A hemeprotein (or haemprotein; also hemoprotein or haemoprotein), or heme protein, is a metalloprotein containing a heme prosthetic group- an organic compound that allows a protein to carry out a function that it cannot do alone.[1] Heme remains bound to the protein permanently, either covalently or noncovalently bound or both.[2]
The heme contains a reduced iron atom, Fe2+ in the center of a highly hydrophobic, planar, porphyrin ring. The iron has six possible coordination bonds. The porphyrin ring has 4 nitrogen atoms that bind to the iron, leaving two other coordination positions of the iron available for bonding to the histidine of the protein and a divalent atom.[2]
Hemeproteins probably evolved to incorporate the iron (Fe) atom contained within the protoporphyrin IX ring of heme into proteins. As it makes hemeproteins responsive to molecules that can bind divalent iron (Fe), this strategy has been maintained throughout evolution as it plays crucial physiological functions. Oxygen (O2) nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) binds to the iron atom in heme proteins. Once bound to the prosthetic heme groups, these molecules can modulate the activity/function of those hemeproteins, affording signal transduction. Therefore, when produced in biologic systems (cells), these gaseous molecules are referred to as gasotransmitters.
Because of their diverse range of biological functions, the structural and functional characterizations of hemeproteins are the most studied classes of biomolecules. This has led to many reported literature. Therefore, for the purpose of global analysis, the data of heme protein stucture-function relationship has been united in a web-based resource, The Heme protein Database (HPD): http://heme.chem.colubmia.edu/heme.php[3] located at a new site http://hemeprotein.info/heme.php[4]
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Hemeproteins have diverse biological functions including:
Hemoglobin and Myoglobin are examples of hemeprotein that are essential in the storing and transports of oxygen in mammals.[2] Hemoglobin is a quaternary protein that occurs in the red blood cell, whereas, myoglobin is a tertiary protein found the muscle cells of mammals. Althought they might differ in location and size, their function are similar. Being hemeproteins, they both contain a prosthetic group.[2]
His-F8 of the myoglobin, also known as the proximal histadine is covalently bonded to the 5th coordination position of the iron. And as an Oxygen binds to the 6th coordination position of the iron, a distal histidine(a histidine that doesn't bond directly with the Iron), His-E7 of the myoglobin binds to the Oxygen that is now covalently bonded to the iron. The same occurs for the hemoglobin, however, being a protein with 4 subunits, the hemoglobin contains 4 heme, therefore, allowing 4 Oxygen atoms to bind to the heme.[2]
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