Citric acid cycle citrate formation

These considerations are supported by the observation that the active oxidation of some member of the citric acid cycle (citrate) is necessary for the maximum incorporation of formate into serine by the cyclo-phorase system of Green. ... 

The toxicity of fluoroacetic acid and of its derivatives has played an historical decisive role at the conceptual level. Indeed, it demonstrates that a fluorinated analogue of a natural substrate could have an activity profile that is far different from that of the nonfluorinated parent compound. The toxicity of fluoroacetic acid is due to its ability to block the citric acid cycle (Krebs cycle), which is an essential process of the respiratory chain. The fluoroacetate is transformed in vivo into 2-fluorocitrate by the citrate synthase. It is generally admitted that aconitase (the enzyme that performs the following step of the Krebs cycle) is inhibited by 2-fluorocitrate the formation of aconitate through elimination of the water molecule is a priori impossible from this substrate analogue (Figure 7.1). 

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... 

The acetyl CoA formed in fatty acid oxidation enters the citric acid cycle only if fat and carbohydrate degradation are appropriately balanced. The reason is that the entry of acetyl CoA into the citric acid cycle depends on the availability of oxaloacetate for the formation of citrate, but the concentration of oxaloacetate is lowered if carbohydrate is unavailable or improperly utilized. Recall that oxaloacetate is normally formed from pyruvate, the product of glycolysis, by pyruvate carboxylase (Section 16.3.1). This is the molecular basis of the adage that fats burn in the flame of carbohydrates. 

Mitochondrial aconitase, enzyme catalyzing the first steps of the citric acid cycle, is known to be reversibly inactivated by very low concentrations of Of This should results in (i) inhibition of supply of the respiratory chain by reducing equivalents and, hence, of the 02 formation, and (ii) accumulation of citrate, an excellent Fe and Fe chelator. 

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... 

The major site for fatty acid synthesis is the cytosol. However, acetyl-CoA, the substrate for fatty acid synthesis, is formed from pyruvate in the mitochondria and, since the inner membrane is impermeable to acetyl-CoA, is translocated to the cytoplasm indirectly as citrate (Fig. 8). When the rate of production of acetyl-CoA from pyruvate is high, the rate of formation of citrate, catalyzed by citrate synthase in the citric acid cycle, is also elevated, and citrate accumulates in mitochondria. Citrate is then translocated into the... 

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... 

As discussed in section 5.6.1, citrate leaves the mitochondria to act as a source of acetyl CoA for fatty acid synthesis only when there is an adequate amount to maintain citric acid cycle activity. As citrate accumulates in the cytosol, it acts as a feed-forward activator of acetyl CoA carboxylase, increasing the formation of malonyl CoA. 

With the use of labeled succinate-1,4-C S succinate-2,3-C labeled a-ketoglutarate, and citrate as substrates for heme formation it was demonstrated that a-ketoglutarate gave rise to the succinyl derivative (27). By blocking succinic dehydrogenase with malonate (28) the succinyl derivative was found to be formed via the citric acid cycle, or to a lesser extent more directly from succinate. The predicted carbon atoms in heme contained the C . 

During the formation of carboxylic acid like lA, there will be shuttling of metabolites within the intracellular compartments, having the capability to utilize the enzymes of the respective compartments. Jaklitsch et al. (1991) reported that the CadA, which is e key enzyme for the biosynthesis of lA, is located in cytosol. Otiier important enzymes, such as citrate synthase and aconitase, are found in the mitochondria, but some residual level of these enzymes are also found in the cytosolic fraction. The depicted mechanism is that the ds-aconitate is transported to cytosol assisted by the malate-citrate antiporter. The biosynthetic pathway of LA in the citric acid cycle is illustrated in Fig. 10.5. 

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