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(Oxaloacetic acid) Net reaction per molecule of glucose: 2 Acetyl-CoA + 6H2O + 2ADP

Chemical reactions of the citric acid cycle, showing the release of carbon dioxide and a number of hydrogen atoms during the cycle.

acetyl portion, however, becomes an integral part of the citric acid molecule. During the successive stages of the citric acid cycle, several molecules of water are added, as shown on the left in the figure, and carbon dioxide and hydrogen atoms are released at other stages in the cycle, as shown on the right in the figure.

The net results of the entire citric acid cycle are given in the explanation at the bottom of Figure 67-6, demonstrating that for each molecule of glucose originally metabolized, 2 acetyl-CoA molecules enter into the citric acid cycle, along with 6 molecules of water. These are then degraded into 4 carbon dioxide molecules, 16 hydrogen atoms, and 2 molecules of coenzyme A. Two molecules of ATP are formed, as follows.

Formation of ATP in the Citric Acid Cycle. Not a great amount of energy is released during the citric acid cycle itself; in only one of the chemical reactions—during the change from a-ketoglutaric acid to succinic acid—is a molecule of ATP formed. Thus, for each molecule of glucose metabolized, two acetyl-CoA molecules pass through the citric acid cycle, each forming a molecule of ATP, or a total of two molecules of ATP formed.

Function of Dehydrogenases and Nicotinamide Adenine Dinu-cleotide in Causing Release of Hydrogen Atoms in the Citric Acid Cycle. As already noted at several points in this discussion, hydrogen atoms are released during different chemical reactions of the citric acid cycle—4 hydrogen atoms during glycolysis, 4 during formation of acetyl-CoA from pyruvic acid, and 16 in the citric acid cycle; this makes a total of 24 hydrogen atoms released for each original molecule of glucose. However, the hydrogen atoms are not simply turned loose in the intracellular fluid. Instead, they are released in packets of two, and in each instance, the release is catalyzed by a specific protein enzyme called a dehydrogenase. Twenty of the 24 hydrogen atoms immediately combine with nicoti-namide adenine dinucleotide (NAD+), a derivative of the vitamin niacin, in accordance with the following reaction:

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