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|Except where noted otherwise, data are given for|
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references
Heme B is the most abundant heme; both hemoglobin and myoglobin are examples of oxygen transport proteins that contain heme B and the peroxidase family of enzymes also contain heme B. The COX-1 and COX-2 enzymes (cyclooxygenase) of recent fame, also contain heme B at one of two active sites.
Generally, heme B is attached to the surrounding protein matrix (known as the apoprotein) through a single coordination bond between the heme iron and an amino acid side-chain.
Both hemoglobin and myoglobin have a coordination bond to an evolutionary conserved histidine, while Nitric oxide synthase and Cytochrome P450 have a coordination bond to an evolutionary conserved cysteine bound to the iron center of heme B.
Since the iron in heme B containing proteins is bound to the four nitrogens of the porphyrin (forming a plane) and a single electron donating atom of the protein, the iron is often in a pentacoordinate state. When bound with oxygen or the toxin carbon monoxide the iron is hexacoordinated.