Heme

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Heme (American English), or haem (British English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecule that serves as a ligand of various proteins, especially as a component of hemoglobin, which carries oxygen in the bloodstream. It is composed of four pyrrole rings with two vinyl and two propionic acid side chains. Heme is biosynthesized in both the bone marrow and the liver. Heme plays a critical role in several 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). 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. The definition is loose, and many depictions omit the axial ligands...

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