If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
Synonyms: removal of superoxide radicals
|Superclasses:||Detoxification → Reactive Oxygen Species Degradation|
All organisms living in an aerobic environment are exposed to reactive oxygen species (ROS) that are formed through metabolic processes and various environmental stresses such as drought, air pollutants, UV light and high light intensities, chilling temperatures and external chemicals [Van99, Alscher02]. For example, reactive oxygen species (ROS) are produced during the β-oxidation of fatty acids or as a result of photorespiration in photosynthetic organisms [Frugoli96]. ROS such as superoxide and hydroxyl radicals as well as hydrogen peroxide can cause significant damage to proteins, nucleic acids and cell organelles.
Most of the aerobic organisms have developed defense systems to face oxidative stress and to scavenge oxidative radicals in the form of enzymes that can detoxify ROS, such as superoxide dismutase (SOD) and catalase/hydroperoxidase (HP) [Beyer87]. SODs represent the first line of defense against ROS, converting superoxide radicals to hydrogen peroxide and water. SODs are differentiated with regard to their metal cofactor. There are iron-dependent, manganese-dependent and copper/zinc-dependent SODs, which differ not only in their metal cofactor, but also in their subcellular location. Catalase is second in the defense line against active oxygen, converting hydrogen peroxide into water and oxygen.
About This Pathway
Gram-negative bacteria commonly synthesize both cytoplasmic and periplasmic isozymes of SOD as their frontline defense against superoxide anion (O2-). E. coli contains two cytoplasmic SOD isozymes, one each of the manganese- and iron-cofactored types (MnSOD and FeSOD), and secretes a copper, zinc-cofactored enzyme (CuZnSOD) to the periplasm. Periplasmic superoxide may be generated by autooxidation of dihydromenaquinone in the cytoplasmic membrane [Korshunov06].
In E. coli, the MnSOD and FeSOD enzymes (encoded by sodA and sodB, respectively) are structurally and kinetically similar. Unlike MnSOD and FeSOD, CuZnSOD is monomeric [Battistoni95, Battistoni96]. Regulation of the three enzymes is complex. Under anaerobic conditions, FeSOD is the only superoxide dismutase enzyme present in E. coli [Hassan77, Kargalioglu94]. MnSOD is induced by aerobic growth and a variety of environmental stress conditions. CuZnSOD constitutes only a small fraction of superoxide dismutase activity in the cell; its expression is induced in stationary phase [Imlay96].
In E. coli, with rising H2O2 concentration, catalase is strongly induced and becomes the primary scavenging enzyme. E. coli expresses two catalases, known as HPI and HPII, that are encoded by katG and katE, respectively. While katG katE mutants could not degrade millimolar concentrations of H2O2, they were subsequently found to retain the ability to degrade H2O2 when it was present at low micromolar concentrations [Seaver01]. This residual activity is due to an enzyme known as alkylhydroperoxide reductase (Ahp). This two-component enzyme had originally been identified as a scavenger of organic hydroperoxides [Jacobson89].
SOD mutants of E. coli are unable to perform normal sulfur metabolism. Both SOD and catalase/peroxidase mutants of E. coli are incapable of synthesizing aromatic products, including amino acids [Imlay08].
SoxRS regulon is turned on by any condition that increases superoxide radical production in E. coli. One of its products is Mn-SOD. Another independent regulon turned on in response to H2O2 is referred to as the OxyR regulon [Fridovich97].
Markus Krummenacker on Tue May 21, 1996:
kr96-5-21:I actually removed WATER again from the primaries for CATAL-RXN ,because it did look more confusing upon closer examination.
Markus Krummenacker on Tue May 21, 1996:
kr96-5-21:added OXYGEN-MOLECULE and WATER on the right side as primariesfor CATAL-RXN . Looks a bit better now.
Battistoni95: Battistoni A, Rotilio G (1995). "Isolation of an active and heat-stable monomeric form of Cu,Zn superoxide dismutase from the periplasmic space of Escherichia coli." FEBS Lett 374(2);199-202. PMID: 7589534
Battistoni96: Battistoni A, Folcarelli S, Gabbianelli R, Capo C, Rotilio G (1996). "The Cu,Zn superoxide dismutase from Escherichia coli retains monomeric structure at high protein concentration. Evidence for altered subunit interaction in all the bacteriocupreins." Biochem J 320 ( Pt 3);713-6. PMID: 9003353
Frugoli96: Frugoli JA, Zhong HH, Nuccio ML, McCourt P, McPeek MA, Thomas TL, McClung CR (1996). "Catalase is encoded by a multigene family in Arabidopsis thaliana (L.) Heynh." Plant Physiol 112(1);327-36. PMID: 8819328
Jacobson89: Jacobson FS, Morgan RW, Christman MF, Ames BN (1989). "An alkyl hydroperoxide reductase from Salmonella typhimurium involved in the defense of DNA against oxidative damage. Purification and properties." J Biol Chem 264(3);1488-96. PMID: 2643600
Kargalioglu94: Kargalioglu Y, Imlay JA (1994). "Importance of anaerobic superoxide dismutase synthesis in facilitating outgrowth of Escherichia coli upon entry into an aerobic habitat." J Bacteriol 176(24);7653-8. PMID: 8002590
Van99: Van Breusegem F, Slooten L, Stassart J-M, Botterman J, Moens T, Van Montagu M, Inze D (1999). "Effects of overproduction of tobacco MnSOD in maize chloroplasts on foliar tolerance to cold and oxidative stress." Journal of Experimental Botany, 50(330), 71-78.
Argaman12: Argaman L, Elgrably-Weiss M, Hershko T, Vogel J, Altuvia S (2012). "RelA protein stimulates the activity of RyhB small RNA by acting on RNA-binding protein Hfq." Proc Natl Acad Sci U S A 109(12);4621-6. PMID: 22393021
Battistoni98: Battistoni A, Donnarumma G, Greco R, Valenti P, Rotilio G (1998). "Overexpression of a hydrogen peroxide-resistant periplasmic Cu,Zn superoxide dismutase protects Escherichia coli from macrophage killing." Biochem Biophys Res Commun 243(3);804-7. PMID: 9501009
Beaumont93: Beaumont MD, Hassan HM (1993). "Characterization of regulatory mutations causing anaerobic derepression of the sodA gene in Escherichia coli K12: cooperation between cis- and trans-acting regulatory loci." J Gen Microbiol 139(11);2677-84. PMID: 8277251
Bebien02: Bebien M, Lagniel G, Garin J, Touati D, Vermeglio A, Labarre J (2002). "Involvement of superoxide dismutases in the response of Escherichia coli to selenium oxides." J Bacteriol 184(6);1556-64. PMID: 11872706
Belkin96: Belkin S, Smulski DR, Vollmer AC, Van Dyk TK, LaRossa RA (1996). "Oxidative stress detection with Escherichia coli harboring a katG'::lux fusion." Appl Environ Microbiol 62(7);2252-6. PMID: 8779563
Benov95a: Benov L, Fridovich I (1995). "A superoxide dismutase mimic protects sodA sodB Escherichia coli against aerobic heating and stationary-phase death." Arch Biochem Biophys 322(1);291-4. PMID: 7574689
Benov97: Benov L, Sage H, Fridovich I (1997). "The copper- and zinc-containing superoxide dismutase from Escherichia coli: molecular weight and stability." Arch Biochem Biophys 340(2);305-10. PMID: 9143335
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