Heme
Chemical coordination complex of an iron ion chelated to a porphyrin / From Wikipedia, the free encyclopedia
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Heme (American English), or haem (Commonwealth English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecular component of hemoglobin, which is necessary to bind oxygen in the bloodstream. It is composed of four pyrrole rings with 2 vinyl and 2 propionic acid side chains.[1] Heme is biosynthesized in both the bone marrow and the liver.[2]
Heme plays a critical role in multiple different redox reactions in mammals, due to its ability to carry the oxygen molecule. Reactions include oxidative metabolism (cytochrome c oxidase, succinate dehydrogenase), xenobiotic detoxification via cytochrome P450 pathways (including metabolism of some drugs), gas sensing (guanyl cyclases, nitric oxide synthase), and microRNA processing (DGCR8).[3][4]
Heme is a coordination complex "consisting of an iron ion coordinated to a tetrapyrrole acting as a tetradentate ligand, and to one or two axial ligands".[5] The definition is loose, and many depictions omit the axial ligands.[6] Among the metalloporphyrins deployed by metalloproteins as prosthetic groups, heme is one of the most widely used[7] and defines a family of proteins known as hemoproteins. Hemes are most commonly recognized as components of hemoglobin, the red pigment in blood, but are also found in a number of other biologically important hemoproteins such as myoglobin, cytochromes, catalases, heme peroxidase, and endothelial nitric oxide synthase.[8][9]
The word haem is derived from Greek αἷμα haima 'blood'.