Oxalacetate from aspartate

In a similar way, transamination from aspartate (Asp, D) to a-ketoglutarate, which, like fumarate, is produced in the tricarboxylic acid cycle (Scheme 11.89), then yields glutamate (Glu, E) and, exactly as in Scheme 12.2, oxalacetate (Equation 12.6). 

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. 

Since the orotic acid required by Lactobacillus could be replaced partially by carbamylaspartic acid, the latter compound was investigated further as a possible pyrimidine precursor labeled carbamylaspartic acid was as effective as orotic acid in labeling the nucleic acid pyrimidines of this microorganism (339). If ureidosuccinic acid were formed from oxalacetate or aspartate, it would be possible to outline the formation of the pyrimi-... 

These syntlieses give no indication as to the structure of aspartic acid, the constitutional formula of which is based upon Kolbe s work, that it is amino-succinic acid the only synthesis of aspartic acid which confirms this constitution appears to be that by Piutti in 1887. Sodium oxalacetic ester, prepared from oxalic ester and acetic ester in the presence of sodium ethylate —... 

Since in the citric acid cycle there is no net production of its intermediates, mechanisms must be available for their continual production. In the absence of a supply of oxalacetic acid, acctaic" cannot enter the cycle. Intermediates for the cycle can arise from the carinxylation of pyruvic acid with CO, (e.g., to form malic acid), the addition of CO > to phosphcnnlpyruvic acid to yield oxalacetic acid, the formation of succinic acid from propionic acid plus CO, and the conversion of glutamic acid and aspartic acid to alpha-ketoglutaric acid and oxalacetic acid, respectively. See Fig. 3. 

The procedure reported in Scheme 13.11 describes deracemization of an amino acid involving oxidation with an L-specific enzyme and transamination with a D-amino transferase using D-aspartate 10, which is generated from L-aspartate 11 by aspartate racemase, as the amino donor. The oxidative enzyme is defined as an L-amino acid deaminase, a flavoprotein from Proteus myxofadens [34]. The transamination reaction is shifted towards the product since the oxalacetate 12 formed decarboxylates spontaneously to give pyruvate and carbon dioxide. 

Kumagai and coworkers11131 developed an enzymatic procedure to produce d-alanine from fumarate by means of aspartase (E. C. 4.3.1.1), aspartate racemase, and D-amino acid aminotransferase (Fig. 17-12). Aspartase catalyzes conversion of fumarate into L-aspartate, which is racemized to form D-aspartate. D-Amino acid aminotransferase catalyzes transamination between D-aspartate and pyruvate to produce D-alanine and oxalacetate. This 2-oxo acid is easily decarboxylated spontaneously to form pyruvate in the presence of metals. Thus, the transamination proceeds exclusively toward the direction of D-alanine synthesis, and total conversion of fumarate into D-alanine was achieved. 

Aminooxyacetate, an inhibitor of glutamate— oxalacetate transaminase, inhibits the formation of aspartate. Soling Kleinicke (1976) observed that aminooxyacetate did not inhibit the formation of glucose from lactate and, therefore, concluded that the malate-aspartate shuttle was not essential for the lactate gluconeogenesis in avian liver. However, Ochs Harris (1980) found that aminooxyacetate did block lactate gluconeogenesis when lower concentrations of pyruvate were used and incubation was for longer than 15 min. They concluded that the malate-aspartate shuttle was required. 

The aspartate entering the cycle is produced by reaction of glutamate with ox-alacetate, the former being produced from a-ketoglutarate plus ammonia released by deamination of an amino acid. The oxalacetate is derived from the fumarate released in the production of arginine from ai inosuccinate, which enters the tricarboxylic acid cycle and is converted to malate and then oxalacetate. We then have a second associated cycle linking the luea and the tricarboxylic acid cycles, which may be visualised as shown in Fig. 9.13. 

Ti-Aspartic Oxidase. Aspartase and transaminases account for a major part of the metabolism of L-aspartic acid. n-Aspartic acid is oxidized by an enzyme present in liver and kidney. This is an oxidase that converts aspartate to oxalacetate and ammonia while reducing oxygen to hydrogen peroxide. The oxidase was resolved by ammonium sulfate precipitation and dialysis to a protein that could be reactivated by FAD but not by FMN. The enzyme differs from n-amino acid oxidase in its insensitivity to benzoate. The only other known substrate for the partially purified D-aspartic oxidase is D-glutamate, but since the relative rates of oxidation of the two amino acids vary during the preparation of the enzyme, it is... 

In the dark, it is assumed that stored starch or other glucan is hydrolyzed to sugar-phosphates which are metabolized to PEP via glycolysis and perhaps to some extent by pentose metabolism. Carboxylation of PEP results largely from atmospheric CO2 to give oxalacetate, which is immediately reduced to malate. Some oxalacetate will ultimately go into aspartate synthesis. Some of the malate will be decarboxylated to pyruvate (or perhaps PEP), where the 3-carbon frag-... 

Hence, those enzymes involving PEP carboxylation and regeneration, and malate production and degradation plus synthesis of the amino acids aspartate and alanine from the keto acids oxalacetate and pyruvate are deemed most interesting for our understanding in CAM. 

The cycle was originally proposed as a mechanism for the terminal oxidation of carbohydrate. It was always obvious that it must also apply to parts of the protein molecule because several amino acids yield members of the cycle directly—glutamic acid, aspartic acid, and alanine—or indirectly—histidine, proline, arginine, and others. Work carried out during the last decade with the help of specially prepared tissue extracts and of isotopes has produced conclusive evidence in support of the conception that the tricarboxylic acid cycle is also the terminal mechanism of the oxidation of fatty acids and ketone bodies. These substances all form the same derivative of acetic acid—acetyl coenzyme A— which can condense with oxalacetate to form citrate. The pathway leading from various... 

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