Editing Superoxide dismutase (section)

== Biochemistry ==

SOD out-competes damaging reactions of superoxide, thus protecting the cell from superoxide toxicity.
The reaction of superoxide with non-radicals is [[selection rule|spin-forbidden]]. In biological systems, this means that its main reactions are with itself (dismutation) or with another biological radical such as [[nitric oxide]] (NO) or with a transition-series metal. The superoxide anion radical ({{chem|O|2|-}}) spontaneously dismutes to O<sub>2</sub> and hydrogen peroxide ({{chem|H|2|O|2}}) quite rapidly (~10<sup>5</sup> M<sup>−1</sup>s<sup>−1</sup> at pH 7).{{Citation needed|reason=Couldn't find reliable source backing the rate constant|date=July 2017}} SOD is necessary because superoxide reacts with sensitive and critical cellular targets. For example, it reacts with the NO radical, and makes toxic [[peroxynitrite]].

Because the uncatalysed dismutation reaction for superoxide requires two superoxide molecules to react with each other, the dismutation rate is second-order with respect to initial superoxide concentration. Thus, the half-life of superoxide, although very short at high concentrations (e.g., 0.05 seconds at 0.1mM) is actually quite long at low concentrations (e.g., 14 hours at 0.1 nM). In contrast, the reaction of superoxide with SOD is first order with respect to superoxide concentration. Moreover, superoxide dismutase has the largest ''k''<sub>cat</sub>/''K''<sub>M</sub> (an approximation of catalytic efficiency) of any known enzyme (~7 × 10<sup>9</sup> M<sup>−1</sup>s<sup>−1</sup>),<ref name="isbn3-540-32680-4">{{cite book | vauthors = Heinrich PC, Löffler G, Petrifies PE | title = Biochemie und Pathobiochemie (Springer-Lehrbuch) | edition = German| publisher = Springer | location = Berlin | year = 2006 | pages = 123 | isbn = 978-3-540-32680-9 }}</ref> this reaction being limited only by the frequency of collision between itself and superoxide. That is, the reaction rate is "diffusion-limited".

The high efficiency of superoxide dismutase seems necessary: even at the subnanomolar concentrations achieved by the high concentrations of SOD within cells, superoxide inactivates the citric acid cycle enzyme [[aconitase]], can poison energy metabolism, and releases potentially toxic iron. Aconitase is one of several iron-sulfur-containing (de)hydratases in metabolic pathways shown to be inactivated by superoxide.<ref name="pmid7768942">{{cite journal | vauthors = Gardner PR, Raineri I, Epstein LB, White CW | title = Superoxide radical and iron modulate aconitase activity in mammalian cells | journal = The Journal of Biological Chemistry | volume = 270 | issue = 22 | pages = 13399–13405 | date = June 1995 | pmid = 7768942 | doi = 10.1074/jbc.270.22.13399 | doi-access = free }}</ref>