Hemerythrin

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Hemerythrin-like family
SCOP2
2HMZ / SCOPe / SUPFAM
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
Trimeric Hemerythrin Protein Complex (PDB: 1HMO​)

Hemerythrin (also spelled haemerythrin;

priapulids, brachiopods, and in a single annelid worm genus, Magelona. Myohemerythrin is a monomeric
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 a heme. The names of the blood oxygen transporters hemoglobin, 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 to innate immunity and anterior tissue regeneration in certain worms.[1]

O2 binding mechanism

The mechanism of dioxygen binding is unusual. Most O2 carriers operate via formation of

aspartates as well as through five histidine
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 a hydroperoxide (OOH) complex. The binding of O2 is roughly described in this diagram:

Active site of hemerythrin before and after oxygenation.

Deoxyhemerythrin contains two high-spin ferrous ions bridged by

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).[2][3]

Quaternary structure and cooperativity

Hemerythrin homooctamer with a single monomer highlighted in yellow. PDB: 1HMO

Hemerythrin typically exists as a homooctamer or heterooctamer composed of α- and β-type subunits of 13–14 kDa 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 lack

brachiopods
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 for carbon 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.

Hemerythrin/HHE cation-binding domain

The hemerythrin/HHE cation-binding

ligands. A protein from Cryptococcus neoformans (Filobasidiella neoformans) that contains haemerythrin/HHE cation-binding domains is also involved in nitric oxide response.[4] A Staphylococcus aureus protein containing this domain, iron-sulfur cluster repair protein ScdA, has been noted to be important when the organism 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 and chemotaxis. More distantly related ones include H-HxxxE-H-HxxxE proteins (including the

E3 ligase) and animal F-box proteins (H-HExxE-H-HxxxE).[6]

References

Further reading

External links

  • 1HMD - PDB structure of deoxyhemerythrin Themiste dyscrita (sipunculid worm)
  • 1HMO – PDB structure of oxyhemerythrin from Themiste dyscrita
  • 2MHR – PDB structure of azido-met myohemerythrin from Themiste zostericola (sipunculid worm)
  • IPR002063InterPro entry for hemerythrin
This article incorporates text from the public domain Pfam and InterPro: IPR012312