Citric acid cycle, reactions citrate formation

The glyoxylate cycle (Figure 9.13) consists of five reactions. The first two reactions (the synthesis of citrate and isocitrate) are familiar ones, because they also occur in the citric acid cycle. However, the formation of citrate from... 

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... 

Step 1. Formation of Citrate The hrst step of the citric acid cycle is the reaction of acetyl-GoA and oxaloacetate to form citrate and CoA-SH. This reaction is called a condensation because a new carbon-carbon bond is formed. The condensation reaction of acetyl-GoA and oxaloacetate to form citryl-CoA takes place in the first stage of the reaction. The condensation is followed by the hydrolysis of citryl-GoA to give citrate and GoA-SH. 

Four control points exist for the citric acid cycle. One, the pyruvate dehydrogenase reaction, lies outside the cycle proper. The formation of citrate and the two oxidative decarboxylations are the other control points. ATP and NADH are inhibitors of the cycle, and ADP and NAD+ are activators. 

Radioactive acetyl CoA can be generated by direct synthesis from C-acetate or from (3 oxidation of radioactive fatty acids, such as uniformly labeled palmitate. Examination of the reactions of the citric acid cycle reveals that neither of the two carbons that enter citrate horn acetate is removed as carbon dioxide during the first pass through the cycle. Labeled carbon from C-methyl-labeled acetate appears in C-2 and C-3 of oxaloacetate, because succinate is symmetrical, with either methylene carbon in that molecule labeling C-2 or C-3 of oxaloacetate. The conversion of oxaloacetate to phosphoenolpyruvate yields PEP labeled at C-2 or C-3 as well. Formation of glyceraldehyde 3-phosphate and its isomer dihydroxyacetone phosphate gives molecules, both labeled at carbons 2 and... 

The nature of the major C-labeled isoptomers of intermediates formed by consecutive turns of the citric acid cycle are illustrated in Figure 5A for condensation of various labeled oxalacetate species with acetyl-CoA C-2 (AC2) and in Figure 5B for reactions involving acetyl-CoA C-1 (Ac ). Consequently, these reaction schemes illustrate the sequential formation of multiply-labeled intermediates, where C refers to citrate, K to a-ketoglutarate (which in the steady... 

---