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Heme B

Heme B

protoheme IX Heme B or haem B (also known as protoheme IX) 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 (cyclooxygenase) 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 (known as the apoprotein) 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 (forming a plane) 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

These are four concentrated ammonia water solutions. From left to right: nicotinamide adduct of Heme B, carbon monoxide adduct of Heme B, nicotinic acid adduct of Heme B, and Heme B. They are slightly soluble in water and are all highly sensitive to oxygen in the air, requiring oxygen-free storage. Heme B can be prepared by reducing Hemin in pyridine with sodium dithionite. Upon addition of the dithionite, the solution changed color and gelled:

Reduction of Hemin with sodium dithionite

It can also be prepared by reducing Hemin with Raney nickel in ammonia water:

Reduction of Hemin with raney nickel

Degradation

Main article: Heme#Degradation

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.

References

References

  1. Ogun, Aminat S.. (2022). "Biochemistry, Heme Synthesis". StatPearls Publishing.
  2. (1934). "Die Chemie des Pyrrols". [[Akademische Verlagsgesellschaft]].
  3. Epstein, Lawrence M.; Straub, Darel K.; Maricondi, C.. (1967). "Moessbauer spectra of some porphyrin complexes with pyridine, piperidine, and imidazole". Inorganic Chemistry.
  4. Watson, Cecil J.. (1977). "International Symposium on Chemistry and Physiology of Bile Pigments". U.S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health.
  5. (2000). "UDP-Glucuronosyltransferases". Current Drug Metabolism.
  6. (2024-01-03). "BilR is a gut microbial enzyme that reduces bilirubin to urobilinogen". Nature Microbiology.
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