{{More science citations needed|date=December 2025}}{{chembox | ImageFile = Heme B.svg | ImageSize = 150px | ImageAlt = Skeletal formula of heme B | ImageFile1 = Haem-B-based-on-xtal-3D-sf.png | ImageSize1 = 170 | ImageAlt1 = Space-filling model of the heme B complex | IUPACName = Iron(II) 3-[18-(2-carboxyethyl)-8,13-bis(ethenyl)-3,7,12,17-tetramethylporphyrin-21,23-diid-2-yl]propanoic acid | OtherNames = Iron protoporphyrin IX,<br>protoheme IX | Section1 = {{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 16739950 | InChI = 1/C34H34N4O4.Fe/c1-7-21-17(3)25-13-26-19(5)23(9-11-33(39)40)31(37-26)16-32-24(10-12-34(41)42)20(6)28(38-32)15-30-22(8-2)18(4)27(36-30)14-29(21)35-25;/h7-8,13-16H,1-2,9-12H2,3-6H3,(H4,35,36,37,38,39,40,41,42);/q;+2/p-2/b25-13-,26-13-,27-14-,28-15-,29-14-,30-15-,31-16-,32-16-;/rC34H32FeN4O4/c1-7-21-17(3)25-13-29-20(6)24(10-12-34(42)43)32-16-28-23(9-11-33(40)41)18(4)26(37-28)14-31-22(8-2)19(5)30(15-27(21)36-25)38(31)35-39(29)32/h7-8,13-16H,1-2,9-12H2,3-6H3,(H,40,41)(H,42,43)/b25-13-,26-14-,27-15-,28-16-,29-13-,30-15-,31-14-,32-16- | InChIKey = KABFMIBPWCXCRK-SMDPYJEOBB | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/C34H34N4O4.Fe/c1-7-21-17(3)25-13-26-19(5)23(9-11-33(39)40)31(37-26)16-32-24(10-12-34(41)42)20(6)28(38-32)15-30-22(8-2)18(4)27(36-30)14-29(21)35-25;/h7-8,13-16H,1-2,9-12H2,3-6H3,(H4,35,36,37,38,39,40,41,42);/q;+2/p-2/b25-13-,26-13-,27-14-,28-15-,29-14-,30-15-,31-16-,32-16-; | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = KABFMIBPWCXCRK-RGGAHWMASA-L | CASNo_Ref = {{cascite|correct|CAS}} | CASNo = 14875-96-8 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 42VZT0U6YR | PubChem = 444098 | SMILES = OC(=O)CC/C6=C(\C)/C=3/N=C6/C=C2/C(/CCC(O)=O)=C(/C)\C1=C\C5=N\C(=C/c4n([Fe]N12)c(C=3)c(C=C)c4C)C(\C=C)=C5\C | MeSHName = Heme+b | SMILES1 = OC(=O)CC/c6c(\C)c3n7c6cc2c(/CCC(O)=O)c(/C)c1cc5n8c(cc4n([Fe]78n12)c(c=3)c(C=C)c4c)c(\C=C)c5\C }} | Section2 = {{Chembox Properties | Formula = C<sub>34</sub>H<sub>32</sub>O<sub>4</sub>N<sub>4</sub>Fe | MolarMass = 616.487 | Appearance = Burgundy like solid #64293D (100,41,61) | Density = | MeltingPt = | BoilingPt = | SolubleOther = Slightly soluble in ammonia water and pyridine }} | Section3 = {{Chembox Hazards | MainHazards = | FlashPt = | AutoignitionPt = }} | Section4 = {{Chembox Related | OtherCompounds = Hemin, Hematin }} }} '''Heme B''' or '''haem B''' (also known as '''protoheme IX''') is the most abundant heme.<ref>{{Citation |last=Ogun |first=Aminat S. |title=Biochemistry, Heme Synthesis |date=2022 |url=https://www.ncbi.nlm.nih.gov/books/NBK537329/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30726014 |access-date=2023-01-03 |last2=Joy |first2=Neena V. |last3=Valentine |first3=Menogh}}</ref> 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.<ref>{{cite book | last1 = Fischer|first1=H. | last2 = Orth|first2=H. | title =Die Chemie des Pyrrols | location = Leipzig | publisher = Akademische Verlagsgesellschaft | date =1934 |title-link=Die Chemie des Pyrrols }}</ref>
== Chemical properties == {{Multiple image | direction = vertical | align = left | image1 = Ammonia Solutions of 4 kinds of Heme-B adduct. (Precipitated).jpg | image2 = Ammonia Solutions of 4 kinds of Heme-B adduct. (Suspended).jpg | width = 200 | caption1 = (Precipitated) | caption2 = (Suspended)<br/>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: <ref>{{cite journal|author=Epstein, Lawrence M.; Straub, Darel K.; Maricondi, C.|title=Moessbauer spectra of some porphyrin complexes with pyridine, piperidine, and imidazole|journal=Inorganic Chemistry|year=1967|volume=6|issue=9|pages=1720–1724|doi=10.1021/ic50055a024}}</ref> 795x795px|Reduction of Hemin with sodium dithionite|center
It can also be prepared by reducing Hemin with Raney nickel in ammonia water: 650x650px|Reduction of Hemin with raney nickel|center
==Degradation== {{Main|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.<ref name=Watson>{{Cite book |last=Watson |first=Cecil J.|chapter-url=https://books.google.com/books?id=PZfDTn6BTEAC&q=irving+london+bilirubin&pg=PA9 |title=International Symposium on Chemistry and Physiology of Bile Pigments |date=1977 |publisher=U.S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health |editor-last=Berk|editor-first=Paul D.|pages=3–16 |chapter=Historical Review of Bilirubin Chemistry}}</ref>
{{chemrxn|width=65%| {{chemrxn/sub||class=skin-invert-image|caption=heme B }} {{chemrxn/arw|direction=forward }} {{chemrxn/cpd|biliverdin|upright=4 }} {{chemrxn/arw|direction=forward }} {{chemrxn/cpd|bilirubin|upright=4 }} }}
Bilirubin is either rendered water soluble by forming bilirubin diglucuronide and excreted from the liver in bile,<ref>{{cite journal|doi=10.2174/1389200003339171|title=UDP-Glucuronosyltransferases |year=2000 |last1=King |first1=C. |last2=Rios |first2=G. |last3=Green |first3=M. |last4=Tephly |first4=T. |journal=Current Drug Metabolism|volume=1|issue=2|pages=143–161|pmid=11465080}}</ref> or further reduced to urobilinogen.<ref name=Watson/><ref>{{Cite journal |last1=Hall |first1=Brantley |last2=Levy |first2=Sophia |last3=Dufault-Thompson |first3=Keith |last4=Arp |first4=Gabriela |last5=Zhong |first5=Aoshu |last6=Ndjite |first6=Glory Minabou |last7=Weiss |first7=Ashley |last8=Braccia |first8=Domenick |last9=Jenkins |first9=Conor |last10=Grant |first10=Maggie R. |last11=Abeysinghe |first11=Stephenie |last12=Yang |first12=Yiyan |last13=Jermain |first13=Madison D. |last14=Wu |first14=Chih Hao |last15=Ma |first15=Bing |date=2024-01-03 |title=BilR is a gut microbial enzyme that reduces bilirubin to urobilinogen |journal=Nature Microbiology |volume=9 |issue=1 |pages=173–184 |language=en |doi=10.1038/s41564-023-01549-x |issn=2058-5276|doi-access=free |pmid=38172624 |pmc=10769871 }}</ref>
{{Enzyme cofactors}} {{tetrapyrroles}}
==References== {{Reflist}}
Category:Tetrapyrroles Category:Biomolecules