Editing Catalase (section)

=== Molecular mechanism ===
While the complete mechanism of catalase is not currently known,<ref name=Boon_b/> the [[chemical reaction|reaction]] is believed to occur in two stages:

: H<sub>2</sub>O<sub>2</sub> + Fe(III)-E → H<sub>2</sub>O + O=Fe(IV)-E(.+)

: H<sub>2</sub>O<sub>2</sub> + O=Fe(IV)-E(.+) → H<sub>2</sub>O + Fe(III)-E + O<sub>2</sub><ref name=Boon_b>{{cite web |vauthors=Boon EM, Downs A, Marcey D | title = Proposed Mechanism of Catalase | work = Catalase: H<sub>2</sub>O<sub>2</sub>: H<sub>2</sub>O<sub>2</sub> Oxidoreductase: Catalase Structural Tutorial | url = http://biology.kenyon.edu/BMB/Chime/catalase/frames/cattx.htm#Proposed%20Mechanism%20of%20Catalase | access-date = 2007-02-11}}</ref>

Here Fe()-E represents the [[iron]] center of the [[heme]] group attached to the enzyme. Fe(IV)-E(.+) is a mesomeric form of Fe(V)-E, meaning the iron is not completely oxidized to +V, but receives some stabilising electron density from the heme ligand, which is then shown as a radical cation (.+).

As hydrogen peroxide enters the [[active site]], it does not interact with the [[amino acid]]s Asn148 ([[asparagine]] at position 148) and [[histidine|His75]], causing a [[proton]] (hydrogen [[ion]]) to transfer between the oxygen atoms. The free oxygen atom coordinates, freeing the newly formed water molecule and Fe(IV)=O. Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen.<ref name=Boon_b /> The reactivity of the iron center may be improved by the presence of the phenolate [[ligand]] of [[tyrosine|Tyr358]] in the fifth coordination position, which can assist in the [[oxidation]] of the Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His75 and Asn148 with [[reaction intermediates]].<ref name=Boon_b /> The decomposition of hydrogen peroxide by catalase proceeds according to first-order kinetics, the rate being proportional to the hydrogen peroxide concentration.<ref>{{cite book | vauthors = Aebi H | title = Oxygen Radicals in Biological Systems | chapter = Catalase in vitro | series = Methods in Enzymology | volume = 105 | pages = 121–126 | date = 1984 | pmid = 6727660 | doi = 10.1016/S0076-6879(84)05016-3 | isbn = 9780121820053 }}</ref>

Catalase can also catalyze the oxidation, by [[hydrogen peroxide]], of various metabolites and toxins, including [[formaldehyde]], [[formic acid]], [[phenols]], [[acetaldehyde]] and [[alcohols]]. It does so according to the following reaction:

: H<sub>2</sub>O<sub>2</sub> + H<sub>2</sub>R → 2H<sub>2</sub>O + R

The exact mechanism of this reaction is not known.

Any heavy metal ion (such as copper cations in [[copper(II) sulfate]]) can act as a [[noncompetitive inhibitor]] of catalase.  However, "Copper deficiency can lead to a reduction in catalase activity in tissues, such as heart and liver."<ref>{{cite journal | vauthors = Hordyjewska A, Popiołek Ł, Kocot J | title = The many "faces" of copper in medicine and treatment | journal = Biometals | volume = 27 | issue = 4 | pages = 611–621 | date = August 2014 | pmid = 24748564 | pmc = 4113679 | doi = 10.1007/s10534-014-9736-5 }}</ref> Furthermore, the poison [[cyanide]] is a noncompetitive inhibitor<ref>{{cite journal | vauthors = Kremer ML | title = Nonstationary inhibition of enzyme action. The cyanide inhibition of catalase. | journal = The Journal of Physical Chemistry | date = April 1981 | volume = 85 | issue = 7 | pages = 835–839 | doi = 10.1021/j150607a021 }}</ref> of catalase at high concentrations of [[hydrogen peroxide]].<ref>{{cite journal | vauthors = Ogura Y, Yamazaki I | title = Steady-state kinetics of the catalase reaction in the presence of cyanide | journal = Journal of Biochemistry | volume = 94 | issue = 2 | pages = 403–408 | date = August 1983 | pmid = 6630165 | doi = 10.1093/oxfordjournals.jbchem.a134369 }}</ref>
[[Arsenate]] acts as an [[Enzyme activator|activator]].<ref>{{cite journal | vauthors = Kertulis-Tartar GM, Rathinasabapathi B, Ma LQ | title = Characterization of glutathione reductase and catalase in the fronds of two Pteris ferns upon arsenic exposure | journal = Plant Physiology and Biochemistry | volume = 47 | issue = 10 | pages = 960–965 | date = October 2009 | pmid = 19574057 | doi = 10.1016/j.plaphy.2009.05.009 | bibcode = 2009PlPB...47..960K }}</ref>  Three-dimensional [[protein structure]]s of the peroxidated catalase intermediates are available at the [[Protein Data Bank]].