THE CITRIC ACID CYCLE

The citric acid cycle is a sequence of reactions in which the two carbon atoms of acetyl-CoA are ultimately oxidized to C02. It is the central pathway for the release of energy from acetyl-CoA, which is produced from the catabolism of carbohydrates (Chap. 11), fatty acids (Chap. 13), and some amino acids (Chap. 15) and is closely involved with two other processes, namely, electron transport and oxidative phosphorylation (Chap. 14). 

The name acetyl-CoA is an abbreviation for the compound acetyl coenzyme A, which has the structure shown in Fig. 12-1. Coenzyme A has three components ADP with an additional 3  

The degradation of carbohydrates (such as glucose) in the glycolytic pathway produces pyruvate, but how does the acetyl-CoA originate in carbohydrate metabolism  

Pyruvate is converted into acetyl-CoA by a group of enzymes known as the pyruvate dehydrogenase complex (see Example 12.3 and Chap. 5). Acetyl-CoA and the enzymes that catalyze the steps of the citric acid cycle are situated within the matrix of the mitochondria, except for one enzyme that is located in the inner mitochondrial membrane. 

The name stems from the first step in the cycle, which is a condensation of oxaloacetate with acetyl-CoA to form citric acid. However, as this product is a tricarboxylic acid, the cycle has an alternative name, the tricarboxylic acid cycle. 

1 The Central Role of the Citric Acid Cycle in Metabolism 

The citric acid cycle is amphibolic. It plays a role in both catabolism and anabolism. It is the central metabolic pathway. 

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Metabolism consists of catabolism, which is the oxidative breakdown of nutrients, and anabolism, which is reductive synthesis of biomolecules. The citric acid cycle is amphibolic, meaning that it plays a role in both catabolism and anabolism. Although the citric acid cycle is a part of the pathway of aerobic oxidation of nutrients (a catabolic pathway see Section 19.7), some of the molecules that are included in this cycle are the starting points of biosynthetic (anabolic) pathways (see Section 19.8). Metabolic pathways operate simultaneously, even though we talk about them separately. We should always keep this point in mind. 

The citric acid cycle has two other common names. One is the Krebs cycle, after Sir Hans Krebs, who first investigated the pathway (work for which he received a Nobel Prize in 1953). The other name is the tricarboxylic acid cycle (or TCA cycle), from the fact that some of the molecules involved are acids with three carboxyl groups. We shall start our discussion with a general overview of the pathway and then go on to discuss specific reactions. 

NADPH is a reducing agent it can be oxidized to NADP (nicotinamide-adenine dinucleotide cation, with change in the ring at the left (the reduced nicotinamide ring)  

In the cells of the human body and of other animals a reaction occurs that is the reverse of photosynthesis, the oxidation of glucose. In this reaction the liberated energy is used to convert ADP to ATP, and probably also to favor the reduction of NADP to NADPH. These energy-rich molecules then serve to fuel the many physiological mechanisms of the body. 

In the period between 1935 and 1950 there was discovered a principal way in which the oxidation of carbohydrates to water and carbon dioxide is carried out with production of a number of high-energy molecules for each molecule of carbon dioxide formed. This biochemical mechanism is called the citric acid cycle or the Krehs cycle. It was in large part formu- lated by 1943 by the British biochemist Hans Adolf Krebs (born 1900), after Albert Szent-Gyorgyi in 1935 had discovered that enzymes from muscle could catalyze the oxidation of dicarboxylic four-carbon acids (succinic, fumaric, malic, and oxaloacetic acid). 

The cycle involves the addition of two carbon atoms (acetic acid. 

CH3COOH) to oxaloacetic acid, HOOCCH2COCOOH, to form citric acid, a six-carbon tricarboxylic acid  

Describe the oxidation of acetyl-CoA in the citric acid cycle. 

The citric acid cycle is a series of reactions that connects the intermediate acetyl-CoA from the metabolic pathways in stages 1 and 2 with electron transport and the synthesis of ATP in stage 3. As a central pathway in metabolism, the citric acid cycle uses the two-carbon acetyl group in acetyl-CoA to prodnce CO2, NADH + and FADH2. The citric acid cycle is named for the citrate ion from citric acid (C6Hg07), a tricarboxylic acid, which forms in the first reaction. The citric acid cycle is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, named for H. A. Krebs, who recognized it in 1937 as the major metabolic pathway for the prodnction of energy. 

FIGURE 18.11 In the citric acid cycle, two carbon atoms are removed as CO2 from six-carbon citrate to give four-carbon succinyl-CoA, which is converted to four-carbon oxaloacetate. 

FIGURE 18.12 In the citric acid cycle, oxidation reactions produce two CO2 and reduced coenzymes NADH and FADH2, and it also regenerates oxaloacetate. 

In the first reaction of the citric acid cycle, the acetyl group (2C) from acetyl-CoA bonds with oxaloacetate (4C) to yield citrate (6C). 

Anaerobic respiration occurs when oxygen is absent. It is not as efficient as aerobic respiration as less ATP is produced overall. It begins in the same way as aerobic respiration with glycolysis in the cytoplasm to produce pyruvic acid. The citric acid cycle cannot take place because the presence of oxygen is necessary and so the pyruvic acid undergoes fermentation. This may be alcohol fermentation as occurs when yeast is grown in anaerobic conditions and ethanol is produced from pyruvic acid. Alternatively, pyruvic acid may be converted to lactic acid as occurs when oxygen is in short supply in the human body and anaerobic respiration takes place. 

The evolution of aerobic respiration was predated by the development of photosynthesis as this increased the concentration of carbon dioxide in the atmosphere (see Section 3.6.1). Indeed, respiration of some sort must have been characteristic of the earliest life forms because they required energy to survive and reproduce. The evolution of the eukaryotes also appears to have been dependent on earlier life forms such as the archaea and bacteria. As discussed in Section 3.4.1 cell organelles in bacteria, such as chloroplasts and mitochondria, became isolated as they developed membranes around them and subsequently the bacteria were incorporated into organisms to produce eukaryotes. This will be considered in Chapter 4. 

When organisms die, the organic compounds of which they are made undergo the processes of decomposition. This is as complex a process as the accumulation of organic material through photosynthesis or metabolic activities such as respiration. The various components and processes have been examined by Aber and Melillo, 2001, and Smith and Smith, 2001. Decomposition occius throughout the biosphere and comprises many chemical reactions. A host of different organisms may be involved as they 

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Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

FIGURE 18.2 The metabolic map as a set of dots and lines. The heavy dots and lines trace the central energy-releasing pathways known as glycolysis and the citric acid cycle. 

The combustion of the acetyl groups of acetyl-CoA by the citric acid cycle and oxidative phosphorylation to produce COg and HgO represents stage 3 of catabolism. The end products of the citric acid cycle, COg and HgO, are the ultimate waste products of aerobic catabolism. As we shall see in Chapter 20, the oxidation of acetyl-CoA during stage 3 metabolism generates most of the energy produced by the cell. 

Certain of the central pathways of intermediary metabolism, such as the citric acid cycle, and many metabolites of other pathways have dual purposes—they serve in both catabolism and anabolism. This dual nature is reflected in the designation of such pathways as amphibolic rather than solely catabolic or anabolic. In any event, in contrast to catabolism—which converges to the common intermediate, acetyl-CoA—the pathways of anabolism diverge from a small group of simple metabolic intermediates to yield a spectacular variety of cellular constituents. 

Catabolism converges via the citric acid cycle to three principal end products water, carbon dioxide, and ammonia. 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

Aconitase catalyzes the citric acid cycle reaction citrate isocitrate... 

The next steps of glucose catabolism are called the citric acid cycle. The pyruvic acid formed in glycolysis is transported into the mitochondria, which arc subcellular organelles with double (inner and outer) membranes. They are referred to as the powerhous-... 

ATP from the 8 N7VDH produced in the citric acid cycle (4 NADH per pyruvic acid),... 

The breakdown of glycogen to glucose. The glucose then enters glycolysis and the citric acid cycle, providing energy in the form of ATP. 

The citric acid cycle, a nine-step process, also diverts chemical energy to the production of ATP and the reduction of NAD and FAD. In each step of the citric acid cycle (also known as the Krebs cycle) a glucose metabolite is oxidized while one of the carrier molecules, NAD or FAD, is reduced. Enzymes, nature s chemical catalysts, do a remarkable job of coupling the oxidation and reduction reactions so that energy is transferred with great efficiency. 

Vlaleic acid has a dipole moment, but the closely related fumaric acid, a substance involved in the citric acid cycle by which food molecules are metabolized, does not. Explain. 

Problem 5.9 Predict the products of the following polar reaction, a step in the citric acid cycle for food metabolism, by interpreting the flow of elections indicated by Uie curved arrows ... 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

In contrast to laboratory reactions, enzyme-catalyzed reactions often give a single enantiomer of a chiral product, even when the substrate is achiral. One step in the citric acid cycle of food metabolism, for instance, is the aconitase-catalyzed addition of water to (Z)-aconitate (usually called ris-aconitate) to give isocitrate. 

A large number of biological reactions involve prochiral compounds. One of the steps in the citric acid cycle by which food is metabolized, for instance, is... 

Elucidating the stereochemistry of reaction at prochirality centers is a powerful method for studying detailed mechanisms in biochemical reactions. As just one example, the conversion of citrate to (ds)-aconitate in the citric acid cycle has been shown to occur with loss of a pro-R hydrogen, implying that the reaction takes place by an anti elimination mechanism. That is, the OH and H groups leave from opposite sides of the molecule. 

Problem 9.26 The aconitase-catalyzed addition of water to ds-aconitate in the citric acid cycle occurs with the following stereochemistry. Does the addition of the OH group occur on the Re or the Si face of the substrate What about the addition of the H Does the reaction have syn or anti stereochemistry ... 

The dehydration of citrate to yield c/s-aconitate, a step jn the citric acid cycle, involves the pro-R "arm" of citrate rather than the pio-S arm. Which of the following two products is formed ... 

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