O2 Binding Mechanism
The mechanism of dioxygen binding is unusual. Most O2 carriers operate via formation of Dioxygen complexes, but hemerythrin holds the O2 as a hydroperoxide. The site that binds O2 consists of a pair of iron centres. The iron atoms are bound to the protein through the carboxylate side chains of a glutamate and aspartates as well as through five histidine residues. Hemerythrin and myohemerythrin are often described according to oxidation and ligation states of the iron centre:
| Fe2+—OH—Fe2+ | deoxy (reduced) |
| Fe2+—OH—Fe3+ | semi-met |
| Fe3+—O—Fe3+—OOH- | oxy (oxidized) |
| Fe3+—OH—Fe3+— (any other ligand) | met (oxidized) |
The uptake of O2 by hemerythrin is accompanied by two-electron oxidation of the diferrous centre to produce a hydroperoxide (OOH-) complex. The binding of O2 binding is roughly described in scheme:
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 O2 substrate. This proton-transfer result in the formation of a single oxygen atom (μ-oxo) bridge in oxy- and methemerythrin. O2 binds to the pentacoordinate Fe2+ centre at the vacant coordination site (B). Then electrons are transferred from the ferrous ions to generate the binuclear ferric (Fe3+,Fe3+) centre with bound peroxide (C).
Read more about this topic: Hemerythrin
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