Carbon dioxide production citric acid cycle

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. 

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

Ethanol is oxidized by alcohol dehydrogenase (in the presence of nicotinamide adenine dinucleotide [NAD]) or the microsomal ethanol oxidizing system (MEOS) (in the presence of reduced nicotinamide adenine dinucleotide phosphate [NADPH]). Acetaldehyde, the first product in ethanol oxidation, is metabolized to acetic acid by aldehyde dehydrogenase in the presence of NAD. Acetic acid is broken down through the citric acid cycle to carbon dioxide (CO2) and water (H2O). Impairment of the metabolism of acetaldehyde to acetic acid is the major mechanism of action of disulfiram for the treatment of alcoholism. 

The citric acid cycle is at the heart of aerobic cellular metabolism, or respiration. This is true of both prokaryotic and eukaryotic organisms, of plants and animals, of organisms large and small. Here is the main point. On the one hand, the small molecule products of catabolism of carbohydrates, lipids, and amino acids feed into the citric acid cycle. There they are converted to the ultimate end products of catabolism, carbon dioxide and water. On the other hand, the molecules of the citric acid cycle are intermediates for carbohydrate, lipid, and amino acid synthesis. Thus, the citric acid cycle is said to be amphibolic, involved in both catabolism and anabolism. It is a sink for the products of degradation of carbohydrates, lipids, and proteins and a source of building blocks for them as well. 

The Krebs-citric acid cycle is the final common pathway for the oxidation of fuel molecules amino acids, fatty acids and carbohydrates. Most fuel molecules enter the cycle as a breakdown product, acetyl coenzyme A (acetyl CoA), which reacts with oxaloacetate (a four-carbon compound) to produce citrate (a six-carbon compound), which is then converted in a series of enzyme-catalysed steps back to oxaloacetate. In the process, two molecules of carbon dioxide and four energy-rich molecules are given off, and these latter are the precursors of the energy-rich molecule ATP, which is subsequently formed and which acts as the fuel source for all aerobic organisms. 

Aerobic glycolysis first involves a ten-step conversion of glucose to pyruvic acid or pyruvate, called the Embden-Meyerhoff-Pamas pathway, followed by its further conversion to carbon dioxide and water via what is variously called the tricarboxylic acid cycle, or citric acid cycle, or Krebs cycle after its discoverer. The net products discharged from the cycle are carbon dioxide and water, with recycle of a further product called oxaloacetic acid or oxaloacetate. Successive organic acids that contain three carboxyl groups (-COOH), are initially involved in the cycle starting with citric acid or a neutral salt of citric acid (citrate). Hence the designator tricarboxylic. 

While this book is not the appropriate place for a detailed discussion of the ADP-ATP cycle, perhaps a bit more mechanistic detail is useful. The aerobic metabolism of glucose to produce ATP from ADP can be-considered to consist of three parts the fermentation of glucose to form pyruvate (and a small amount of ATP from ADP), the conversion of the pyruvate to carbon dioxide in the citric acid cycle in which NAD" " and FAD (the oxidized form of flavin adenine dinucleotide) are converted to NADH and FADH2, and their oxidation in the respiratory chain, resulting in the formation of a larger quantity of ATP from ADP. If oxygen is not available, so the reaction is anaerobic, only the first step, formation of pyruvate, occurs with a small amount of ATP production. 

In animals TPP-dependent decarboxylation reactions are essential to the production of energy needed for cell metahohsm. In these reactions a-ketoacids are converted to acyl CoA molecules and carbon dioxide. The reactions (e.g., the conversion of pyruvate to acetyl CoA) are an important part of the breakdown of carbohydrates, and of the conversion of several classes of molecules (carbohydrates, fats, and proteins) to energy, carbon dioxide, and water in the citric acid cycle. In other organisms, in addition to its participation in the above reactions, TPP is a required coenzyme in alcohol fermentation, in the carbon fixation reactions of photosynthesis, and in the hiosynthesis of the amino acids leucine and valine. 

The citric acid cycle is shown in schematic form in Figure 19.3. Under aerobic conditions, pyruvate produced by glycolysis is oxidized further, with carbon dioxide and water as the final products. First, the pyruvate is oxidized to one carbon dioxide molecule and to one acetyl group, which becomes linked to an intermediate, coenzyme A (GoA) (Section 15.7). The acetyl-GoA enters the citric acid cycle. In the citric acid cycle, two more molecules of carbon dioxide are produced for each molecule of acetyl-GoA that enters the cycle, and electrons are transferred in the process. The immediate electron acceptor in all cases but one is NAD which is reduced to NADH. In the one case in which there is another intermediate electron acceptor, FAD (flavin adenine dinucleotide). 

When a fatty acid with an even number of carbon atoms undergoes successive rounds of the p-oxidation cycle, the product is acetyl-CoA. (Fatty acids with even numbers of carbon atoms are the ones normally found in nature, so acetyl-CoA is the usual product of fatty-acid catabolism.) The number of molecules of acetyl-CoA produced is equal to half the number of carbon atoms in the original fatty acid. For example, stearic acid contains 18 carbon atoms and gives rise to 9 molecules of acetyl-CoA. Note that the conversion of one 18-carbon stearic acid molecule to nine 2-carbon acetyl rmits requires eight, not nine, cycles of P-oxidation (Figure 21.7). The acetyl-GoA enters the citric acid cycle, with the rest of the oxidation of fetty acids to carbon dioxide and water taking... 

The major pathway of catabolism of monosaccharides, such as glucose and fructose, is glycolysis. Monosaccharides are converted into pyruvate with the generation of ATP [6]. Pyruvate is an intermediate in several metabolic pathways, but the majority of it is converted to acetyl coenzyme A, which enters the citric acid cycle. Although more ATP is generated in the citric acid cycle, the most important product is NADH, which is derived from NAD+ as the acetyl coenzyme A is oxidized. This oxidation releases carbon dioxide as a waste product. An alternative route for glucose catabolism is the pentose phosphate pathway, in which pentose sugars such as ribose is produced. 

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). 

These products are absorbed across the wall of the small intestine into the blood and are transported to cells. Once in the cells, glucose and other monosaccharides, fatty acids, some amino acids, and glycerol enter the mitochondria and feed into a complex series of reactions called the citric acid cycle, or Krebs cycle. The citric acid cycle produces carbon dioxide and other molecules, such as NADH (not to be confused with NADPH) and ATP. This NADH and ATP then move through another set of reactions to produce more ATP and water. 

When taken up into the human body from the diet, the 22 standard amino acids are either used to synthesize proteins and other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase, the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis. 

The acetaldehyde produced from ethanol in the liver is further oxidized to acetic acid, which is eventually converted to carbon dioxide and water in the citric acid cycle. However, the intermediate products can cause considerable damage while they are present within the cells of the hver. 

This oxygen-requiring (aerobic) metabolic process, which is also known as the Krebs cycle, is the means by which metabolic products of fats, carbohydrates, and protein are converted to water and carbon dioxide, plus energy. The citric acid cycle operates in the mitochondria of cells, where the appropriate enzymes are located. 

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