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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]
Heme is bound to the protein either covalently or noncovalently or both. [2] The heme consists of iron cation bound at the center of the conjugate base of the porphyrin, as well as other ligands attached to the "axial sites" of the iron. The porphyrin ring is a planar dianionic, tetradentate ligand.
The name hemoglobin (or haemoglobin) is derived from the words heme (or haem) and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme group. Each heme group contains one iron atom, that can bind one oxygen molecule through ion-induced dipole forces.
Like hemoglobin, myoglobin is a cytoplasmic protein that binds oxygen on a heme group. It harbors only one globulin group, whereas hemoglobin has four. Although its heme group is identical to those in Hb, Mb has a higher affinity for oxygen than does hemoglobin but fewer total oxygen-storage capacities. [22]
Heme B or haem B (also known as protoheme IX) is the most abundant heme. [1] Hemoglobin and myoglobin are examples of oxygen transport proteins that contain heme B. 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.
Haem is derived from Greek meaning blood, [18] [19] and globin is Latin derived from globus typically accepted to mean glob/spherical/round object; the terms are conjoined with an -o-. Regarding haem, the use of "ae / æ" remains prevalent in British English in modern day [20] whereas the American English spelling evolved to heme from hema. [19]
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.
The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myoglobin and hemoglobin. Both of these proteins reversibly bind oxygen via a heme prosthetic group.