Isocitric Pyruvate metabolism

ADP, acetyl-CoA, succinyl-CoA, and citrate. The major known sites for regulation of the cycle include two enzymes outside the cycle (pyruvate dehydrogenase and pyruvate carboxylase) and three enzymes inside the cycle (citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase). All of these sites of regulation represent important metabolic branchpoints. 

After comparing the protein profiles of myocardial mitochondria between a chronic restraint stress group and a control group, 11 protein spots were found to change, of which seven were identified. Five of these proteins, carnitine palmitoyltransferase 2, mitochondrial acyl-CoA thioesterase 1, isocitrate dehydrogenase 3 (NAD ) alpha, fumarate hydratase 1, and pyruvate dehydrogenase beta, were foimd to decrease in abimdance following chronic restraint stress with fimctional roles in the Krebs cycle and lipid metabolism in mitochondria. The other two proteins, creatine kinase and prohibitin, increased after chronic restraint stress (liu et ak, 2004). 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

We have also looked at the reaction of chromate with a series of other carboxylates under these same conditions, including formate, oxalate, succinate, /5-hydroxybutyrate, glycolate, malate, pyruvate and isocitrate. None of these candidate reductants reduced chromate at pH 7.4. These results emphasize the fact that chromate reduction under neutral or slightly alkaline conditions is under kinetic control, since all of the above carboxylates have favorable reduction potentials (see Table 1) and are thermodynamically capable of reducing chromate. Thus none of the carboxylic acids that have been examined, appears to be able to reduce chromate at pH 7.4 (1 M Tris HCl) and 25 °C. These results again point to the significance of the SH group in chromate metabolism. 

As pyruvic acid decarboxylation constitutes the link between glycolysis and the Krebs cycle, a-ketoglutaric decarboxylation divides the reactions involving 6-carbon acids (citrate, isocitrate, and oxalosuccinate) and those involving 4-carbon acids (succinate, fumarate, and malate). The analogy between the two reactions is not restricted to their role in intermediate metabolism, but extends also to the mechanism of action of the two multiple-enzyme systems. In a-ketoglutaric decarboxylation, the overall reaction leads to the formation of CO2 and succinate. CoA, NAD, thiamine, lipoic acid, and magnesium are requirements for this multiple-enzyme system activity. 

Figure 11.5 Metabolic pathways related to glutamic acid production in C glutamicum. ICDH, isocitrate dehydrogenase, ODHC, 2-oxoglutarate dehydrogenase complex GDH, glutamate dehydrogenase PPC, phosphoenolpyruvate carboxylase and PC, pyruvate carboxylase.

Figure 17.1 Anaerobic metabolic pathways involved in SA production in wild-type Actinobacillus succinogenes. PEP, phos-phoenolpyruvate OAA, oxaloacetate MAL, malate FUM, fumarate ICT, IsocItrate CIT, citrate acetyl-P, acetyl-phosphate pck, PEP carboxykinase QOH, menaquinol Idh, lactate dehydrogenase pfi, pyruvate formatelyase ...

Figure 17.10 Metabolic pathways involved in SA production in the metabolically engineered Escherichia coii SBS550MG strain. NADH competing pathways are knocked out with the knockout of acetate pathway, while glyoxylate shunt is activated by knockout of the icIR with introduction of the Lactococcus lactis pyc gene. Black lines indicate overexpression. X indicate knocked out genes. PEP, phosphoenolpyruvate OAA, oxaloac-etate MAL, malate FUM, fumarate Suc-CoA, succinyl-CoA a-KG, a-ketoglutarate ICT, isocitrate CIT, citrate PYR, pyruvate AcCoA, acetyl-CoA idhA, lactate dehydrogenase pta,...

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