Editing Reactive oxygen species (section)

==Antioxidant enzymes==

===Superoxide dismutase===
{{Main|Superoxide dismutase}}
[[Superoxide dismutase]]s (SOD) are a class of enzymes that catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important [[antioxidant]] defense in nearly all cells exposed to oxygen. In mammals and most chordates, three forms of superoxide dismutase are present. SOD1 is located primarily in the cytoplasm, SOD2 in the mitochondria and SOD3 is extracellular. The first is a dimer (consists of two units), while the others are tetramers (four subunits). SOD1 and SOD3 contain copper and zinc ions, while SOD2 has a manganese ion in its reactive centre. The genes are located on chromosomes 21, 6, and 4, respectively (21q22.1, 6q25.3 and 4p15.3-p15.1).{{citation needed|date=September 2024}}

The SOD-catalysed [[dismutation]] of [[superoxide]] may be written with the following half-reactions:
:<math chem>\begin{align}
& \ce{M}^{(n+1)+}  + \ce{O2- ->[SOD]}\  \ce{M}^{n+} + \ce{O2} \\
& \ce{M}^{n+} + \ce{O2- + 2H+ ->[] [SOD]}\ \ce{M}^{(n+1)+} + \ce{H2O2}
\end{align}</math>

where M&nbsp;=&nbsp;[[Copper|Cu]] ({{math|1=''n'' = 1}}); [[manganese|Mn]] ({{math|1=''n'' = 2}}); [[iron|Fe]] ({{math|1=''n'' = 2}}); [[nickel|Ni]] ({{math|1=''n'' = 2}}). In this reaction the [[oxidation state]] of the metal cation oscillates between {{mvar|n}} and {{math|''n'' + 1}}.

[[Catalase]], which is concentrated in [[peroxisomes]] located next to mitochondria, reacts with the hydrogen peroxide to catalyze the formation of water and oxygen. [[Glutathione peroxidase]] reduces hydrogen peroxide by transferring the energy of the reactive peroxides to a sulfur-containing [[tripeptide]] called [[glutathione]]. The sulfur contained in these enzymes acts as the reactive center, carrying reactive electrons from the peroxide to the glutathione. [[Peroxiredoxins]] also degrade {{chem2|H2O2}}, within the mitochondria, cytosol, and nucleus.

:<math chem>\begin{align}
& \ce{2 H2O2 ->[\text{catalase}] 2 H2O{} + O2} \\
& \ce{2GSH{} + H2O2 ->[] [\text{glutathione} \atop \text{peroxidase}] GS-SG{} + 2H2O}
\end{align}</math>