Acids citric acid cycle, fractionation

In the second stage of fatty acid oxidation, the acetyl-CoA is oxidized to C02 in the citric acid cycle. A large fraction of the theoretical yield of free energy from fatty acid oxidation is recovered as ATP by oxidative phosphorylation, the final stage of the oxidative pathway. 

Only a fraction of the energy of glucose is released in its anaerobic conversion into ethanol or lactate. Much more energy can be extracted aerobically by means of the citric acid cycle and the electron-transport chain. The entry point to this oxidative pathway is acetyl coenzyme A (acetyl CoA), which is formed inside mitochondria by the oxidative decarboxylation of pyruvate. 

Chlorobiaceae photosynthesis is via reversal of the citric acid cycle tchemotrophes include sulphur and ammonium oxidizing bacteria (see Box 1.7) teach additional heterotrophic step involves a similar fractionation methanogenesis generally leads to a fractionation of c.40%o in freshwater conditions but c.70%o under marine conditions. 

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

From fig. 1 it can be seen that in addition to the labile protein-bound fraction, metal-transferrin complexes in the case of actinides, there is also a low molecular mass fraction in which metal ions are bound to, and transported with, ligands such as the anions of amino and carboxylic acids. For the hard cations of the actinide elements, these complexing agents are predominantly carboxylic acids (from the citric acid cycle) (Duffield and Taylor 1986, Popplewell et al. 1975, Duffield et al. 1984, Metivier 1973) or inorganic anions such as carbonate, e.g., for uranium(VI) (Stevens et al. 1980). 

The basic construction of the mathematical model using simplified metabolic networks to describe the reactions of the citric acid cycle and associated transamination reactions between pyruvate and alanine, oxalacetate and aspartate and a-ketoglutarate and glutamate, and the use of the FACSIMILE program (Chance et al., 1977) to solve the rather large number of simultaneous differential equations generated by the model was the same as previously described (Chance etal., 1983). For the present experiments the model was expanded to include an input flux at the level of succinate to represent propionate metabolism to succinyl-CoA, and a dilution of the aspartate pool to represent net proteolysis. These input fluxes required an output flux of carbon from the citric acid cycle in order to maintain a steady state carbon balance, for which the conversion of malate to pyruvate via malic enzyme was chosen. The model calculates the unknown flux parameters to provide a minimum least squares fit of the C fractional enrichments of specific carbon atoms of metabolic intermediates as measured by C NMR spectroscopy. 

It had been known that propionic acid could give rise, in the presence of liver cell fractions, to citric acid cycle compounds and eventually carbohydrates. 

It was first observed that adrenal homogenates and crudely fractionated particles require or can be activated by various cofactors and combinations of cofactors and metabolic intermediates, such as glucose, fumarate and other citric acid cycle components, adenosine triphosphate (ATP) or adenylic acid, nicotinamide or diphospho-pyridine nucleotide (DPN) (334,386,570,721) or triphosphopyridine nucleotide (TPN) in place of DPN plus ATP (335,786). The activity of meticulously prepared adrenal mitochondria is inhibited by low concentrations of dinitrophenol it was therefore suggested that lljS-hydroxylation requires concurrent oxidative phosphorylation (112,386). 

During the formation of carboxylic acid like lA, there will be shuttling of metabolites within the intracellular compartments, having the capability to utilize the enzymes of the respective compartments. Jaklitsch et al. (1991) reported that the CadA, which is e key enzyme for the biosynthesis of lA, is located in cytosol. Otiier important enzymes, such as citrate synthase and aconitase, are found in the mitochondria, but some residual level of these enzymes are also found in the cytosolic fraction. The depicted mechanism is that the ds-aconitate is transported to cytosol assisted by the malate-citrate antiporter. The biosynthetic pathway of LA in the citric acid cycle is illustrated in Fig. 10.5. 

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