| Hemerythrin-like family | |||||||||
|---|---|---|---|---|---|---|---|---|---|
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| Identifiers | |||||||||
| Symbol | Hemerythrin | ||||||||
| Pfam | PF01814 | ||||||||
| InterPro | IPR035938 | ||||||||
| PROSITE | PDOC00476 | ||||||||
| CATH | 1HMO | ||||||||
| SCOP2 | 2HMZ /SCOPe /SUPFAM | ||||||||
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Hemerythrin (also spelledhaemerythrin;Ancient Greek:αἷμα,romanized: haîma,lit. 'blood',Ancient Greek:ἐρυθρός,romanized: erythrós,lit. 'red') is anoligomericprotein responsible foroxygen (O2) transport in themarine invertebratephyla ofsipunculids,priapulids,brachiopods, and in a singleannelid worm genus,Magelona.Myohemerythrin is amonomeric O2-binding protein found in the muscles of marine invertebrates. Hemerythrin and myohemerythrin are essentially colorless when deoxygenated, but turn a violet-pink in the oxygenated state.
Hemerythrin does not, as the name might suggest, contain aheme. The names of the blood oxygen transportershemoglobin,hemocyanin, and hemerythrin do not refer to the heme group (only found in globins). Instead, these names are derived from the Greek word for blood. Hemerythrin may also contribute toinnate immunity and anteriortissue regeneration in certain worms.[1]
The mechanism of dioxygen binding is unusual. Most O2 carriers operate via formation ofdioxygen complexes, but hemerythrin holds the O2 as ahydroperoxide (HO2, or -OOH−). 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 aglutamate andaspartates as well as through fivehistidine residues. Hemerythrin and myohemerythrin are often described according to oxidation and ligation states of the iron center:
| 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 ahydroperoxide (OOH−) complex. The binding of O2 is roughly described in this diagram:
Deoxyhemerythrin contains two high-spin ferrous ions bridged byhydroxyl group (A). One iron is hexacoordinate and another is pentacoordinate. Ahydroxyl group serves as abridging 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).[2][3]
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Hemerythrin typically exists as a homooctamer or heterooctamer composed of α- and β-type subunits of 13–14kDa each, although some species have dimeric, trimeric and tetrameric hemerythrins. Each subunit has a four-α-helix fold binding a binuclear iron centre. Because of its size hemerythrin is usually found in cells or "corpuscles" in the blood rather than free floating.
Unlike hemoglobin, most hemerythrins lackcooperative binding to oxygen, making it roughly 1/4 as efficient as hemoglobin. In somebrachiopods though, hemerythrin shows cooperative binding of O2. Cooperative binding is achieved by interactions between subunits: the oxygenation of one subunit increases the affinity of a second unit for oxygen.
Hemerythrin affinity forcarbon monoxide (CO) is actually lower than its affinity for O2, unlike hemoglobin which has a very high affinity for CO. Hemerythrin's low affinity for CO poisoning reflects the role of hydrogen-bonding in the binding of O2, a pathway mode that is incompatible with CO complexes which usually do not engage in hydrogen bonding.
The hemerythrin/HHE cation-bindingdomain occurs as a duplicated domain in hemerythrins, myohemerythrins and related proteins. This domain bindsiron in hemerythrin, but can bind other metals in related proteins, such ascadmium in theNereis diversicolor hemerythrin. It is also found in the NorA protein fromCupriavidus necator, this protein is a regulator of response tonitric oxide, which suggests a different set-up for its metalligands. A protein fromCryptococcus neoformans (Filobasidiella neoformans) that contains haemerythrin/HHE cation-binding domains is also involved in nitric oxide response.[4] AStaphylococcus aureus protein containing this domain, iron-sulfur cluster repair protein ScdA, has been noted to be important when theorganism switches to living in environments with low oxygen concentrations; perhaps this protein acts as an oxygen store or scavenger.[5]
Hemerythrin/HHE (H-HxxxE-HxxxH-HxxxxD) proteins found in bacteria are implicated in signal transduction andchemotaxis. More distantly related ones include H-HxxxE-H-HxxxE proteins (including theE3 ligase) and animalF-box proteins (H-HExxE-H-HxxxE).[6]
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