Citric acid cycle fumarate formation

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... 

SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle and of the aerobic respiratory chain (Saraste, 1999). QFR participates in anaerobic respiration with fumarate as the terminal electron acceptor (Kroger, 1978 Kroger etal., 2002) and is part of the electron transport chain catalyzing the oxidation ofvarious donor substrates (e.g., H2 or formate) by fumarate. These reactions are coupled via an electrochemical proton potential (Ap) to ADP phosphorylation with inorganic phosphate by ATP synthase (Mitchell, 1979). 

The answer is b. (Murray, pp 182-189. Scriver, pp 1521-1552. Sack, pp 121-138. Wilson, pp 287-317.) Reducing equivalents are produced at four sites in the citric acid cycle. NADH is produced by the isocitrate dehydrogenase-catalyzed conversion of a-ketoglutarate to succinyl CoA and by the malate dehydrogenase-catalyzed conversion of malate to oxaloacetate. FADH, is produced by the succinate dehydrogenase-catalyzed conversion of succinate to fumarate. Succinyl CoA synthetase catalyzes the formation of succinate from succinyl CoA, with the concomitant phosphorylation of GDP to GTP... 

Succinate is an intermediate of the citric acid cycle (and the glyoxylate cycle) produced by action of the enzyme succinyl-CoA synthetase on succinyl-CoA. Succinate is converted to fumarate by action of the enzyme succinate dehydrogenase (with formation of FADH2)... 

Step 6. Formation of Fumarate—FAD-Linked Oxidation Succinate is oxidized to fumarate, a reaction that is catalyzed by the enzyme succinate dehydrogenase. This enzyme is an integral protein of the inner mitochondrial membrane. We shall have much more to say about the enzymes bound to the inner mitochondrial membrane in Ghapter 20. The other individual enzymes of the citric acid cycle are in the mitochondrial matrix. The electron acceptor, which is FAD rather than NADA is covalently bonded to the enzyme succinate dehydrogenase is also called a flavoprotein because of the presence of FAD with its flavin moiety. In the succinate dehydrogenase reaction, FAD is reduced to FADHo and succinate is oxidized to fumarate. 

The formation of glycine is equivalent to 9ATP, making the total 15ATP. Fumarate is converted back to aspartate via the citric acid cycle and transamination and does not require any ATP equivalents... 

The efficiency of an enzyme can be reduced or can even become negligible in the presence of certain substances, known as inhibitors. Many inhibitors have structural resemblances with the substrates and compete with them for the formation of complexes with the enzyme. This is the case of the inactivation of cytochrome c oxidase by the cyanide ion, which blocks the mitochondrial electron-transport chain to oxygen. Similarly, the inactivation of the succinate dehydrogenase by malonate involves its inhibition of the conversion of succinate to fumarate in the citric acid cycle. In the latter case, the mechanism for competitive inhibition is... 

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