Subsystem: Putrescine utilization pathways
This subsystem's description is:
Putrescine is present in virtually all living cells, from bacteria to eukaryotes and is essential for cell structure and physiology [1,2]. It can bind to nucleic acids, stabilize membrane and stimulate activity of several enzymes [2–4]. The intracellular putrescine deficiency leads to poor cell growth rate and survival . On the other hand, despite the proved necessity of intracellular polyamine for cellular metabolism, polyamine accumulation can lead to inhibition of cellular growth and protein synthesis (Samsonova et al., 2005). Thus, it is clear that the cell mechanisms for maintenance of the optimum intracellular putrescine contents at normal environment as well as at stress conditions should exist. According to the current data, steady state concentration of the intracellular putrescine in Escherichia coli is generally mediated by specific influx/effux transport systems [see SS: Polyamine Metabolism] and by coupling action of putrescine synthesis from arginine or ornithine [see SS: Arginine and Ornithine Degradation] and its degradation [this SS]. There are two routs of putrescine degradation to gamma-aminobutyric acid (GABA, see diagram):
(1) via putrescine:a-ketoglutarate aminotransferase (PAT) and gamma-aminobutyraldehyde dehydrogenase (ABADH) (Shaibe et al., 1985)
(2) via gamma-glutamylated intermediates (Kurihara et al., 2005)
The first pathway has been elucidated over 20 years ago. In it putrescine is converted to gamma-aminobutyraldehyde by putrescine transaminase and then to be oxidized to gamma-aminobutyrate (GABA) by aminobutyraldehyde dehydrogenase (Shaibe et al., 1985). However, genes encoding these enzymes have been identified only recently (Samsonova et al., 2005;
The second bacterial putrescine utilization pathway has been discovered recently. This novel pathway involves six sequential steps as follows: 1) import of putrescine; 2) ATP-dependent gamma-glutamylation of putrescine; 3) oxidization of gamma-glutamylputrescine; 4) dehydrogenation of gamma-glutamyl-gamma-aminobutyraldehyde; 5) hydrolysis of the gamma-glutamyl linkage of gamma-glutamyl-gamma-aminobutyrate; and 6) transamination of gamma-aminobutyrate to form the final product of this pathway, succinate semialdehyde, which is the precursor of succinate (Kurihara et al., 2005). Genes, encoding these enzymes, as well as a putrescine importer, and a transcriptional regulator are invariably localized in a cluster.
GABA produced in either pathway is converted to succinate semialdehyde by GABA aminotransferase (gabaT) and is then oxidized to succinate by succinate semialdehyde dehydrogenase [SSADH or SSADH(P)]. See SS: Gamma-aminobutyrate (GABA) metabolism for further details.
1. Shaibe E, Metzer E, Halpern YS. 1985. Metabolic pathway for the utilization of L-arginine, L-ornithine, agmatine, and putrescine as nitrogen sources in Escherichia coli K-12. J Bacteriol, 163(3):933-7.
2. Kurihara S, Oda S, Kato K, Kim HG, Koyanagi T, Kumagai H, Suzuki H. 2005. A novel putrescine utilization pathway involves gamma-glutamylated intermediates of Escherichia coli K-12. J Biol Chem, 280(6):4602-8
3. Samsonova NN, Smirnov SV, Novikova AE, Ptitsyn LR. 2005. Identification of Escherichia coli K12 YdcW protein as a gamma-aminobutyraldehyde dehydrogenase. FEBS Lett, 579(19):4107-12.
4. Samsonova NN, Smirnov SV, Altman IB, Ptitsyn LR 2003. Molecular cloning and characterization of Escherichia coli K12 ygjG gene. BMC Microbiol, 3(1):2
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