Citrate synthesis

J. M. de la Fuente, V. Ramirez-Rodn guez, J. L. Cabrcra-Ponce, and L. Herrera-Estrella, Aluminum tolerance in transgenic plants by alteration of citrate synthesis. Science 276 1566 (1997). 

The stereochemistry of enzymic citrate synthesis and cleavage has been described in detail 39>. In microorganisms, it has been found that the stereospecificity of citrate synthase from different sources is different. 

Figure 9.3 k summary of the Physiological pathway of the Krebs cycle. The pathway starts with acetyl-CoA, since citrate synthesis is the flux-generating step. The physiological pathway includes the electron transfer chain, since there is no flux-generating step in this chain. The pathway is indicated by the broader lines. The pathway, therefore, starts with acetyl-CoA and finishes with CO2 and H2O, which are lost to the environment. Acetyl-CoA is formed from a variety of precursors glucose and fatty acids are presented in this figure. 

The condensation of acetyl CoA and oxaloacetate to form citrate is catalyzed by citrate synthase (Figure 9.5). This aldol condensation has an equilibrium far in the direction of citrate synthesis. Citrate synthase is allosterically activated by Ca2+ and ADP, and inhibited by ATP, NADH, succinyl CoA, and fatty acyl CoA derivatives (see Figure 9.9). However, the primary mode of regulation is also deter mined by the availability of its substrates, acetyl CoA and oxaloac etate. [Note Citrate, in addition to being an intermediate in the TCA cycle, provides a source of acetyl CoA for the cytosolic synthesis of... 

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.

The problem of reducing sintering temperature is also crucial for Bao sSrg sTiOj, which possesses ferroelectric properties but is used more often as a material with a positive temperature coefficient of resistance (PTCR). The citrate synthesis method is often considered as an alternative to freeze-drying synthesis, but in the following case the solution of barium, strontium, and titanium citrates was dried by various methods (i.e., oven drying and freeze-drying). Careful... 

The TCA cycle starts from citrate synthesis by acetyl-CoA and oxaloacetate as seen in Figure 5. This reaction, however, has a negative free energy change (AG0 = -32.3 kJ) therefore, oxaloacetate must react very quickly with acetyl-CoA and be kept nearly 0 concentration normally. 

Poly(Diol Citrate) Synthesis 260 1 6.3.2 Orthopedic Applications 273... 

Rapid p-oxidation of fatty acids in perfused liver (DeBeer et a/., 1974) and in isolated mitochondria (Lopes-Cardozo and Van den Bergh, 1972) has been shown to suppress the operation of citric acid cycle apparently from the elevation of mitochondrial NADH/NAD ratio which restricts oxaloaceta-te availability for citrate synthase and simultaneously inhibits isocitrate oxidation (Lenartowicz et a/., 1976). Considerable support for an earlier postulate that oxaloacetate availability normally determines the rate of citrate synthesis has become available. Thus, because of marked protein binding, the concentration of free, as opposed to total, oxaloacetate in matrix of liver mitochondria is now estimated to be near the of citrate synthase (Siess et al., 1976 Brocks eta ., 1980). The antiketogenic effect of alanine (Nosadini et a/., 1980) and of 3-mercaptopicolinate, an inhibitor of phosphoenolpy-ruvate carboxykinase (Blackshear et a/., 1975), is believed to be exerted, at least in part, from their ability to raise hepatic oxaloacetate concentration. And, in pyruvate carboxylase deficiency, expected to impair oxaloacetate supply, concentration of ketone bodies is elevated (Saudubray et a/., 1976). 

Fig.2. Active forms of acyl-CoA (thioesters). 2a Synthesis of acetylphosphate (activated carboxyl group). 2 b Citrate synthesis (activated a-methy-lene group).

The a-methyl group in acetyl-CoA undergoes numerous condensation reactions, e.g. 1. carboxylation of acetyl-C)OA to malonyl-CoA by biotin-dependent acetyl- A carboxylase (EC 6.4.1.2) in fatty acid biosynthesis, 2. aldol condensations, e.g. in citrate synthesis (Fig. 2 b) in the tricarboxylic acid cycle. When acetoacetyl-CoA is synthesized from 2 molecules of acelyl-CoA (ester condensation), one molecule enters the reaction as an electrophile, the other as a nucleophile. 

Fe catalyses most redox reactions in the cell (see Cytochromes). It is involved in the reduction of ribonucleotides to deoxyribonucleotides, is a coenzyme for aconitase (EC 4.2.1.3) in the tricarboxylic acid cycle, and is a component of a number of metalloflavo-proteins. It has a regulatory role in many microorganisms, e g. as an inhibitor of citrate synthesis in Aspergillus niger, and as a promoter of antibiotic synthesis by Streptomyces species. 

Kolah, A. K., Asthana, N. S., Vu, D. T., Lira, C. T., Miller, D. J. (2008b). Triethyl citrate synthesis by reactive distillation. Industrial Engineering Chemistry Research, 47, 1017—1025. 

Cleavage of Tricarboxylic Adds. Both citric and isocitric acids are reversibly split by enzymes found in various bacteria. Citritase splits citrate to oxalacetate and acetate (XXII). CoA does not participate in this reaction. Unlike the reaction catalyzed by the citrate condensing enzyme, which greatly favors citrate synthesis, the standard free enei ... 

Poke J., Ahner T. T., Delissen F., et al. Mechanism of gold nanoparticle formation in the classical citrate synthesis method derived from coupled In situ XANES and SAXS evaluation. J. Am. Chem. Soc. 132 no. 4 (2010) 1296-1301. 

KIT-6 aged at 100°C with the same composition synthesized by other methods (RG reactive grinding citrate synthesis using citrate complexes) [26]. 

Deng YF, Jiang YQ, Hong QM, Zhou ZH (2007) Speciation of water-soluble titanium citrate synthesis, structural, spectroscopic properties and biological relevance. Polyhedron 26 1561-1569... 

Lisensky, G. Citrate synthesis of gold nanoparticles. http //www.nusec.wisc.edu/Edetc/nanolab/gold/index.html (July, 2007). 

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