Editing Hemerythrin (section)

==O<sub>2</sub> binding mechanism==
The mechanism of dioxygen binding is unusual. Most O<sub>2</sub> carriers operate via formation of [[dioxygen complex]]es, but hemerythrin holds the O<sub>2</sub> as a [[hydroperoxide]] (HO<sub>2</sub>, or -OOH<sup>−</sup>). The site that binds O<sub>2</sub> consists of a pair of iron centres. The iron atoms are bound to the protein through the carboxylate side chains of a [[glutamate]] and [[aspartate]]s as well as through five [[histidine]] residues. Hemerythrin and myohemerythrin are often described according to oxidation and ligation states of the iron center:

{| class=wikitable
|Fe<sup>2+</sup>—OH—Fe<sup>2+</sup>
|deoxy (reduced)
|-
|Fe<sup>2+</sup>—OH—Fe<sup>3+</sup>
|semi-met
|-
|Fe<sup>3+</sup>—O—Fe<sup>3+</sup>—OOH<sup>−</sup>
|oxy (oxidized)
|-
|Fe<sup>3+</sup>—OH—Fe<sup>3+</sup>— (any other ligand)
|met (oxidized)
|}

The uptake of O<sub>2</sub> by hemerythrin is accompanied by two-electron oxidation of the di[[ferrous]] centre to produce a [[hydroperoxide]] (OOH<sup>−</sup>) complex. The binding of O<sub>2</sub> is roughly described in this diagram:

:[[File:O2+hemerythrin.svg|thumb|center|440px|class=skin-invert-image|Active site of hemerythrin before and after oxygenation.]]

Deoxyhemerythrin contains two high-spin ferrous ions bridged by [[hydroxyl]] group ('''A'''). One iron is hexacoordinate and another is pentacoordinate. A [[hydroxyl]] group serves as a [[bridging ligand]] but also functions as a proton donor to the O<sub>2</sub> substrate.  This proton-transfer result in the formation of a single oxygen atom (μ-oxo) bridge in oxy- and methemerythrin. O<sub>2</sub> binds to the pentacoordinate Fe<sup>2+</sup> centre at the vacant coordination site ('''B'''). Then electrons are transferred from the ferrous ions to generate the binuclear ferric (Fe<sup>3+</sup>,Fe<sup>3+</sup>) centre with bound peroxide ('''C''').<ref>D. M. Kurtz, Jr. "Dioxygen-binding Proteins" in Comprehensive Coordination Chemistry II 2003, Volume 8, Pages 229–260. {{doi|10.1016/B0-08-043748-6/08171-8}}</ref><ref>{{cite journal | last1 = Friesner | first1 = R. A. | last2 = Baik | first2 = M.-H. | last3 = Gherman | first3 = B. F. | last4 = Guallar | first4 = V. | last5 = Wirstam | first5 = M. | last6 = Murphy | first6 = R. B. | last7 = Lippard | first7 = S. J. | year = 2003 | title = How iron-containing proteins control dioxygen chemistry: a detailed atomic level description via accurate quantum chemical and mixed quantum mechanics/molecular mechanics calculations | journal = Coord. Chem. Rev. | volume = 238–239 | pages = 267–290 | doi = 10.1016/S0010-8545(02)00284-9 }}</ref>