Heme B

Heme B or haem B is the most abundant heme. 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 of recent fame, also contain heme B at one of two active sites. Isolated heme B is slightly soluble in ammonia water and is extremely sensitive to oxygen, instantly oxidizing to a dark green to black-brown color when exposed to air.
Generally, heme B is attached to the surrounding protein matrix through a single coordination bond between the heme iron and an amino-acid side-chain.
Both hemoglobin and myoglobin have a coordination bond to an evolutionarily-conserved histidine, while nitric oxide synthase and cytochrome P450 have a coordination bond to an evolutionarily-conserved cysteine bound to the iron center of heme B.
Since the iron in heme B containing proteins is bound to the four nitrogens of the porphyrin and a single electron donating atom of the protein, the iron is often in a pentacoordinate state. When oxygen or the toxic carbon monoxide is bound the iron becomes hexacoordinated.
The correct structures of heme B and heme S were first elucidated by German chemist Hans Fischer.

Chemical properties

Heme B can be prepared by reducing Hemin in pyridine with sodium dithionite. Upon addition of the dithionite, the solution changed color and gelled:
It can also be prepared by reducing Hemin with Raney nickel in ammonia water:

In humans, heme B is degraded by the enzyme heme oxygenase to give biliverdin, which is then converted to bilirubin by biliverdin reductase.
Bilirubin is either rendered water soluble by forming bilirubin diglucuronide and excreted from the liver in bile, or further reduced to urobilinogen.