Glutamate ornithine synthesis

Ornithine Biosynthesis. Two reaction sequences leading to ornithine synthesis from glutamic acid have been described. In Neurospora and the yeast Tondopsis utilis the sequence involves the simple transamination of glutamic semialdehyde (IV). The reaction proceeds much... 

The precursor for ornithine synthesis is N-acetylglutamate, which is also an obligatory activator of carbamyl phosphate synthetase. This provides a regulatory mechanism — if N-acetylglutamate is not available for ornithine synthesis (and hence there would be impaired activity of the urea synthesis cycle), then ammonium is not incorporated into carbamyl phosphate. This can be a cause of hyperammonaemia in a variety of metabolic disturbances that lead to either a lack of acetyl CoA for N-acetyl glutamate synthesis or an accumulation of propionyl CoA, which is a poor substrate for, and hence an inhibitor of, N-acetylglutamate synthetase. 

Figure 33. Synthesis of alkaloids from ornithine. Aikaioids are derived via putrescine or glutamic semialdehyde. At least two enzymes, ODL (Ornithine decarboxylase) or PDL (Pyrroline...

Fig. 1.3 Reactions showing synthesis of glutamate in brain. Aspartate aminotransferase (1) glu-taminase (2) glutamate dehydrogenase (3) GABA aminotransferase (4) alanine aminotransferase (5) ornithine aminotransferase (6) Al-pyrroline 5-carboxylic acid dehydrogenase (7) and asparagine synthetase (8)...

Note in (c), (d), and (e), these urea cycle intermediates will contain low levels of 14C as a result of a very weak synthesis of ornithine from glutamate. 

Grisolia and Towne (20) first proposed that an intermediate was formed prior to the synthesis of carbamyl-P. The idea of an intermediate was further strengthened by later evidence (35) regarding the increase in synthetic activity that follows the preincubation of the rat liver enzyme with acetyl glutamate, ATP, and Mg+2, and by the demonstration that, after preincubation of the enzyme with acetyl glutamate, ATP, Mg+2, and HC03", followed by the removal of ATP from the incubation mixtures, some citrulline was still synthesized upon addition of ornithine and ornithine transcarbamylase. 

Pyridoxine plays a role in (1) the control of the hypothalamo-pituitary end-organ system, (2) melatonin synthesis, and (3) convulsive seizure activity. Neurological deficits resulting from pyridoxine deficiency can largely be explained by decreased activity of glutamic acid decarboxylase, 5-hydroxytryptophan decarboxylase, and ornithine decarboxylase (Dakshinamurti et al., 1990). The products of these... 

Ornithine and citrulline are amino acids, but they are not used as building blocks of proteins. The formation of NH4 + by glutamate dehydrogenase, its incorporation into carbamoyl phosphate, and the subsequent synthesis of citrulline take place in the mitochondrial matrix. In contrast, the next three reactions of the urea cycle, which lead to the formation of urea, take place in the cytosol. 

Branched-chain amino acids apparently stimulate the urea cycle. Carbamoylphosphate synthetase, which channels ammonia into the urea cycle, is induced by ornithine and N-acetylglutamate as a cofactor of urea synthesis. Here, BCAA follow two modes of action (i.) they stimulate the synthesis of N-acetylglutamate via synthetase formed from glutamate and acetyl CoA, and (2.) they inhibit omithine-keto acid transferase, which is the enzyme responsible for ornithine degradation, leading to an increase in ornithine concentration. Ammonia detoxication is thus stimuiated by two regu-iatory mechanisms, (s. fig. 40.2)... 

The biomolecular modes of action of ornithine have been the subject of several experimental investigations. Ornithine activates the enzymes carbamylphosphate synthetase and ornithine carbamyl transferase, which are necessary for the liver-specific process of urea synthesis (133,139) this occurs mainly in the periportal hepa-tocytes (= definitive ammonia detoxification). Glutamine synthesis (binding of ammonia to glutamate) takes place predominantly in the perivenous hepatocytes (= transitory ammonia detoxification). Large amounts of glutamate are necessary for this. Aspartate, ornithine... 

Unlike fractions of pig-kidney protein, Neurospora crassa extracts can use L-glutamine, but not ammonium salts nor ammonium salts plus adeno-sine-5-triphosphoric acid. No synthesis of D-glucosamine is stimulated in Neurospora extracts by L-glutamic acid, L-aspartic acid, L-asparagine, L-alanine, glycine, L-valine, L-leucine, L-lysine, L-arginine, L-serine, L-cys-teine, L-citrulline, L-ornithine, butyramide, putrescine, or urea. Recently, a protein fraction has been discovered, in rat liver, that converts D-glucose... 

Fig. I. Metabolic map for synthesis and metabolism of glutamate and aspartate. AAT = aspartate aminotransferase AS = asparagine synthetase GAD = glutamic acid decarboxylase GDH = glutamate dehydrogenase GS = glutamine synthetase OAT = ornithine D-aminotransferase P5CDH = l-pyrroline-5-carboxylate dehydrogena.se PAG = phosphate-activated glutaminase PO = proline oxidase TCA = tricarboxylic acid.

In a recent optimization study, an alternative coupling system has been developed [29 c], The chemical yield in the synthesis of two non-proteinogenic a-amino acids was remarkably improved by applying a coupled transamination process using additionally an ornithine co-aminotransferase to couple L-ornithine co-trans-amination to L-glutamate a-transamination [29 c],... 

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