Conversion of oxaloacetate

The oxaloacetate is then transported from mitochondrion into the cytosol but not directly, since there is no transporter for oxaloacetate in the mitochondrial membrane. This problem is solved by conversion of oxaloacetate to aspartate, by transamination, and it is the aspartate that is transported across the inner mitochondrial membrane to the cytosol, where oxaloacetate is regenerated from aspartate by a cytosolic aminotransferase enzyme. 

However, the urea cycle also causes a net conversion of oxaloacetate to fumarate (via aspartate), and the regeneration of oxaloacetate (Fig. 18-12) produces NADH in the malate dehydrogenase reaction. Each NADH molecule can generate up to 2.5 ATP during mitochondrial... 

The urea cycle results in a net conversion of oxaloacetate to fumarate, both of which are intermediates in the citric acid cycle. The two cycles are thus interconnected. 

As mentioned in Section 4, glyoxylate can be converted to oxaloacetate by condensation with acetyl-CoA (Fig. 17-16) and the oxaloacetate can be decarboxylated to pyruvate. This sequence of reactions resembles that of the conversion of oxaloacetate to 2-oxoglutarate in the citric acid cycle (Fig. 17-4). Doth... 

Figure 20-4. Biochemical pathways for gluconeogenesis in the liver. Alanine, a major gluconeogenic substrate, is used to synthesize oxaloacetate. The carbon skeletons of glutamine and other glucogenic amino acids feed into the TCA cycle as a-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate and thus also provide oxaloacetate. Conversion of oxaloacetate to phosphoenolpyruvate and ultimately to glucose limits the availability of oxaloacetate for citrate synthesis and thus greatly diminishes flux through the initial steps of the TCA cycle (dashed lines). Concurrent P-oxidation of fatty acids provides reducing equivalents (NADH and FADH2) for oxidative phosphorylation but results in accumulation of acetyl-CoA.

A third fate of pyruvate is its carboxylation to oxaloacetate inside mitochondria, the first step in gluconeogenesis. This reaction and the subsequent conversion of oxaloacetate into phosphoenolpyruvate bypass an irreversible step of glycolysis and hence enable glucose to be synthesized from pyruvate. The carboxylation of pyruvate is also important for replenishing intermediates of the citric acid cycle. Acetyl CoA activates pyruvate carboxylase, enhancing the synthesis of oxaloacetate, when the citric acid cycle is slowed by a paucity of this intermediate. 

There will be no labeled carbons. The CO2 added to pyruvate (formed from the lactate) to form oxaloacetate is lost with the conversion of oxaloacetate into phosphoenolpyruvate. 

D-Tartaric acid dehydratase (E.C. 4.2.1.81) and the stereochemical counterpart l-tartaric acid dehydratase (E.C. 4.2.1.32) are able to catalyze the conversion of oxaloacetic acid to d- and L-tartaric acid respectively. The actual addition of water to the C-C double bond is most likely to occur at the enol tautomer, and the resulting tartaric acid has the 2S,3S (D-stereo isomer made by E.C. 4.2.1.81) or 2R,3R (l-tartaric acid dehydratase) configuration. Despite the stereochemistry of the reactions catalyzed, the lack of available enzyme and the instability of the enzymes in presence of oxygen131 have hampered their application in organic synthesis thus far. 

The citric-acid cycle, a mitochondrial process, starts with citrate synthase catalyzing the conversion of oxaloacetate and acetyl CoA to citrate. This enzyme can be inhibited weakly by ATP and more strongly by fatty acyl CoA. 

The conversion of oxaloacetate to PEP by PEP-carboxykinase (PEPCK, Eq. 14-43 Fig. 17-20) is another control point in gluconeogenesis. Insulin inhibits gluconeogenesis by decreasing transcription of the mRNA for this enzyme. Glucagon and cAMP stimulate its transcription. The activity of PEP carboxykinase " is also enhanced by Mn + and by very low concentrations of Fe +. However, the enzyme is readily inactivated by Fe " and oxygen. Any regulatory significance is uncertain. 

Oxaloacetate, generated from pyruvate by pyruvate carboxylase or from amino acids that form intermediates of the TCA cycle, does not readily cross the mitochondrial membrane. It is either decarboxylated to form PEP by the mitochondrial PEPCK or it is converted to malate or aspartate (see Figs. 31.7B and 31.7C). The conversion of oxaloacetate to malate requires NADH. PEP, malate, and aspartate can be transported into the cytosol. 

F. 31.7. The generation of PEP from gluconeogenic precursors. A. Conversion of oxaloacetate to phosphoenolpyruvate, using PEP carboxykinase. B. Interconversion of oxaloacetate and malate. C. Transamination of aspartate to form oxaloacetate. Note that the cytosolic reaction is the reverse of the mitochondrial reaction as shown in Eigure 31.5. 

The conversion of oxaloacetate to phosphoenolpyruvate is catalyzed by the enzyme phosphoenolpyruvate carboxykinase (PEPCK), which is found in the... 

Describe the transport of acetyl groups across the inner mitochondrial membrane in the form of citrate and explain its purpose. Account for the synthesis of NADPH during the conversion of oxaloacetate into pyruvate in the cytosol. 

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... 

Is the conversion of oxaloacetate to phosphoenol pyruvate more likely to occur in the mitochondria or the cytoplasm of skeletal myocytes, and or hepatocytes ... 

The TCA-cycle is linked to Gluconeogenesis (see) by the conversion of oxaloacetate to phosphoeno/pyr-... 

L-Leucine is derived from L-valine by the x-keto acid elongation system, i.e., via x-ketoisovaleric acid and -carboxy-j8-hydroxyisocaproic acid as outlined in Fig. 199. The transformation of ot-ketoisovaleric acid to (X-ketoisocaproic acid closely resembles the conversion of oxaloacetic acid to (X-ketoglutaric acid in the tricarboxylic acid cycle (D 5). x-Ketbacid elongation systems also participate in the formation of L-lysine via the x-aminoadipic acid pathway (D 18) and in the biosynthesis of a number of secondary amino acids which are precursors of gluco-sinolates (D 9.4). 

The conversion of oxaloacetate to succinate is catalyzed by enzymes of the citric acid cycle malate dehydrogenase, fumarase and succinate dehydrogenase. These enzymes were isolated from the cells of P. shermanii... 

Allen et al., 1964), where they are known to catalyze the conversion of oxaloacetate to succinate at a high rate (Krebs and Egglestone, 1941). In propionibacteria, the succinate dehydrogenase is not inhibited by malonate, in contrast with succinate dehydrogenases of the Krebs cycle (Ichikawa, 1955). 

The irreversible step of glycolysis catalysed by pyruvate kinase is bypassed in gluconeogenesis by conversion of pyruvate first to oxaloacetate, then conversion of oxaloacetate to phosphoenolpyruvate by phosphoenolpyru-vate carboxykinase. The transfer of the amino group from glutamate to oxaloacetate produces aspartate, catalysed by the enzyme aspartate aminotransferase. Malate dehydrogenase converts oxaloacetate to malate in the malate-aspartate shutde. 

Oxaloacetate is an intermediate of many metabolic pathways. It also plays a role in the malate-aspartate shuttle, which transfers high energy electrons into mitochondria. Citrate is formed by the condensation of oxaloacetate with acetyl CoA. A transamination reaction transfers an amino group from an amino acid to an a-keto acid. Transfer of the amino group from aspartate to a-ketoglutarate forms oxaloacetate and glutamate. In gluconeogenesis, pyruvate is carboxylated in mitochondria to form oxaloacetate. After transfer to the cytosol, the enzyme phosphoenolpyruvate carboxykinase catalyses the conversion of oxaloacetate to phosphoenolpyruvate. 

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