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Last Updated: 2018-12-21
In the ornithine biosynthesis pathway of E. coli, L-glutamate is acetylated to N-acetylglutamate by the enzyme N-acetylglutamate synthase, encoded by the argA gene. The acetyl donor for this reaction is acetyl-CoA. N-acetylglutamic acid is then phosphorylated via an ATP driven acetylglutamate kinase which yields a N-acetyl-L-glutamyl 5-phosphate. This compound undergoes a NADPH dependent reduction resulting in N-acetyl-L-glutamate 5-semialdehyde. This compound reacts with L-glutamic acid through a acetylornithine aminotransferase / N-succinyldiaminopimelate aminotransferase to produce a N-acetylornithine which is then deacetylated through a acetylornithine deacetylase which yield an ornithine. Ornithine interacts with hydrogen ion through a Ornithine decarboxylase resulting in a carbon dioxide release and a putrescine. Putrescine can be metabolized by reaction with either l-glutamic acid or oxoglutaric acid. If putrescine reacts with L-glutamic acid, it reacts through an ATP mediated gamma-glutamylputrescine producing a hydrogen ion, ADP, phosphate and gamma-glutamyl-L-putrescine. This compound is reduced by interacting with oxygen, water and a gamma-glutamylputrescine oxidoreductase resulting in ammonium, hydrogen peroxide and 4-gamma-glutamylamino butanal. This compound is dehydrogenated through a NADP mediated reaction lead by gamma-glutamyl-gamma-aminobutaryaldehyde dehydrogenase resulting in hydrogen ion, NADPH and 4-glutamylamino butanoate. In turn, the latter compound reacts with water through a gamma-glutamyl-gamma-aminobutyrate hydrolase resulting in L-glutamic acid and Gamma aminobutyric acid. On the other hand, if putrescine reacts with oxoglutaric acid through a putrescine aminotransferase, it results in L-glutamic acid, and a 4-aminobutyraldehyde. This compound reacts with water through a NAD dependent gamma aminobutyraldehyde dehydrogenase resulting in hydrogen ion, NADH and gamma-aminobutyric acid. Gamma Aaminobutyric acid reacts with oxoglutaric acid through 4-aminobutyrate aminotransferase resulting in L-glutamic acid and succinic acid semialdehyde. This compound in turn can react with with either NADP or NAD to result in the production of succinic acid through succinate-semialdehyde dehydrogenase or aldehyde dehydrogenase-like protein yneI respectively. Succinic acid can then be integrated in the TCA cycle.
Ornithine Metabolism References
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