Citric acid cycle reactions succinate dehydrogenase

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

Three modifications of the conventional oxidative citric acid cycle are needed, which substitute irreversible enzyme steps. Succinate dehydrogenase is replaced by fumarate reductase, 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarate oxidoreductase (2-oxoglutarate synthase), and citrate synthase by ATP-citrate lyase [3, 16] it should be noted that the carboxylases of the cycle catalyze the reductive carboxylation reactions. There are variants of the ATP-driven cleavage of citrate as well as of isocitrate formation [7]. The reductive citric acid... 

Acetyl-CoA is oxidized to C02 by the Krebs cycle, also called the tricarboxylic acid cycle or citric acid cycle. The origin of the acetyl-CoA may be pyruvate, fatty acids, amino acids, or the ketone bodies. The Krebs cycle may be considered the terminal oxidative pathway for all foodstuffs. It operates in the mitochondria, its enzymes being located in their matrices. Succinate dehydrogenase is located on the inner mitochondrial membrane and is part of the oxidative phosphorylation enzyme system as well (Chapter 17). The chemical reactions involved are summarized in Figure 18.7. The overall reaction from pyruvate can be represented by Equation (18.5) ... 

Succinate dehydrogenase catalyzes the so-called trans elimination of two H s. This is the only reaction in the citric acid cycle involving FAD, and succinate dehydrogenase is the only enzyme in the cycle that is membrane-bound. The importance of this will be discussed in Chap. 14. 

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

In the succinate dehydrogenase-catalyzed reaction, why is the appropriate electron acceptor FAD rather than NAD+, which is used in the other redox reactions of the citric acid cycle (Chap. 12) ... 

Succinate-CoQ Reductase (Complex II) Succinate dehydrogenase, the enzyme that oxidizes a molecule of succinate to fumarate in the citric acid cycle, is an integral component of the succinate-CoQ reductase complex. The two electrons released in conversion of succinate to fumarate are transferred first to FAD, then to an iron-sulfur cluster, and finally to CoQ (see Figure 8-17). The overall reaction catalyzed by this complex is... 

Succinate dehydrogenase (also called succinate-coenzyme Q reductase or Complex II) is an enzyme of the citric acid cycle and glyoxylate cycle that catalyzes the reaction below ... 

Succinyl-CoA is an intermediate of the citric acid cycle produced by decarboxylation of oi-ketoglutarate. The reaction is catalyzed by the a-ketoglutarate dehydrogenase enzyme complex. Succinyl-CoA is converted to succinate (with formation of GTP) in a reaction catalyzed by the enzyme succinyl-CoA synthetase. 

Fumarate is an intermediate of the citric acid cycle and the glyoxylate cycle, produced by action of the enzyme succinate dehydrogenase on succinate. FADH2 is produced from FAD in the reaction. Fumarate is converted to L-malate by addition of water to the molecule catalyzed by the enzyme fumarate hydratase. 

Oxidation/reduction reactions involve the transfer of electrons from a molecule being oxidized (the electron donor) to a molecule being reduced (the electron acceptor). Because one or more electrons are transferred, neither oxidation nor reduction can occur without the other occurring simultaneously. An example of an oxidation/reduction reaction is the following reaction, catalyzed by succinate dehydrogenase, from the citric acid cycle ... 

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