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This is a€?Energy Metabolisma€?, chapter 20 from the book Introduction to Chemistry: General, Organic, and Biological (v. This content was accessible as of December 29, 2012, and it was downloaded then by Andy Schmitz in an effort to preserve the availability of this book. PDF copies of this book were generated using Prince, a great tool for making PDFs out of HTML and CSS.
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DonorsChoose.org helps people like you help teachers fund their classroom projects, from art supplies to books to calculators. The discovery of the link between insulin and diabetes led to a period of intense research aimed at understanding exactly how insulin works in the body to regulate glucose levels.
The insulin receptor is located in the cell membrane and consists of four polypeptide chains: two identical chains called I± chains and two identical chains called I? chains. The thousands of coordinated chemical reactions that keep cells alive are referred to collectively as metabolismThe thousands of coordinated chemical reactions that keep cells alive.. These two equations summarize the biological combustion of a carbohydrate and a lipid by the cell through respiration.
Like the combustion of the common fuels we burn in our homes and cars (wood, coal, gasoline), respiration uses oxygen from the air to break down complex organic substances to carbon dioxide and water.
Adenosine triphosphate (ATP), a nucleotide composed of adenine, ribose, and three phosphate groups, is perhaps the most important of the so-called energy-rich compounds in a cell. Energy-rich compounds are substances having particular structural features that lead to a release of energy after hydrolysis. The pyrophosphate bond, symbolized by a squiggle (~), is hydrolyzed when ATP is converted to adenosine diphosphate (ADP).
Energy is released because the products (ADP and phosphate ion) have less energy than the reactants [ATP and water (H2O)].
The hydrolysis of ATP releases energy that can be used for cellular processes that require energy. We have said that animals obtain chemical energy from the fooda€”carbohydrates, fats, and proteinsa€”they eat through reactions defined collectively as catabolism.
In stage II, these monomer units (or building blocks) are further broken down through different reaction pathways, one of which produces ATP, to form a common end product that can then be used in stage III to produce even more ATP. Carbohydrate digestion begins in the mouth (Figure 20.5 "The Principal Events and Sites of Carbohydrate Digestion"), where salivary I±-amylase attacks the I±-glycosidic linkages in starch, the main carbohydrate ingested by humans. Protein digestion begins in the stomach (Figure 20.6 "The Principal Events and Sites of Protein Digestion"), where the action of gastric juice hydrolyzes about 10% of the peptide bonds. The pain of a gastric ulcer is at least partially due to irritation of the ulcerated tissue by acidic gastric juice. Aminopeptidases in the intestinal juice remove amino acids from the N-terminal end of peptides and proteins possessing a free amino group. This diagram illustrates where in a peptide the different peptidases we have discussed would catalyze hydrolysis the peptide bonds. Lipid digestion begins in the upper portion of the small intestine (Figure 20.9 "The Principal Events and Sites of Lipid (Primarily Triglyceride) Digestion"). The monoglycerides and fatty acids cross the intestinal lining into the bloodstream, where they are resynthesized into triglycerides and transported as lipoprotein complexes known as chylomicrons.
The further metabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in stages II and III of catabolism. In what section of the digestive tract does most of the carbohydrate, lipid, and protein digestion take place? Aminopeptidase catalyzes the hydrolysis of amino acids from the N-terminal end of a protein, while carboxypeptidase catalyzes the hydrolysis of amino acids from the C-terminal end of a protein. During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids.
Using chemical equations, describe the chemical changes that triglycerides undergo during digestion. What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment?
What are the expected products from the enzymatic action of trypsin on each amino acid segment?
Chymotrypsin is found in the small intestine and catalyzes the hydrolysis of peptide bonds following aromatic amino acids.
Pepsin is found in the stomach and catalyzes the hydrolysis of peptide bonds, primarily those that occur after aromatic amino acids. Bile salts aid in digestion by dispersing lipids throughout the aqueous solution in the small intestine. Emulsification is important because lipids are not soluble in water; it breaks lipids up into smaller particles that can be more readily hydrolyzed by lipases.
A metabolic pathwayA series of biochemical reactions by which an organism converts a given reactant to a specific end product.
A metabolic pathway is a series of biochemical reactions by which an organism converts a given reactant to a specific end product. The acetyl group enters a cyclic sequence of reactions known collectively as the citric acid cycle (or Krebs cycle or tricarboxylic acid [TCA] cycle)A cyclic sequence of reactions that brings about the oxidation of a two-C unit to carbon dioxide and water.. At first glance, the citric acid cycle appears rather complex (Figure 20.12 "Reactions of the Citric Acid Cycle").
In the first reaction, acetyl-CoA enters the citric acid cycle, and the acetyl group is transferred onto oxaloacetate, yielding citrate. Isocitrate then undergoes a reaction known as oxidative decarboxylation because the alcohol is oxidized and the molecule is shortened by one carbon atom with the release of carbon dioxide (decarboxylation).
Comment: So far, in the first four steps, two carbon atoms have entered the cycle as an acetyl group, and two carbon atoms have been released as molecules of carbon dioxide. In the fifth reaction, the energy released by the hydrolysis of the high-energy thioester bond of succinyl-CoA is used to form guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and inorganic phosphate in a reaction catalyzed by succinyl-CoA synthetase. Succinate dehydrogenase then catalyzes the removal of two hydrogen atoms from succinate, forming fumarate. In the following step, a molecule of water is added to the double bond of fumarate to form L-malate in a reaction catalyzed by fumarase. One revolution of the cycle is completed with the oxidation of L-malate to oxaloacetate, brought about by malate dehydrogenase.
Respiration can be defined as the process by which cells oxidize organic molecules in the presence of gaseous oxygen to produce carbon dioxide, water, and energy in the form of ATP. Figure 20.14 "The Mitochondrial Electron Transport Chain and ATP Synthase" illustrates the organization of the electron transport chain. In the oxidation half-reaction, two hydrogen (H+) ions and two electrons are removed from the substrate.
Electrons from FADH2, formed in step 6 of the citric acid cycle, enter the electron transport chain through complex II. Complexes III and IV include several iron-containing proteins known as cytochromesA protein that contains an iron porphyrin in which iron can alternate between Fe(II) and Fe(III).. Each intermediate compound in the electron transport chain is reduced by the addition of one or two electrons in one reaction and then subsequently restored to its original form by delivering the electron(s) to the next compound along the chain. Looking again at Figure 20.14 "The Mitochondrial Electron Transport Chain and ATP Synthase", we see that as electrons are being transferred through the electron transport chain, hydrogen (H+) ions are being transported across the inner mitochondrial membrane from the matrix to the intermembrane space. In cells that are using energy, the turnover of ATP is very high, so these cells contain high levels of ADP. Mitochondria are small organelles with a double membrane that contain the enzymes and other molecules needed for the production of most of the ATP needed by the body. The reduced coenzymes (NADH and FADH2) produced by the citric acid cycle are reoxidized by the reactions of the electron transport chain.
The pH gradient produced by the electron transport chain drives the synthesis of ATP from ADP. From the reactions in Exercises 1 and 2, select the equation(s) by number and letter in which each type of reaction occurs. Both molecules serve as electron shuttles between the complexes of the electron transport chain. Cytochromes are proteins in the electron transport chain and serve as one-electron carriers. Describe how the presence or absence of oxygen determines what happens to the pyruvate and the NADH that are produced in glycolysis. Determine the amount of ATP produced by the oxidation of glucose in the presence and absence of oxygen.

In stage II of catabolism, the metabolic pathway known as glycolysisThe metabolic pathway in which glucose is broken down to two molecules of pyruvate with the corresponding production of ATP. The 10 reactions of glycolysis, summarized in Figure 20.16 "Glycolysis", can be divided into two phases. When glucose enters a cell, it is immediately phosphorylated to form glucose 6-phosphate, in the first reaction of phase I. See the license for more details, but that basically means you can share this book as long as you credit the author (but see below), don't make money from it, and do make it available to everyone else under the same terms.
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Hormones in general act by binding to some protein, known as the hormonea€™s receptor, thus initiating a series of events that lead to a desired outcome.
The I± chains, positioned on the outer surface of the membrane, consist of 735 amino acids each and contain the binding site for insulin.
Animals, for example, require heat energy to maintain body temperature, mechanical energy to move their limbs, and chemical energy to synthesize the compounds needed by their cells. RespirationThe process by which cells oxidize organic molecules in the presence of gaseous oxygen to produce carbon dioxide, water, and energy in the form of ATP.
But the energy released in the burning of wood is manifested entirely in the form of heat, and excess heat energy is not only useless but also injurious to the living cell. As a result, these compounds are able to supply energy for biochemical processes that require energy. Several others are listed in Table 20.1 "Energy Released by Hydrolysis of Some Phosphate Compounds". We can think of catabolism as occurring in three stages (Figure 20.4 "Energy Conversions"). The secretion of I±-amylase in the small intestine converts any remaining starch molecules, as well as the dextrins, to maltose. Gastric juiceA mixture of water, inorganic ions, hydrochloric acid, and various enzymes and proteins found in the stomach. Pancreatic juice, carried from the pancreas via the pancreatic duct, contains inactive enzymes such as trypsinogen and chymotrypsinogen. Figure 20.8 "Hydrolysis of a Peptide by Several Peptidases" illustrates the specificity of these protein-digesting enzymes. A hormone secreted in this region stimulates the gallbladder to discharge bile into the duodenum. Phospholipids and cholesteryl esters undergo similar hydrolysis in the small intestine, and their component molecules are also absorbed through the intestinal lining.
Chymotrypsin catalyzes the hydrolysis of peptide bonds following aromatic amino acids, while trypsin catalyzes the hydrolysis of peptide bonds following lysine and arginine. The acetyl unit, derived (as we will see) from the breakdown of carbohydrates, lipids, and proteins, is attached to coenzyme A, making the acetyl unit more reactive. The cyclical design of this complex series of reactions, which bring about the oxidation of the acetyl group of acetyl-CoA to carbon dioxide and water, was first proposed by Hans Krebs in 1937. All the reactions, however, are familiar types in organic chemistry: hydration, oxidation, decarboxylation, and hydrolysis.
In this reaction, a tertiary alcohol, which cannot be oxidized, is converted to a secondary alcohol, which can be oxidized in the next step.
The reaction is catalyzed by isocitrate dehydrogenase, and the product of the reaction is I±-ketoglutarate. This time I±-ketoglutarate is converted to succinyl-CoA, and another molecule of NAD+ is reduced to NADH. The remaining reactions of the citric acid cycle use the four carbon atoms of the succinyl group to resynthesize a molecule of oxaloacetate, which is the compound needed to combine with an incoming acetyl group and begin another round of the cycle. This step is the only reaction in the citric acid cycle that directly forms a high-energy phosphate compound. This oxidation-reduction reaction uses flavin adenine dinucleotide (FAD), rather than NAD+, as the oxidizing agent.
We have seen that two carbon atoms enter the citric acid cycle from acetyl-CoA (step 1), and two different carbon atoms exit the cycle as carbon dioxide (steps 3 and 4). A cell may contain 100a€“5,000 mitochondria, depending on its function, and the mitochondria can reproduce themselves if the energy requirements of the cell increase. The components of the chain are organized into four complexes designated I, II, III, and IV. In the reduction half-reaction, the NAD+ molecule accepts both of those electrons and one of the H+ ions.
Succinate dehydrogenase, the enzyme in the citric acid cycle that catalyzes the formation of FADH2 from FAD is part of complex II.
The iron in these enzymes is located in substructures known as iron porphyrins (Figure 20.15 "An Iron Porphyrin"). The coenzymes NADH and FADH2 are oxidized by the respiratory chain only if ADP is simultaneously phosphorylated to ATP. The concentration of H+ is already higher in the intermembrane space than in the matrix, so energy is required to transport the additional H+ there. They must therefore consume large quantities of oxygen continuously, so as to have the energy necessary to phosphorylate ADP to form ATP.
Table 20.2 "Maximum Yield of ATP from the Complete Oxidation of 1 Mol of Acetyl-CoA" summarizes the theoretical maximum yield of ATP produced by the complete oxidation of 1 mol of acetyl-CoA through the sequential action of the citric acid cycle, the electron transport chain, and oxidative phosphorylation.
For each acetyl-CoA that enters the citric acid cycle, 2 molecules of carbon dioxide, 3 molecules of NADH, 1 molecule of ATP, and 1 molecule of FADH2 are produced.
This series of reactions also produces a pH gradient across the inner mitochondrial membrane.
In the first 5 reactionsa€”phase Ia€”glucose is broken down into two molecules of glyceraldehyde 3-phosphate.
The phosphate donor in this reaction is ATP, and the enzymea€”which requires magnesium ions for its activitya€”is hexokinase.
You may also download a PDF copy of this book (72 MB) or just this chapter (5 MB), suitable for printing or most e-readers, or a .zip file containing this book's HTML files (for use in a web browser offline). In the early 1970s, the insulin receptor was purified, and researchers began to study what happens after insulin binds to its receptor and how those events are linked to the uptake and metabolism of glucose in cells.
Living cells remain organized and functioning properly only through a continual supply of energy.
The oxidation process ultimately converts the lipid or carbohydrate to carbon dioxide (CO2) and water (H2O). Living organisms instead conserve much of the energy respiration releases by channeling it into a series of stepwise reactions that produce adenosine triphosphate (ATP) or other compounds that ultimately lead to the synthesis of ATP. One reason for the amount of energy released is that hydrolysis relieves the electron-electron repulsions experienced by the negatively charged phosphate groups when they are bonded to each other (Figure 20.3 "Hydrolysis of ATP to Form ADP").
Notice, however, that the energy released when ATP is hydrolyzed is approximately midway between those of the high-energy and the low-energy phosphate compounds. In stage I, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids.
HCl helps to denature food proteins; that is, it unfolds the protein molecules to expose their chains to more efficient enzyme action. The amino acids that are released by protein digestion are absorbed across the intestinal wall into the circulatory system, where they can be used for protein synthesis. The principal constituents of bile are the bile salts, which emulsify large, water-insoluble lipid droplets, disrupting some of the hydrophobic interactions holding the lipid molecules together and suspending the resulting smaller globules (micelles) in the aqueous digestive medium. Each reaction of the citric acid cycle is numbered, and in Figure 20.12 "Reactions of the Citric Acid Cycle", the two acetyl carbon atoms are highlighted in red. An important reaction linked to this is the reduction of the coenzyme nicotinamide adenine dinucleotide (NAD+) to NADH. GTP can readily transfer its terminal phosphate group to adenosine diphosphate (ADP) to generate ATP in the presence of nucleoside diphosphokinase.
Succinate dehydrogenase is the only enzyme of the citric acid cycle located within the inner mitochondrial membrane. Oxaloacetate can accept an acetyl group from acetyl-CoA, allowing the cycle to begin again.
Yet nowhere in our discussion of the citric acid cycle have we indicated how oxygen is used.
Thus there are two compartments in mitochondria: the intermembrane space, which lies between the membranes, and the matrix, which lies inside the inner membrane. These electrons come from NADH, which is formed in three reactions of the citric acid cycle.

The other H+ ion is transported from the matrix, across the inner mitochondrial membrane, and into the intermembrane space. The iron ions in the FeA·S centers are in the Fe(III) form at first, but by accepting an electron, each ion is reduced to the Fe(II) form. Like the FeA·S centers, the characteristic feature of the cytochromes is the ability of their iron atoms to exist as either Fe(II) or Fe(III).
The currently accepted model explaining how these two processes are linked is known as the chemiosmotic hypothesis, which was proposed by Peter Mitchell, resulting in Mitchell being awarded the 1978 Nobel Prize in Chemistry. Consider, for example, that resting skeletal muscles use about 30% of a resting adulta€™s oxygen consumption, but when the same muscles are working strenuously, they account for almost 90% of the total oxygen consumption of the organism.
The individual reactions in glycolysis were determined during the first part of the 20th century. In the last five reactionsa€”phase IIa€”each glyceraldehyde 3-phosphate is converted into pyruvate, and ATP is generated. The binding of insulin to its receptor stimulates the I? chains to catalyze the addition of phosphate groups to the specific side chains of tyrosine (referred to as phosphorylation) in the I? chains and other cell proteins, leading to the activation of reactions that metabolize glucose.
This means that the hydrolysis of ATP can provide energy for the phosphorylation of the compounds below it in the table. One part of stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer unitsa€”which occurs in the mouth, stomach, and small intestinea€”and is referred to as digestionThe breakdown of food molecules by hydrolysis reactions into the individual monomer units in the mouth, stomach, and small intestine..
Disaccharides such as sucrose and lactose are not digested until they reach the small intestine, where they are acted on by sucrase and lactase, respectively. The principal digestive component of gastric juice is pepsinogen, an inactive enzyme produced in cells located in the stomach wall. For example, it may be used as the starting material for the biosynthesis of lipids (such as triglycerides, phospholipids, or cholesterol and other steroids).
The citric acid cycle produces adenosine triphosphate (ATP), reduced nicotinamide adenine dinucleotide (NADH), reduced flavin adenine dinucleotide (FADH2), and metabolic intermediates for the synthesis of needed compounds. Each intermediate in the cycle is a carboxylic acid, existing as an anion at physiological pH. The NADH is ultimately reoxidized, and the energy released is used in the synthesis of ATP, as we shall see.
Recall, however, that in the four oxidation-reduction steps occurring in the citric acid cycle, the coenzyme NAD+ or FAD is reduced to NADH or FADH2, respectively. The outer membrane is permeable, whereas the inner membrane is impermeable to most molecules and ions, although water, oxygen, and carbon dioxide can freely penetrate both membranes. The metal ions can be reduced and then oxidized repeatedly as electrons are passed from one component to the next. Leta€™s use step 8 as an example, the reaction in which L-malate is oxidized to oxaloacetate and NAD+ is reduced to NADH. The NADH diffuses through the matrix and is bound by complex I of the electron transport chain. Because each FeA·S center can transfer only one electron, two centers are needed to accept the two electrons that will regenerate FMN.
Thus, each cytochrome in its oxidized forma€”Fe(III)a€”can accept one electron and be reduced to the Fe(II) form. The process that links ATP synthesis to the operation of the electron transport chain is referred to as oxidative phosphorylationThe process that links ATP synthesis to the operation of the electron transport chain.. It was the first metabolic pathway to be elucidated, in part because the participating enzymes are found in soluble form in the cell and are readily isolated and purified.
Notice that all the intermediates in glycolysis are phosphorylated and contain either six or three carbon atoms.
The presence of such a reaction in a catabolic pathway that is supposed to generate energy may surprise you. In this chapter we will look at the pathway that breaks down glucosea€”in response to activation by insulina€”for the purpose of providing energy for the cell. Section 20.1 "ATPa€”the Universal Energy Currency" examines the structure of ATP and begins to explore its role as the chemical energy carrier of the body. For example, the hydrolysis of ATP provides sufficient energy for the phosphorylation of glucose to form glucose 1-phosphate.
The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose.
When food enters the stomach after a period of fasting, pepsinogen is converted to its active forma€”pepsina€”in a series of steps initiated by the drop in pH.
Chymotrypsin preferentially attacks peptide bonds involving the carboxyl groups of the aromatic amino acids (phenylalanine, tryptophan, and tyrosine).
Most importantly for energy generation, it may enter the citric acid cycle and be oxidized to produce energy, if energy is needed and oxygen is available. All the reactions occur within the mitochondria, which are small organelles within the cells of plants and animals. As such, it prevents the cycle from operating in the reverse direction, in which acetyl-CoA would be synthesized from carbon dioxide. The matrix contains all the enzymes of the citric acid cycle with the exception of succinate dehydrogenase, which is embedded in the inner membrane.
Recall from Chapter 5 "Introduction to Chemical Reactions", Section 5.5 "Oxidation-Reduction (Redox) Reactions", that a compound is reduced when it gains electrons or hydrogen atoms and is oxidized when it loses electrons or hydrogen atoms. This change in oxidation state is reversible, so the reduced form can donate its electron to the next cytochrome, and so on.
The buildup of H+ ions in the intermembrane space results in an H+ ion gradient that is a large energy source, like water behind a dam (because, given the opportunity, the protons will flow out of the intermembrane space and into the less concentrated matrix). The pathway is structured so that the product of one enzyme-catalyzed reaction becomes the substrate of the next.
On the other hand, a green plant is able to absorb radiant energy from the sun, the most abundant source of energy for life on the earth.
By the same token, the hydrolysis of compounds, such as creatine phosphate, that appear above ATP in the table can provide the energy needed to resynthesize ATP from ADP. Trypsin attacks peptide bonds involving the carboxyl groups of the basic amino acids (lysine and arginine). We will look more closely at the structure of mitochondria in Section 20.5 "Stage II of Carbohydrate Catabolism". The enzymes that are needed for the reoxidation of NADH and FADH2 and ATP production are also located in the inner membrane. By passing the electrons along, NADH is oxidized back to NAD+ and FMN is reduced to FMNH2 (reduced form of flavin mononucleotide). Complex III contains cytochromes b and c, as well as FeA·S proteins, with cytochrome c acting as the electron shuttle between complex III and IV.
Current research indicates that the flow of H+ down this concentration gradient through a fifth enzyme complex, known as ATP synthase, leads to a change in the structure of the synthase, causing the synthesis and release of ATP.
Plants use this energy first to form glucose and then to make other carbohydrates, as well as lipids and proteins. It has a fairly broad specificity but acts preferentially on linkages involving the aromatic amino acids tryptophan, tyrosine, and phenylalanine, as well as methionine and leucine. Pancreatic juice also contains procarboxypeptidase, which is cleaved by trypsin to carboxypeptidase.
Instead, oxygen participation and significant ATP production occur subsequent to the citric acid cycle, in two pathways that are closely linked: electron transport and oxidative phosphorylation. They are arranged in specific positions so that they function in a manner analogous to a bucket brigade. The latter is an enzyme that catalyzes the hydrolysis of peptide linkages at the free carboxyl end of the peptide chain, resulting in the stepwise liberation of free amino acids from the carboxyl end of the polypeptide. This highly organized sequence of oxidation-reduction enzymes is known as the electron transport chain (or respiratory chain)An organized sequence of oxidation-reduction reactions that ultimately transports electrons to oxygen, reducing it to water.. This enzyme has the ability to transfer electrons to molecular oxygen, the last electron acceptor in the chain of electron transport reactions.
They must eat plants or other animals to get carbohydrates, fats, and proteins and the chemical energy stored in them (Figure 20.2 "Some Energy Transformations in Living Systems"). Once digested and transported to the cells, the nutrient molecules can be used in either of two ways: as building blocks for making new cell parts or repairing old ones or a€?burneda€? for energy.

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