Aspartate synthesis

Answer Oxaloacetate might be withdrawn for aspartate synthesis or for gluconeogenesis. Oxaloacetate is replenished by the anaplerotic reactions catalyzed by PEP carboxykinase, PEP carboxylase, malic enzyme, or pyruvate carboxylase (see Fig. 16-15, p. 632). 

Aspartate, like glutamate, is an excitatory neurotransmitter, but it functions in far fewer pathways. It is synthesized from the TCA cycle intermediate oxaloacetate via transamination reactions. Like glutamate synthesis, aspartate synthesis uses oxaloacetate that must be replaced through anaplerotic reactions. Aspartate cannot pass through the blood-brain barrier. 

Compound (LVII) is very rapidly excreted into the kidney. It is metabolised to 5-(3 -hydroxy-4 -aminophenyl)cytosine and its O-glucuronide, as well as derivatives acetylated on both amino groups [343]. It is a competitive inhibitor of succinate dehydrogenase (K = 9-2 x 10 m K,- = 5 0 X IO m), and it has been considered that this is its mode of action [343]. One role of this enzyme involves aspartate synthesis, via fumarate. This pyrimidine enhances the incorporation of orotic acid into nucleic acid but is not itself incorporated. 

Reichard found that carbamyl aspartate synthesis would take place in crude liver preparations if carbamyl phosphate and aspartate were provided and showed that the enzyme responsible, aspartate carbamyltransferase, was not the same as that which formed citrulline, but apparently shared carbamyl phosphate with it. 

Amino acids are absorbed by the mammary gland in quantities sufficient to account for the protein synthesised within it. Considerable interconversion of amino acids occurs before synthesis, and certain amino acids are important as sources of others. Thus, ornithine, which does not appear in milk protein, is absorbed and retained in large quantities by the mammary gland and has been shown to be a precursor of proline, glutamate and aspartate. Synthesis of the carbohydrate moieties of the proteins takes place in the mammary gland, as does phosphorylation of serine and threonine before their incorporation into the caseins. 

It is now generally accepted that the metabolic pathway for the majority of the malate and aspartate synthesis in succulent plants 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-... 

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