H P O O O – O – CH 2 O C CH P O O O – – O AT P ADP O – H 2 O AT P ADP 1 P O COO – CH 3 OO – O Fructose 1,6-bisphosphate 2-Phosphoglycerate Phosphoenolpyruvate 3-Phosphoglyceraldehyde 5 COOH P O OH CH 2 O CH O – O – O – 10 AT P ADP 7 P O COO – OHCH 2 OCH O – O – C OC CH 2 COO – NADH + H + NAD + OHCH COO – CH 3 Pyruvate To Krebs cycle (anaerobic) (aerobic) 3-Phosphoglycerate 1,3-Bisphosphoglycerate Lactate 9 NAD + OH NADH + H + FIGURE 4–19 Glycolytic pathway. Under anaerobic conditions, there is a net synthesis of two molecules of ATP for every molecule of glucose that enters the pathway. Note that at the pH existing in the body, the products produced by the various glycolytic steps exist in the ionized, anionic form (pyruvate, for example). They are actually produced as acids (pyruvic acid, for example) that then ionize. reactions produce a net gain of two molecules of ATP and four atoms of hydrogen, two of which are trans- ferred to NAD ϩ and two are released as hydrogen ions: Glucose ϩ 2 ADP ϩ 2 P i ϩ 2 NAD ϩ 88n (4–1) 2 Pyruvate ϩ 2 ATP ϩ 2 NADH ϩ 2 H ϩ ϩ 2 H 2 O These 10 reactions, none of which utilizes molecular oxy- gen, take place in the cytosol. Note (Figure 4–19) that all the intermediates between glucose and the end product pyruvate contain one or more ionized phos- phate groups. As we shall learn in Chapter 6, plasma membranes are impermeable to such highly ionized Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 72 PART ONE Basic Cell Functions C COO – CH 3 O OH C LactatePyruvate COO – cycle Reaction 6 (anaerobic) (aerobic) H 2NADH + 2H + 2NAD + Glucose CH 3 Krebs 22 FIGURE 4–20 Under anaerobic conditions, the coenzyme NAD ϩ utilized in the glycolytic reaction 6 (see Figure 4–19) is regenerated when it transfers its hydrogen atoms to pyruvate during the formation of lactate. molecules, and thus these molecules remain trapped within the cell. Note that the early steps in glycolysis (reactions 1 and 3) each use, rather than produce, one molecule of ATP, to form phosphorylated intermediates. In addi- tion, note that reaction 4 splits a six-carbon intermedi- ate into two three-carbon molecules, and reaction 5 converts one of these three-carbon molecules into the other so that at the end of reaction 5 we have two mol- ecules of 3-phosphoglyceraldehyde derived from one molecule of glucose. Keep in mind, then, that from this point on, two molecules of each intermediate are involved. The first formation of ATP in glycolysis occurs dur- ing reaction 7 when a phosphate group is transferred to ADP to form ATP. Since, as stressed above, two in- termediates exist at this point, reaction 7 produces two molecules of ATP, one from each of them. In this reac- tion, the mechanism of forming ATP is known as substrate-level phosphorylation since the phosphate group is transferred from a substrate molecule to ADP. As we shall see, this mechanism is quite different from that used during oxidative phosphorylation, in which free inorganic phosphate is coupled to ADP to form ATP. A similar substrate-level phosphorylation of ADP occurs during reaction 10, where again two molecules of ATP are formed. Thus, reactions 7 and 10 generate a total of four molecules of ATP for every molecule of glu- cose entering the pathway. There is a net gain, however, of only two molecules of ATP during glycolysis because two molecules of ATP were used in reactions 1 and 3. The end product of glycolysis, pyruvate, can pro- ceed in one of two directions, depending on the avail- ability of molecular oxygen, which, as we stressed ear- lier, is not utilized in any of the glycolytic reactions themselves. If oxygen is present—that is, if aerobic conditions exist—pyruvate can enter the Krebs cycle and be broken down into carbon dioxide, as described in the next section. In contrast, in the absence of oxygen (anaerobic conditions), pyruvate is converted to lac- tate (the ionized form of lactic acid) by a single enzyme- mediated reaction. In this reaction (Figure 4–20) two hydrogen atoms derived from NADH ϩ H ϩ are trans- ferred to each molecule of pyruvate to form lactate, and NAD ϩ is regenerated. These hydrogens had orig- inally been transferred to NAD ϩ during reaction 6 of glycolysis, so the coenzyme NAD ϩ shuttles hydrogen between the two reactions during anaerobic glycoly- sis. The overall reaction for anaerobic glycolysis is Glucose ϩ 2 ADP ϩ 2 P i 88n (4–2) 2 Lactate ϩ 2 ATP ϩ 2 H 2 O As stated in the previous paragraph, under aerobic conditions pyruvate is not converted to lactate but rather enters the Krebs cycle. Therefore, the mechanism just described for regenerating NAD ϩ from NADH ϩ H ϩ by forming lactate does not occur. (Compare Equa- tions 4–1 and 4–2.) Instead, as we shall see, H ϩ and the hydrogens of NADH are transferred to oxygen during oxidative phosphorylation, regenerating NAD ϩ and producing H 2 O. In most cells, the amount of ATP produced by gly- colysis from one molecule of glucose is much smaller than the amount formed under aerobic conditions by the other two ATP-generating pathways—the Krebs cycle and oxidative phosphorylation. There are special cases, however, in which glycolysis supplies most, or even all, of a cell’s ATP. For example, erythrocytes con- tain the enzymes for glycolysis but have no mito- chondria, which, as we have said, are required for the other pathways. All of their ATP production occurs, therefore, by glycolysis. Also, certain types of skeletal muscles contain considerable amounts of glycolytic en- zymes but have few mitochondria. During intense muscle activity, glycolysis provides most of the ATP in these cells and is associated with the production of large amounts of lactate. Despite these exceptions, most cells do not have sufficient concentrations of gly- colytic enzymes or enough glucose to provide, by gly- colysis alone, the high rates of ATP production neces- sary to meet their energy requirements and thus are unable to function for long under anaerobic conditions. Our discussion of glycolysis has focused upon glu- cose as the major carbohydrate entering the glycolytic pathway. However, other carbohydrates such as fruc- tose, derived from the disaccharide sucrose (table sugar), and galactose, from the disaccharide lactose Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 (milk sugar), can also be catabolized by glycolysis since these carbohydrates are converted into several of the intermediates that participate in the early portion of the glycolytic pathway. In some microoganisms (yeast cells, for example), pyruvate is converted under anaerobic conditions to carbon dioxide and alcohol (CH 3 CH 2 OH) rather than to lactate. This process is known as fermentation and forms the basis for the production of alcohol from ce- real grains rich in carbohydrates. Table 4–5 summarizes the major characteristics of glycolysis. Krebs Cycle The Krebs cycle, named in honor of Hans Krebs, who worked out the intermediate steps in this pathway (also known as the citric acid cycle or tricarboxylic acid cycle), is the second of the three pathways in- volved in fuel catabolism and ATP production. It uti- lizes molecular fragments formed during carbohy- drate, protein, and fat breakdown, and it produces carbon dioxide, hydrogen atoms (half of which are bound to coenzymes), and small amounts of ATP. The enzymes for this pathway are located in the inner mi- tochondrial compartment, the matrix. The primary molecule entering at the beginning of the Krebs cycle is acetyl coenzyme A (acetyl CoA): Coenzyme A (CoA) is derived from the B vitamin pan- tothenic acid and functions primarily to transfer acetyl groups, which contain two carbons, from one molecule to another. These acetyl groups come either from pyruvate, which, as we have just seen, is the end prod- CoA S O CH 3 C uct of aerobic glycolysis, or from the breakdown of fatty acids and some amino acids, as we shall see in a later section. Pyruvate, upon entering mitochondria from the cytosol, is converted to acetyl CoA and CO 2 (Figure 4–21). Note that this reaction produces the first mole- cule of CO 2 formed thus far in the pathways of fuel catabolism, and that hydrogen atoms have been trans- ferred to NAD ϩ . The Krebs cycle begins with the transfer of the acetyl group of acetyl CoA to the four-carbon mole- cule, oxaloacetate, to form the six-carbon molecule, ci- trate (Figure 4–22). At the third step in the cycle a mol- ecule of CO 2 is produced, and again at the fourth step. Thus, two carbon atoms entered the cycle as part of the acetyl group attached to CoA, and two carbons (al- though not the same ones) have left in the form of CO 2 . Note also that the oxygen that appears in the CO 2 is not derived from molecular oxygen but from the car- boxyl groups of Krebs-cycle intermediates. In the remainder of the cycle, the four-carbon mol- ecule formed in reaction 4 is modified through a series of reactions to produce the four-carbon molecule ox- aloacetate, which becomes available to accept another acetyl group and repeat the cycle. 73 Protein Activity and Cellular Metabolism CHAPTER FOUR Entering substrates Glucose and other monosaccharides Enzyme location Cytosol Net ATP production 2 ATP formed directly per molecule of glucose entering pathway Can be produced in the absence of oxygen (anaerobically) Coenzyme production 2 NADH ϩ 2 H ϩ formed under aerobic conditions Final products Pyruvate—under aerobic conditions Lactate—under anaerobic conditions Net reaction Aerobic: Glucose ϩ 2 ADP ϩ 2 P i ϩ 2 NAD ϩ 88n 2 pyruvate ϩ 2 ATP ϩ 2 NADH ϩ 2 H ϩ ϩ 2 H 2 O Anaerobic: Glucose ϩ 2 ADP ϩ 2 P i 88n 2 lactate ϩ 2 ATP ϩ 2 H 2 O TABLE 4–5 Characteristics of Glycolysis NAD + NADH + H + Pyruvic acid Acetyl coenzyme A OC COOH CH 3 OC CH 3 CO 2 CoA CoAS + SH + FIGURE 4–21 Formation of acetyl coenzyme A from pyruvic acid with the formation of a molecule of carbon dioxide. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 74 PART ONE Basic Cell Functions Now we come to a crucial fact: In addition to pro- ducing carbon dioxide, intermediates in the Krebs cy- cle generate hydrogen atoms, most of which are trans- ferred to the coenzymes NAD ϩ and FAD to form NADH and FADH 2 . This hydrogen transfer to NAD ϩ occurs in each of steps 3, 4, and 8, and to FAD in re- action 6. These hydrogens will be transferred from the coenzymes, along with the free H ϩ , to oxygen in the next stage of fuel metabolism—oxidative phosphory- lation. Since oxidative phosphorylation is necessary for regeneration of the hydrogen-free form of these coen- zymes, the Krebs cycle can operate only under aerobic con- ditions. There is no pathway in the mitochondria that can remove the hydrogen from these coenzymes un- der anaerobic conditions. So far we have said nothing of how the Krebs cy- cle contributes to the formation of ATP. In fact, the Krebs cycle directly produces only one high-energy nu- cleotide triphosphate. This occurs during reaction 5 in which inorganic phosphate is transferred to guanosine 6 H CH 3 CoA SH S O C 2 CH 2 Oxidative phosphorylation Malate C H CH 2 CoA HO COO – COO – COO – 3 Acetyl coenzyme A Oxaloacetate α-Ketoglutarate Citrate CO 2 O CH 2 COO – C COO – OH CH 2 COO – H COO – OHC COO – CH 2 COO – C 1 8 7 4 C OC COO – COO – CH 2 CH 2 NADH + H + H 2 O NADH + H + COO – NADH + H + CO 2 Isocitrate OC AT P GDP Fumarate CoA FADH 2 COO – P i COO – COO – 5 CH 2 CH 2 CH Succinyl coenzyme A CH Succinate ADP GTP H 2 O CoA CoA COO – COO – CH 2 CH 2 H 2 O S FIGURE 4–22 The Krebs-cycle pathway. Note that the carbon atoms in the two molecules of CO 2 produced by a turn of the cycle are not the same two carbon atoms that entered the cycle as an acetyl group (identified by the dashed boxes in this figure). Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 diphosphate (GDP) to form guanosine triphosphate (GTP). The hydrolysis of GTP, like that of ATP, can pro- vide energy for some energy-requiring reactions. In ad- dition, the energy in GTP can be transferred to ATP by the reaction This reaction is reversible, and the energy in ATP can be used to form GTP from GDP when additional GTP is required for protein synthesis (Chapter 5) and sig- nal transduction (Chapter 7). To reiterate, the formation of ATP from GTP is the only mechanism by which ATP is formed within the Krebs cycle. Why, then, is the Krebs cycle so impor- tant? Because the hydrogen atoms transferred to coen- zymes during the cycle (plus the free hydrogen ions generated) are used in the next pathway, oxidative phosphorylation, to form large amounts of ATP. The net result of the catabolism of one acetyl group from acetyl CoA by way of the Krebs cycle can be written: Acetyl CoA ϩ 3 NAD ϩ ϩ FAD ϩ GDP ϩ P i ϩ 2H 2 O 88n 2 CO 2 ϩ CoA ϩ 3 NADH ϩ 3H ϩ ϩ FADH 2 ϩ GTP (4–3) One more point should be noted: Although the ma- jor function of the Krebs cycle is to provide hydrogen atoms to the oxidative-phosphorylation pathway, some of the intermediates in the cycle can be used to synthesize organic molecules, especially several types of amino acids, required by cells. Oxaloacetate is one of the intermediates used in this manner. When a mol- ecule of oxaloacetate is removed from the Krebs cycle in the process of forming amino acids, however, it is not available to combine with the acetate fragment of acetyl CoA at the beginning of the cycle. Thus, there must be a way of replacing the oxaloacetate and other Krebs-cycle intermediates that are consumed in syn- GTP ϩ ADP GDP ϩ ATP thetic pathways. Carbohydrates provide one source of oxaloacetate replacement by the following reaction, which converts pyruvate into oxaloacetate. Pyruvate ϩ CO 2 ϩ ATP 88n Oxaloacetate ϩ ADP ϩ P i (4–4) Certain amino acid derivatives, as we shall see, can also be used to form oxaloacetate and other Krebs- cycle intermediates. Table 4–6 summarizes the characteristics of the Krebs cycle reactions. Oxidative Phosphorylation Oxidative phosphorylation provides the third, and quantitatively most important, mechanism by which energy derived from fuel molecules can be transferred to ATP. The basic principle behind this pathway is sim- ple: The energy transferred to ATP is derived from the energy released when hydrogen ions combine with molecular oxygen to form water. The hydrogen comes from the NADH ϩ H ϩ and FADH 2 coenzymes gener- ated by the Krebs cycle, by the metabolism of fatty acids (see below), and, to a much lesser extent, during aerobic glycolysis. The net reaction is ᎏ 1 2 ᎏ O 2 ϩ NADH ϩ H ϩ 8n H 2 O ϩ NAD ϩ ϩ 53 kcal/mol The proteins that mediate oxidative phosphorylation are embedded in the inner mitochondrial membrane unlike the enzymes of the Krebs cycle, which are sol- uble enzymes in the mitochondrial matrix. The pro- teins for oxidative phosphorylation can be divided into two groups: (1) those that mediate the series of reactions by which hydrogen ions are transferred to molecular oxygen, and (2) those that couple the energy released by these reactions to the synthesis of ATP. 75 Protein Activity and Cellular Metabolism CHAPTER FOUR Entering substrate Acetyl coenzyme A—acetyl groups derived from pyruvate, fatty acids, and amino acids Some intermediates derived from amino acids Enzyme location Inner compartment of mitochondria (the mitochondrial matrix) ATP production 1 GTP formed directly, which can be converted into ATP Operates only under aerobic conditions even though molecular oxygen is not used directly in this pathway Coenzyme production 3 NADH ϩ 3 H ϩ and 2 FADH 2 Final products 2 CO 2 for each molecule of acetyl coenzyme A entering pathway Some intermediates used to synthesize amino acids and other organic molecules required for special cell functions Net reaction Acetyl CoA ϩ 3 NAD ϩ ϩ FAD ϩ GDP ϩ P i ϩ 2 H 2 O 88n 2 CO 2 ϩ CoA ϩ 3 NADH ϩ 3 H ϩ ϩ FADH 2 ϩ GTP TABLE 4–6 Characteristics of the Krebs Cycle Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 Most of the first group of proteins contain iron and copper cofactors, and are known as cytochromes (be- cause in pure form they are brightly colored). Their structure resembles the red iron-containing hemoglo- bin molecule, which binds oxygen in red blood cells. The cytochromes form the components of the electron transport chain, in which two electrons from the hy- drogen atoms are initially transferred either from NADH ϩ H ϩ or FADH 2 to one of the elements in this chain. These electrons are then successively transferred to other compounds in the chain, often to or from an iron or copper ion, until the electrons are finally transferred to molecular oxygen, which then combines with hydrogen ions (protons) to form water. These hydrogen ions, like the electrons, come from the free hydrogen ions and the hydrogen-bearing co- enzymes, having been released from them early in the transport chain when the electrons from the hydrogen atoms were transferred to the cytochromes. Importantly, in addition to transferring the coen- zyme hydrogens to water, this process regenerates the hydrogen-free form of the coenzymes, which then be- come available to accept two more hydrogens from in- termediates in the Krebs cycle, glycolysis, or fatty acid pathway (as described below). Thus, the electron trans- port chain provides the aerobic mechanism for regen- erating the hydrogen-free form of the coenzymes, whereas, as described earlier, the anaerobic mechanism, which applies only to glycolysis, is coupled to the for- mation of lactate. At each step along the electron transport chain, small amounts of energy are released, which in total account for the full 53 kcal/mol released from a direct reaction between hydrogen and oxygen. Because this energy is released in small steps, it can be linked to the synthesis of several molecules of ATP, each of which requires only 7 kcal/mol. ATP is formed at three points along the electron transport chain. The mechanism by which this occurs is known as the chemiosmotic hypothesis. As elec- trons are transferred from one cytochrome to another along the electron transport chain, the energy released is used to move hydrogen ions (protons) from the ma- trix into the compartment between the inner and outer mitochondrial membranes (Figure 4–23), thus pro- ducing a source of potential energy in the form of a hydrogen-ion gradient across the membrane. At three points along the chain, a protein complex forms a chan- nel in the inner mitochondrial membrane through which the hydrogen ions can flow back to the matrix side and in the process transfer energy to the forma- tion of ATP from ADP and P i . FADH 2 has a slightly lower chemical energy content than does NADH ϩ H ϩ and enters the electron transport chain at a point 76 PART ONE Basic Cell Functions Cytochromes in electron transport chain NADH + H + FADH 2 NAD + + 2H + FAD + 2H + Matrix H 2 O H + 2 e – 2 e – 2 e – Inner mitochondrial membrane Outer mitochondrial membrane 1 2 O 2 +2 ADP P i H + ATP ADP P i H + ATP H + H + H + ADP P i H + ATP FIGURE 4–23 ATP is formed during oxidative phosphorylation by the flow of hydrogen ions across the inner mitochondrial membrane. Two or three molecules of ATP are produced per pair of electrons donated, depending on the point at which a particular coenzyme enters the electron transport chain. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I. Basic Cell Functions 4. Protein Activity and Cellular Metabolism © The McGraw−Hill Companies, 2001 beyond the first site of ATP generation (Figure 4–23). Thus, the transfer of its electrons to oxygen produces only two ATP rather than the three formed from NADH ϩ H ϩ . To repeat, the majority of the ATP formed in the body is produced during oxidative phosphorylation as a result of processing hydrogen atoms that originated largely from the Krebs cycle, during the breakdown of carbohydrates, fats, and proteins. The mitochondria, where the oxidative phosphorylation and the Krebs- cycle reactions occur, are thus considered the power- houses of the cell. In addition, as we have just seen, it is within these organelles that the majority of the oxy- gen we breathe is consumed, and the majority of the carbon dioxide we expire is produced. Table 4–7 summaries the key features of oxidative phosphorylation. Reactive Oxygen Species As we have just seen, the formation of ATP by oxida- tive phosphorylation involves the transfer of electrons and hydrogen to molecular oxygen. Several highly reactive transient oxygen derivatives can also be formed during this process—hydrogen peroxide and the free radicals superoxide anion and hydroxyl radical. Although most of the electrons transferred along the electron transport chain go into the formation of water, small amounts can combine with oxygen to O 2 O 2 – • OH – H 2 O 2 + OH • 2 OH – 2 H 2 O 2 H + 2 H + e – e – e – e – Superoxide anion Hydrogen peroxide Hydroxyl radical form reactive oxygen species. These species can react with and damage proteins, membrane phospholipids, and nucleic acids. Such damage has been implicated in the aging process and in inflammatory reactions to tissue injury. Some cells use these reactive molecules to kill invading bacteria, as described in Chapter 20. Reactive oxygen molecules are also formed by the action of ionizing radiation on oxygen and by reactions of oxygen with heavy metals such as iron. Cells con- tain several enzymatic mechanisms for removing these reactive oxygen species and thus providing protection from their damaging effects. Carbohydrate, Fat, and Protein Metabolism Having described the three pathways by which energy is transferred to ATP, we now consider how each of the three classes of fuel molecules—carbohydrates, fats, and proteins—enters the ATP-generating pathways. We also consider the synthesis of these fuel molecules and the pathways and restrictions governing their con- version from one class to another. These anabolic path- ways are also used to synthesize molecules that have functions other than the storage and release of energy. For example, with the addition of a few enzymes, the pathway for fat synthesis is also used for synthesis of the phospholipids found in membranes. Carbohydrate Metabolism Carbohydrate Catabolism In the previous sections, we described the major pathways of carbohydrate ca- tabolism: the breakdown of glucose to pyruvate or lac- tate by way of the glycolytic pathway, and the metab- olism of pyruvate to carbon dioxide and water by way of the Krebs cycle and oxidative phosphorylation. 77 Protein Activity and Cellular Metabolism CHAPTER FOUR Entering substrates Hydrogen atoms obtained from NADH ϩ H ϩ and FADH 2 formed (1) during glycolysis, (2) by the Krebs cycle during the breakdown of pyruvate and amino acids, and (3) during the breakdown of fatty acids Molecular oxygen Enzyme location Inner mitochondrial membrane ATP production 3 ATP formed from each NADH ϩ H ϩ 2 ATP formed from each FADH 2 Final products H 2 O—one molecule for each pair of hydrogens entering pathway. Net reaction ᎏ 1 2 ᎏ O 2 ϩ NADH ϩ H ϩ ϩ 3 ADP ϩ 3 P i 88n H 2 O ϩ NAD ϩ ϩ 3 ATP TABLE 4–7 Characteristics of Oxidative Phosphorylation [...]... transcript and link the exon-derived segments together to form the mRNA molecule that passes through the nuclear pores to the cytosol The lengths of the intron- and exon-derived segments represent the relative lengths of the base sequences in these regions 95 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition 96 I Basic Cell Functions © The McGraw−Hill Companies, 20 01 5 Genetic... coenzyme A Cytochromes 12 H2O 6 ( NADH + H + ) 6 H2O 6 O2 2 FADH 2 2 ATP 4 CO2 C 6 H 12 0 6 + 6 O 2 + 38 ADP + 38 P i 6 CO 2 + 6 H 2 O + 38 ATP FIGURE 4 24 Pathways of aerobic glucose catabolism and their linkage to ATP formation The amount of energy released during the catabolism of glucose to carbon dioxide and water is 686 kcal/mol of glucose: C6H12O6 ϩ 6 O2 88n 6 H2O ϩ 6 CO2 ϩ 686 kcal/mol As noted... FOUR (CH2)14 CH3 CH2 CH2 COOH C18 Fatty acid CoA ATP H2O AMP + 2Pi (CH2)14 CH3 SH O CH2 CH2 C S CoA FAD FADH 2 H2O NAD+ NADH + H+ O CH3 CoA (CH2)14 C O CH2 C S CoA SH O O CH3 (CH2)14 C S CoA + CH3 C S CoA Acetyl CoA O2 Krebs cycle Coenzyme—2H Oxidative phosphorylation H2O CO2 9 ATP 139 ATP FIGURE 4 27 Pathway of fatty acid catabolism, which takes place in the mitochondria The energy equivalent of two... Sequence of events during the synthesis of a protein by a ribosome G C C G G U U A A Anticodon 97 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition 98 I Basic Cell Functions © The McGraw−Hill Companies, 20 01 5 Genetic Information and Protein Synthesis PART ONE Basic Cell Functions the ribosome reaches a termination sequence in mRNA specifying the end of the protein, the link... to the cytosol as part of the ribosomal subunits The specific location of a protein is determined by binding sites on the protein that bind to specific sites at the protein’s destination For example, in the case of the ribosomal proteins, they bind to sites on the nuclear pores that control access to the nucleus 101 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition 1 02. .. genes Of each pair, one chromosome was inherited from the mother and one from the father, with each potentially able to code for the same type of protein The development of an individual is determined by the controlled expression of the set of genes inherited at the time of conception Growth occurs through the successive division of cells to form the trillions of cells that make up the adult human body. .. et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition 78 I Basic Cell Functions © The McGraw−Hill Companies, 20 01 4 Protein Activity and Cellular Metabolism PART ONE Basic Cell Functions Glycolysis Oxidative phosphorylation Glucose (cytosol) (mitochondria) 2 (NADH + H+) 2 ATP 34 ATP 2 Pyruvate 2 ( NADH + H + ) Krebs cycle (mitochondria) 10 ATP 12 ATP 12 ATP 2 CO2 ATP ATP ATP 2 Acetyl... together Thus, both nucleotide chains contain a specifically ordered sequence of bases, one chain being complementary to the other This specificity of base pairing, as we shall see, forms the basis of the transfer of information from DNA to RNA and of the duplication of DNA during cell division Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I Basic Cell Functions © The. .. glucose molecule yields a maximum of 38 ATP molecules Thus, taking into account the difference in molecular weight of the fatty acid and glucose, the amount of ATP formed from the ᎏ catabolism of a gram of fat is about 2 1ᎏ times greater 2 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition I Basic Cell Functions © The McGraw−Hill Companies, 20 01 4 Protein Activity and Cellular... components are synthesized in the cytoplasm and then transported into the mitochondria Regulation of Protein Synthesis Transcription 1 RNA polymerase binds to the promoter region of a gene and separates the two strands of the DNA double helix in the region of the gene to be transcribed 2 Free ribonucleotide triphosphates base-pair with the deoxynucleotides in the template strand of DNA 3 The ribonucleotides . reaction Aerobic: Glucose ϩ 2 ADP ϩ 2 P i ϩ 2 NAD ϩ 88n 2 pyruvate ϩ 2 ATP ϩ 2 NADH ϩ 2 H ϩ ϩ 2 H 2 O Anaerobic: Glucose ϩ 2 ADP ϩ 2 P i 88n 2 lactate ϩ 2 ATP ϩ 2 H 2 O TABLE 4–5 Characteristics of Glycolysis NAD + NADH. A CH Succinate ADP GTP H 2 O CoA CoA COO – COO – CH 2 CH 2 H 2 O S FIGURE 4 22 The Krebs-cycle pathway. Note that the carbon atoms in the two molecules of CO 2 produced by a turn of the cycle are not the same. whether the vitamin is water- soluble or fat-soluble. As the amount of water-soluble vitamins in the diet is increased, so is the amount ex- creted in the urine; thus the accumulation of these