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arginine biosynthesis

The first five steps of arginine biosynthesis in S. cerevisiae take place in the mitochondrion . This part of the pathway is known as the acetylated derivatives cycle because the acetyl group that is added to L-glutamate in the first step of the pathway is recycled via N-acetylglutamate generated in the fifth step. The enzymes that catalyze the second and third steps are encoded by a single gene (ARG5,6) that is translated into a pre-protein which is imported into mitochondria and cleaved there to yield two enzymes, N-acetylglutamate kinase and N-acetylglutamyl-phosphate reductase . These enzymes form a complex with each other and with N-acetylglutamate synthase, the first enzyme in the pathway, which may have implications for regulation of their activity . The mitochondrial steps of the arginine biosynthesis pathway result in the formation of ornithine, which is exported to the cytoplasm by the transporter Ort1p . In the cytoplasm, L-ornithine is converted to L-arginine in three reactions mediated by ornithine carbamoyltransferase, arginosuccinate synthase, and argininosuccinate lyase. Transcription of genes of the arginine biosynthetic pathway, as well as of other amino acid biosynthetic pathways, is activated by the transcription factor Gcn4p under conditions of amino acid starvation . Transcription of ARG1, ARG3, ARG5,6, and ARG8 is also repressed in the presence of arginine by the ArgR/Mcm1p complex, which consists of Arg80p, Arg81p, Arg82p, and Mcm1p . The transcriptional activator Gcn4p interacts with subunits of the ArgR/Mcm1p repressor, allowing for fine-tuning of transcriptional control in response to arginine availability . General Background (from MetaCyc) Arginine biosynthesis is notable for its complexity and variability at the genetic level, and by its connection with several other pathways, such as pyrimidine and polyamine biosynthesis, and certain degradative pathways. The initial steps of the arginine biosynthetic pathways proceed via N-acetylated intermediates. The presumed reason for this is that the acetylation prevents the spontaneous cyclization of glutamate derivatives, which leads to proline biosynthesis, thus keeping the two pathways separate ( and references therein). About This Pathway (from MetaCyc) The cyclic pathway for arginine biosynthesis has been demonstrated in many organisms, including the prokaryotes Bacillaceae, pseudomonads, cyanobacteria, photosynthetic bacteria and archaebacteria, as well as eukaryotic organisms such as Saccharomyces cerevisiae, Neurospora sp., and Chlamydomonas sp. Organisms that employ this pathway possess the argJ gene, which encodes the enzyme ornithine acetyltransferase (OAT). OAT catalyzes the transfer of the acetyl group from N--acetyl-ornithine to glutamate, generating ornithine and N-acetyl-glutamate (NAG). Since the later is the substrate for another enzyme in this pathway, N-acetyl-glutamate kinase (encoded by argB), which catalyzes a previous step, this reaction effectively converts the otherwise linear pathway to a cyclical pathway . While most of the NAG in these organisms is generated by OAT, some of it is still generated by N-acetyl-ornithine synthase (NAGS, EC, encoded by argA), which fulfils an anaplerotic (replenishment of pathway intermediates) role to replenish NAG that is lost due to degradation or cell division . In some bacteria, such as Bacillus sp. , Nisseria gonorrhoeae , Thermotoga neapolitana and in yeast (Saccharomyces cerevisiae) , OAT can have a dual enzymic function. In these organisms, either N-acetylornithine or acetyl-CoA can be used as acetyl donors for formation of NAG. argJ genes from such organisms can complement both argE and argA mutations in E. coli. However, these organisms still possess separate NAGS enzymes encoded by argA genes. The biological role of duplicate NAGS activity in these organisms remains to be elucidated .

Type: pathway
Taxonomic scope
organism-specific biosystem
Saccharomyces cerevisiae

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