Cycle citric acid

Fig. 3. Biosynthetic pathways for amino acids. HMP = hexose monophosphate pathway CAC = citric acid cycle P = phosphate PP = pyrophosphate ...

L-glutamate dehydrogenase L-glutamate test for 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. 

Citric acid cycle components (from rat heart mitochondria). 

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. 

FIGURE 18.16 Compartmentalization of glycolysis, the citric acid cycle, and oxidative phosphorylation. 

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... 

Lowen.stein, J. M., ed., 1969. Citric Acid Cycle Control and Compartmentation. New York Marcel Dekker. 

Oxidation of 2 molecules of glyceraldehyde-3-phosphate yields 2 NADH Pyruvate conversion to acetyl-CoA (mitochondria) 2 NADH Citric acid cycle (mitochondria) 2 molecules of GTP from 2 molecules of succinyl-CoA + 2 + 2... 

FADHg] from each citric acid cycle produce 1.5 ATP each + 3 + 3... 

NADH from citric acid cycle produce 2.5 ATP each + 15 + 15... 

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),... 

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