Oxalacetic acid reduction

For the determination of keto-acids, e.g. pyruvic, a-ketoglutaric and oxalacetic acids in mixtures and in biological materials, a method has been proposed employing paper electrophoretic separations of the 2,4-dinitrophenylhydrazones of these acids.< ) After elution, the polarographic wave corresponding to the reduction of the nitro-groups (in the 2,4-dinitrophenylhydrazone formed) was followed in 0-1 N HCl and the first wave at —0T4 V measured. 

The sugar 2-amino-2-deoxy-D-gluconic acid inhibits reduction of iron(III) by D-galacturonic acid. Decarboxylation of oxalacetic acid is the final step in a sequence of reactions with the C-bound chromium(III) complex, [(H20)5CrCH2CN] ". The initial steps are coordination and ring closure of the oxalacetate, both by displacement of coordinated H2O with rate constants 0.16 M s and 1 x 10 s respectively at 25.0 °C and 1.0 M ionic strenght. Decarboxylation takes place after protonation of the coordinated substrate. 

The existence of a corresponding enzyme promoting a reductive amination of oxalacetic acid to L-aspartic acid is uncertain. 

A carrier molecule containing four carbon atoms (the C4 unit) takes up a C2 unit (the activated acetic acid ), which is introduced into the cycle. The product is a six-carbon molecule (the C6 unit), citric acid, or its salt, citrate. CO2 is cleaved off in a cyclic process, so that a C5 unit is left this loses a further molecule of CO2 to give the C4 unit, oxalacetate. In the living cell, this process involves ten steps, which are catalysed by eight enzymes. However, the purpose of the TCA cycle is not the elimination of CO2, but the provision of reduction equivalents, i.e., of electrons, and... 

Another model is based on the fact that the genetic code shows a number of regularities, some of which have already been mentioned above. It is suspected that codons beginning with C, A or U code for amino acids which were formed from a-ketoacids (or a-ketoglutarate, 1-KG), oxalacetate (OAA) and pyruvate. This new model, which is quite different from the previous models, assumes that a covalent complex formed from two nucleotides acted as a catalyst for chemical reactions such as the reductive amination of a-ketoacids, pyruvate and OAA. More recent analyses suggest that the rTCA cycle (see Sect. 7.3) could have served as a source of simple a-ketoacids, including glyoxylate, pyruvate, OAA and a-KG. a-Ketoacids could, however, also have been formed via a reductive acetyl-CoA reaction pathway. The bases of the two nucleotides specify the amino acid synthesized and were retained until the modern three-letter codes were established (Copley et al., 2005). 

Further evidence of like character in support of the participation of coenzyme I in reactions associated with the reduction of dehydroascorbic acid has been advanced by Waygood (1950). Cell-free extracts of wheat seedlings were found to contain a malic dehydrogenase enzyme, reducing coenzyme I, as well as ascorbic oxidase and peroxidase enzymes. When to such extracts malic acid, coenzyme I, and ascorbic acid were added, together with a fixative for the oxalacetate formed in the reaction, the system absorbed oxygen in excess of that required for the complete oxidation of ascorbic acid. In this system methylene blue could replace ascorbic acid. 

After weighing the different posibilities to counter the lack of equal labeling in carbons 3 and 6 of lysine, Strassman and Weinhouse 180) propose as the more likely synthetic mechanism, a condensation of an acetyl methyl carbon with the carbonyl carbon of a-ketoglutarate, similar to the condensation of acetyl CoA and oxalacetate to yield citrate. This reaction would form homocitric acid. Upon oxidation and decarboxylation of the latter there would be obtained a-ketoadipic acid. Transamination of a-ketoadipic acid produces a-aminoadipic acid, which can be converted to lysine by reduction to the corresponding semialdehyde, followed by transamination. 

Szent Gyorgyi s conception of the catalytic action of fumarate, oxalacetate, and their precursors cannot account for the conversion of the dicarboxylic acids into succinate in the malonate-poisoned tissue. It follows that in addition to the reactions which Szent Gyorgyi postulates— the reversible oxidation and reduction of oxalacetate and malate—other reactions of the 4-carbon acids must occur, leading to their conversion, by oxidative processes, to succinate. The tricarboxylic acid cycle offers a complete explanation of the behavior of the dicarboxylic acids. 

A similar pathway for the formation of sucdnate in both Actinobadllus sp. 130Z and A. sucdniciproducens has been proposed. The formation of oxalacetate fi-om PEP via CO2 fixation is the first key step. The enzymes malate dehydrogenase, fumarase, and fumarate reductase, all enzymes of the tricarboxylic acid (TCA) cycle, work in a reductive fashion toward succinate (Van der Werf et al. 1997 Samuelov et al. 1991). The reactions catalyzed are as follows ... 

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