N-Acetylornithine

Rajaram, V., Ratna Prasuna, P, Savithri, H.S., and Murthy, M.R. (2008) Structure of biosynthetic N-acetylornithine aminotransferase from Salmonella typhimu-rium Studies on substrate specificity and inhibitor binding. Proteins 70, 429-441. 

Three relevant enz3une activities have been demonstrated in E. coli extracts. These are an acetylase which forms JV-acetylglutamate from glutamic acid and acetyl CoA 129), a transaminase which forms N -acetylornithine 116), and acetylomithinase which hydrolyzes the N -acetylomithine to ornithine 116). It is highly significant that the last named enz3une was absent in extracts of a mutant blocked between iV -acetylomithine and ornithine 116). 

The product of the stoichiometric reaction of acetyl-P with ornithine, catalyzed by ornithine transcarbamylase, has been shown unequivocally to be 6-acetylornithine the transcarbamylases from rat liver, frog liver, and bacteria, however, even though yielding the same product, appear to differ in their ratios of activity with carbamyl-P and acetyl-P (Table n). While it is possible that the synthesis of 6-acetylornithine is catalyzed by other enzymes (16), the different ratios may be due to species differences we know now that the ratios of activity with carbamyl-P and acetyl-P of all ornithine transcarbamylases thus far tested remain constant with purification. Further, the ratio of citrulline to acetylornithine formation does not change with a number of treatments, such as heat inactivation of preparations containing orni-... 

In 1993, a thermostable aminoacylase from Bacillus stearothermophilus was characterized by Sakanyan et al.l51. The enzyme hydrolyzes N-acyl derivatives of aromatic amino acids preferentially and even has some dipeptidase activity. Its optimal reaction temperature is 70 °C after incubation for 15 min, 90% of the original activity was retained. The authors write that the similarity of the B. stearothermophilus enzyme sequence with that of other enzymes such as aminoacylase I, acetylornithine deacetylase and carboxypeptidase G2 suggests a common origin. The aminoacylase from B. stearothermophilus is well characterized the gene has been completely sequenced1511, cloned into E. coli and overexpressed[51> 671 and studied for catalytic and stability properties16 1 the intrinsic one Zn2+ ion per subunit seems to have a predominantly structural role and activity can be restored to the apo-enzyme by Co2+ and particularly by Cd2+ (3-fold activity ) but not by Zn2+. 

Quite another pathw ay has been show n to be follow-ed in the biosynthesis of ornithine from glutamic acid, namehq via A -acetylglutamic acid, iV-acetylglutamic semialdehyde and A "-acetylornithine.""" This leads to the reaction sequence showm below. ... 

This is cited as evidence that molds and vertebrates synthesize ornithine from glutamic y-semialdehyde (131,132), while E. colt and related bacteria utilize the acetylornithine pathway. This interpretation has been contested by Davis (f). He points out that Neurospora may be impermeable to N -acetylomithine or that a block might occur after this compound in the auxotrophs studied. 

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