Tricarboxylate cycle

E. M. T. El Mansi, G. C. Dawson, and C. F. A. Bryce, Steady state modeling of metabolic flux between the tricarboxylic cycle and the glyoxylate bypass in Escherichia coli. CABIOS (now Bioinformatics) 10(3), 295 299 (1994). 

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic acid... 

Referring to reactions, pathways, or processes that replenish or add to intermediates of a metabolic cycle (usually the tricarboxylic cycle). The process itself is referred to as anaplerosis. 

How complex can the proteome of hydrogenosomes in T. vaginalis be expected to be Trichomonad hydrogenosomes have lost many standard metabolic capacities of mitochondria like—and consequently most of the proteins involved in—the tricarboxylic cycle, membrane-bound electron transport and ATP-production (Muller 1993), or fatty acid synthesis (Beach et al. 1990). Because of the absence of a genome (Clemens and Johnson 2000) the complex machineries of DNA replication and repair, gene transcription and protein synthesis are also absent from these organelles. On the other hand, experimental evidence exists for only a small number of metabolic... 

Arsan, O.M. (1986). The role of water temperature in regulation of die processes of glycolysis and the tricarboxylic cycle in fish (In Russian). Gydmbiologicheskii Zhumal 22(5), 71-74. 

Szerb, J.C., Issekutz, B. (1987). Increase in the stimulation-induced overflow of glutamate by fluoroacetate, a selective inhibitor of the glial tricarboxylic cycle. Brain Res. 410 116-20. 

Although Cu would be a more efficient oxidation catalyst, it does also support decarboxylation much more than the lighter 3d-ions. As a result, malate would be converted into pyruvate rather than oxaloacetate, causing the entire tricarboxylate cycle to collapse. As Mn + does hardly cause decarboxylation (Pedersen 1948 Hedrick and SaUach 1961), but favors oxidation, it affords a kind of local optimum for this purpose. 

The tricarboxylate cycle is marked parts of the products are obviated into both metal mobriization and amino acid synthesis (over a couple of intermediates). Neither structures nor chemical classification of secondary chelators in the (larger, upper) roots or shoot are known in general (Clemens et al. 2002). Like commonplace in chemical notation, catalyst ions (metals which become active in metaUoproteins) are denoted by swift brackets M disregarding issues tike speciation or oxidation state (which may periodically change in a catalytic cycle anyway) whereas complexes are shown in square brackets. To yet allow for effective growth, the investment of organics to obtain metal ions from soil must be kept as small as possible... 

Fig. 2.18 Simplified scheme of the cycle/aUocation of magnesium in a green plant. Mg is involved - inter aha - in making peptide bonds, in tricarboxylate cycle, hydrolysis of molecules and binding of CO (ribulose-bisphosphatecarboxylase/oxidase or PEP carboxylase in plants which also employs Mg or Mtf+ in some plants (Kai et al. 2003)). Reaction steps in which Mg takes part as biocatalyst are marked by broken lines/arrows. Citrate and other intermediates of the tricarboxylate cycle, particularly malate, are employed by higher plants for extraction of essential metals, including Mg, Fe and Mn (thus the closed loop) from soil via and by means of the roots. This closed loop depicts a manner of autocatalysis. Amino acids which are required for protein biosynthesis are produced by reductive amination from tricarboxylate cycle intermediates and other 2-oxoadds which likewise eventually...

To give a real example, have a closer look on main functions and cycle of magnesium in green plants. Control on autocatalysis depends on the principal functions of Mg, that is, on photosynthesis when substantial parts of Mg taken up by roots are allocated to chlorophyll and rubisco synthesis, less will be available for other metabolic pathways, reducing the turnovers there unless there are lots of Mg around like in marine plants. In addition, the tricarboxylate cycle (citrate cycle) requires Mg (besides Fe and Mn) to produce the enzymes hence some Mg (as well as Fe, Mn) must be invested to produce the citrate (malate, oxaloacetate (aspartate)) ions delivered by the roots to render Mg (and other metals) in turn bioavailable by means of complexation and resorption of almost neutral complex entities. Furthermore, the tricarboxylate cycle is coupled to biosynthesis of amino acids by redox transamination hence there will be both competition at the metal center(s) and possible extraction of metal ions from enzymes once NHj and electrons are... 

VI. The Tricarboxylic Cycle and Carbon Dioxide Assimilation in Animal... 

COs to form oxalacetate which under anaerobic conditions is reduced to malate. The malate in turn may be converted to fumarate and succinate (Fig, 5). The last step in this series of reactions is blocked by malonate. The second pathway involves the aerobic condensation of pyruvate and oxalacetate followed by oxidation of the condensation product to form -ketoglutarate and succinate. Wood has proposed that the first condensation product of the aerobic tricarboxylic cycle is cfs-aconitic acid which is then converted to succinate by way of isocitric, oxalosuccinic, and a-ketoglutaric acids. The a-ketoglutarate is decarboxylated and oxidized to succinic acid. Isotopic a-ketoglutarate containing isotopic carbon only in the carboxyl group located a to the carbonyl would be expected to yield non-isotopic succinate after decarboxylation. This accounts for the absence of isotopic carbon in succinate isolated from malonate-poisoned liver after incubation with pyruvate and isotopic bicarbonate. 

That fatty acids may be oxidized by way of the tricarboxylic cycle has received serious consideration in recent years. Breusch has suggested that /3-keto acids resulting from the biological oxidation of long-... 

Lynen, and Virtanen and Sundman have reported that in yeast, acetate is metabolized by way of the tricarboxylic cycle. Wieland and Rosenthal have also shown that kidney contains an enzyme system which can convert acetate or acetoacetate to citrate. Upon addition of oxalacetate along with either of these acids, the yield of citrate is significantly increased. In these experiments barium ions were added to inhibit further breakdown of the resulting citrate. These authors have proposed that oxalacetate and acetoacetate condense to form either acetyl citrate or citroyl acetate which are subsequently hydrolyzed to citrate and acetate. 

In this connection it is of interest that Buchanan and coworkers demonstrated that isotopic acetoacetate (CH3 C 0-CH2 C 00H) and acetate (CH3-C OOH) are oxidized in kidney by way of the tricarboxylic cycle. In several experiments of this type, a-ketoglutarate, fumarate and succinate were isolated after incubation with isotopic acetoacetate or acetate. The isolated acids were found to contain sufficient excess of isotope to warrant the belief that acetoacetate and acetate are oxidized in kidney to a major extent by way of the tricarboxylic cycle. A similar result with carboxyl-labelled acetate has been obtained by Weinhouse and coworkers. ... 

A pathway by which acetate and acetoacetate are oxidized is indicated in Fig. 8. The similarity to the mechanism of oxidation of pyruvate is striking. According to this proposed scheme acetate and acetoacetate are converted to a hypothetical two-carbon intermediate which is capable of condensing with oxalacetate or one of the other four-carbon dicarboxylic acids to yield cis-aconitate or isocitrate. The rest of the cycle is identical with the carbohydrate tricarboxylic cycle. Reference to this cycle illustrates several points of interest. It makes clear the pathway by which labelled carbon in acetate or acetoacetate may be transformed into D-glucose or glycogen. Since oxalacetate labelled in either carboxyl position is generated in the cycle, carboxyl-labelled... 

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