Tricarboxylic acid cycle citrate synthase

The consequent interpretation, accepted by Krebs in his review of the tricarboxylic acid cycle in 1943, was therefore that citric acid could not be an intermediate on the main path of the cycle, and that the product of the condensation between oxaloacetate and acetyl CoA would have to be isocitrate, which is asymmetric. This view prevailed between 1941 and 1948 when Ogston made the important suggestion that the embarrassment of the asymmetric treatment of citrate could be avoided if the acid was metabolized asymmetrically by the relevant enzymes, citrate synthase and aconitase. If the substrate was in contact with its enzyme at three or more positions a chiral center could be introduced. 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

Notice the intermediate in the reaction of citrate synthase (fig. 13.7). Do you think at some time in the future evolution will produce a variety of citrate synthase that recovers the energy in the thioester, analogous to the production of GTP (ATP) by succinate thiokinase (page 291) Would this energy recovery have any effect on the thermodynamics of the tricarboxylic acid cycle ... 

The glycolytic pathway includes three such reactions glucose 6-phosphate isomer-ase (1,2-proton transfer), triose phosphate isomerase (1,2-proton transfer), and eno-lase (yS-elimination/dehydration). The tricarboxylic acid cycle includes four citrate synthase (Claisen condensation), aconitase (j5-elimination/dehydration followed by yS-addition/hydration), succinate dehydrogenase (hydride transfer initiated by a-proton abstraction), and fumarase (j5-elimination/dehydration). Many more reactions are found in diverse catabolic and anabolic pathways. Some enzyme-catalyzed proton abstraction reactions are facilitated by organic cofactors, e.g., pyridoxal phosphate-dependent enzymes such as amino acid racemases and transaminases and flavin cofactor-dependent enzymes such as acyl-C-A dehydrogenases others. 

Figure 8.2 Malate Dehydrogenase (MDH). (a) Reversible reaction catalyzed by MDH where NADH is nicotinamide adenine dinucleotide, reduced form (b) ribbon display structure of MDH (porcine heart) (side view) (pdb 4mdh). The homo-dimeric protein consists of two polypeptides chains (yellow and red), with nicotinamide adenine dinudeotide (NAD+) in both independent catalytic sites illustrated in a ball and stick (blue) representation (c) chemical illustration of the tricarboxylic acid cycle (TCA) to demonstrate the importance of MDH catalysis in cycle closure. Enzyme abbreviations are PDH, pyruvate dehyrogensase CS, citrate synthase.

This reaction is essentially irreversible and the thioester bond of acetyl-S-CoA (CHgCO-SCoA) has a high free energy of hydrolysis (AG° = —31 kJ —7 5 kcal). This energy is utilized in a condensation reaction of acetyl CoA with the enol form of oxaloacetic acid to produce citric acid and CoASH is liberated. The enzyme mediating this reaction, citrate synthase (condensing enzyme), is the first enzyme of the tricarboxylic acid cycle (Krebs cycle) (Fig. 17.4). 

Citrate (si)-synthase, citrate condensing enzyme, citrogenase (EC 4.1.3.7) the tricarboxylic acid cycle enzyme which catalyses the aldol condensation of ox-aloacetate and acetyl-CoA to form citrate. C.s. from E. coli (M, 248,000) consists of 4 subunits (M, 98,000). C.s. from pig or rat heart (M, 98,000) consists of 2 subunits (M, 49,000). 

The classic example is the reaction by which acetate carbon enters the tricarboxylic acid cycle, the citrate synthase reaction. Extensive mechanistic studies have established the involvement of the enolate of the acetyl... 

When oxaloacetic acid and acetyl-coenzyme A (acetyl-CoA) labeled with radioactive carbon-14 in position 2 are incubated with citrate synthase, an enzyme of the tricarboxylic acid cycle, only the following enantiomer of [2- C]citric acid is formed stereoselec-tively. Note that citric acid containing only is achiral. Assign an or S configuration to this enantiomer of [2- C]citric acid. Note Carbon-14 has a higher priority than carbon-12.)... 

Figure 3.7. Specific activities of the tricarboxylic acid cycle enzymes in P. shermanii (A) and P. petersonii (B) grown imder aerobic conditions (a), stationary conditions (b), or anaerobic conditions (c). 1 isocitrate dehydrogenase 2 aconitase 3 a-ketoglutarate dehydrogenase 4 citrate synthase 5 succinate thiokinase 6 succinate dehydrogenase 7 fumarase 8 malate dehydrogenase. From Krainova and Bonarceva (1973).

Fig. 5.3 Examples of connections between epigenetics and metabolic pathways. (Abbreviations a-KT a-ketoglutarate AcCoA acetyl coenzyme A AcsCSl acetyl-CoA synthase 1 ACL ATP-citrate lyase ETC electron-transport chain FAD flavin adenine dinucleotide GSH glutathione IDH isocitrate dehydrogenase LDH lactate dehydrogenase NAD nicotinamide adenine dinucleotide SAM5-adenosyl methionine TCA tricarboxylic acid cycle)...

Energy status refers to the mitochondrial ATP/ADP concentration ratio which may regulate the flux through the tricarboxylic acid cycle (B) in several ways, one of which involves changes in the concentrations of oxaloacetate and citrate and hence citrate synthase activity. 

Compoxmd 1080 is absorbed from tixe gastrointestinal tract, respiratory tract, mucous membranes, and woxmds (Holstege et al., 2007). Different routes of exposxxre do not have a remarkable effect on toxicity. The mechanism of action for SMFA is blockage of the tricarboxylic acid cycle. Metabolic activation by the formation of fluorocitrate is reqxxired, a process known as lethal synthesis. Huoroacetate is converted to fluoroacetyl-CoA and then converted by citrate synthase to fluorocitrate. Aconitase catalyzes the... 

Fig. 5.22. Oxidation of acetyl-CoA via the tricarboxylic acid (TCA) cycle. Individual enzymes of the pathway are marked. 1, citrate synthase 2 and 3, cis-aconitate hydratase 4 and 3, isocitrate dehydrogenase 6, a-oxo glutarate dehydrogenas 7, succinate thiokinase 8, succinate...

Citrate is a key intermediate of the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, in the central metabolism of cells. (The set of reactions of the TCA cycle will be considered in some detail in Chapter 6.) One reaction in the cycle is the combination of oxaloacetate (OAA) and the acetyl group from acetyl coenzyme A (ACCOA), in the presence of H2O, to form citrate (CIT), thiol coenzyme A (COASH), and hydrogen ion (H+). The chemical reference reaction for this aldol condensation-hydrolysis reaction catalyzed by citrate synthase is ... 

Citrate synthase is induced, which stimulates tricarboxylic acid (TCA) cycle activity and results in the generation of ATP. 

Fig. 2.7. Tricarboxylic acid or Krebs cycle. 1 = citrate synthase 2-3 = aconitase 4 = isocitrate dehydrogenase 5 = complex a-ketoglutarate dehydrogenase 6 = snccinyl-CoA synthetase, 7 = succinate dehydrogenase 8 = fumarase 9 = malate dehydrogenase GTP = guanosine triphosphate GDP = guanosine diphosphate...

PHB synthesis from glucose using Azotobacter beijerinkii revealed substantial amounts of polymer accumulation under oxygen limitation conditions. The key feature of control in A. beijerinckii is the pool size of acetyl-CoA, which may either be oxidized via the tricarboxylic acid (TCA) cycle or can serve as a substrate for PHB synthesis the diversion depends on environmental conditions, especially oxygen limitation, when the NADH/NAD ratio increases. Citrate synthase and isocitrate dehydrogenase are inhibited by NADH, and as a consequence, acetyl-CoA no longer enters the TCA cycle at the same rate. Instead acetyl-CoA is converted to acetoacetyl-CoA by p-ketothiolase, the first... 

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