Carbon Monoxide Ligand Accepts ... 361 x 202 - 4k - gif user.mc

4. A type of organometallic compound which is crucial in performing a variety of biological functions is the metalloporphyrins. These consist of a metal center and a planar ligand constructed from four pyrrole derivatives which chelates the metal. The behavior of each is determined mostly by the type of metal being chelated and the types of substituents on the edge of the porphyrin. An example of such is the heme complex. This metalloporphrin is a constituent in cytochromes, the proteins responsible for transporting and storing blood oxygen. In myoglobin and hemoglobin, the iron center of heme becomes the center of an octahedral ligand field. It has four coordination sites filled by the porphyrin ring and one by another part of the cytochrome. This leaves one possible position available at which a dioxygen molecule can coordinate

9. In the case of cytochromes, the sixth position is coordinated reversibly in the presence of molecular oxygen. This reversibility, which is necessary for continued oxygen transfer, does not occur with just ordinary heme. Oxygen coordinated to one heme complex will eventually coordinate to another, forming hematin. In the case of cytochromes however, steric hinderance caused by the bulky protein part prevents another heme-protein from coming close enough to the bound oxygen, allowing for reversibility.(19) Another example of a metalloporphyrin is chlorophyll a: Carbon Monoxide Ligand Accepts ...361 x 202 - 4k - gifuser.mc.net