Editing Superoxide dismutase (section)

=== Independent Evolution of SOD Families ===
Studies examining the phylogenetic distribution of SODs support the proposal that the major [[Physiology|physiological]] function of SOD is to act as a protective mechanism among oxygen-metabolizing organisms against the formation of superoxide free radicals (McCord ''et al.'' 1971).<ref>McCord ''et al.'' (1971). An enzyme-based theory of obligate anaerobiosis: the physiological function of superoxide dismutase. ''Proc. Nat. Acad. Sci. USA, 68''(5): 1024-1027. pnas.org/doi/pdf/10.1073/pnas.68.5.1024. </ref> Although the enzyme is functionally limited to the conversion of superoxide radicals into less toxic oxygen-containing molecules, methodological approaches utilizing phylogenetic and structural protein analysis suggest that three SOD [[Protein isoform|isoforms]] have evolved independently to combat superoxide accumulation. Each enzyme family is characterized by a distinct 3D structure, amino acid sequence, and with regard to the metal-binding [[Cofactor (biochemistry)|cofactor]](s) used to support its structural stability and [[Catalysis|catalytic]] activity (Case 2017): either manganese (MnSOD), nickel (NiSOD), or both copper and zinc (Miller 2012).<ref>Miller (2012). Superoxide dismutases: Ancient enzymes and new insights, ''FEBS Open Bio'', 586, doi:10.1016/j.febslet.2011.10.048.</ref> An additional family utilizing an iron cofactor (FeSOD) has also been identified, which is evolutionarily related to MnSOD (Wolfe-Simon ''et al.'' 2005). The evolution of metalliform diversity can likely be explained by changes in heavy metal bioavailability that took place during large compositional changes of the earth’s early atmosphere and oceans.

==== Fe/MnSODs ====
SODs containing Fe (FeSOD), Mn (MnSOD), or may contain either (Fe/MnSOD) are believed to have been the earliest SOD isoforms among life on early earth. During this time, Fe and Mn would have been highly bioavailable. The differences between the [[Redox|oxidation and reduction]] potentials of each metal is thought to have been advantageous to organism survival, allowing them to exist in environments with varying O<sub>2</sub> concentration and metal availability.

==== CuZnSODs ====
The most modern SOD family is believed to utilize both Cu and Zn ions (CuZnSOD). The absence of CuZnSODs from [[Archaea|archaeal]] and [[protist]] genomes coupled with the post-GOE increased bioavailability of Cu and Zn suggests that the development of this isoform took place at a later period in evolutionary time (Banci ''et al.'' 2005; Wilkinson ''et al.'' 2006).<ref>Banci ''et al''. (2005). A prokaryotic superoxide dismutase paralog lacking two Cu ligands: From largely unstructured in solution to ordered in the crystal. ''Proc. Natl. Acad. Sci. USA'', ''102'':7541–7546. doi:10.1073/pnas.0502450102.</ref><ref>Wilkinson ''et al.'' (2006). Functional characterisation of the iron superoxide dismutase gene repertoire in ''Trypanosoma brucei''. ''Free Radic. Biol. Med. 40'':198–209. doi:10.1016/j.freeradbiomed.2005.06.022.</ref>

==== NiSODs ====
The family of Ni-containing SODs (NiSOD) is less understood. Evidence suggests that these isoforms are largely distributed among [[Marine prokaryotes#Marine bacteria|marine bacteria]] and [[algae]] (Wolfe-Simon ''et al.'' 2005; Dupont ''et al.'' 2008).<ref>Wolfe-Simon ''et al.'' (2005). THE ROLE AND EVOLUTION OF SUPEROXIDE DISMUTASES IN ALGAE. ''Journal of Phycology'', 41: 453-465. doi-org.reed.idm.oclc.org/10.1111/j.1529-8817.2005.00086.x.</ref><ref>Dupont ''et al.'' (2008). Diversity, function and evolution of genes coding for putative Ni-containing superoxide dismutases. ''Environ. Microbiol. 10'':1831–1843. doi:10.1111/j.1462-2920.2008.01604.x.</ref> The evolution of NiSOD is currently predicted to have occurred around the time of the GOE when a decrease in aquatic bioavailability of Fe took place.