Editing Heme (section)

==Function==
[[Image:Succinate Dehygrogenase 1YQ3 Haem group.png|thumb|right|The heme group of [[succinate dehydrogenase]] bound to [[histidine]], an [[electron carrier]] in the [[mitochondria]]l [[electron transfer chain]]. The large semi-transparent sphere indicates the location of the [[iron]] [[ion]]. From {{PDB|1YQ3}}.]]

[[Hemoproteins]] have diverse biological functions including the transportation of [[diatomic]] gases, chemical [[catalysis]], diatomic gas detection, and [[electron transfer]]. The heme iron serves as a source or sink of electrons during electron transfer or [[redox]] chemistry. In [[peroxidase]] reactions, the [[porphyrin]] [[molecule]] also serves as an electron source, being able to delocalize radical electrons in the conjugated ring. In the transportation or detection of diatomic gases, the gas binds to the heme iron. During the detection of diatomic gases, the binding of the gas [[ligand]] to the heme iron induces [[conformational change]]s in the surrounding protein.<ref>{{cite journal|last=Milani|first= M.|title=Structural bases for heme binding and diatomic ligand recognition in truncated hemoglobins.|journal=J. Inorg. Biochem.|year=2005|volume=99|issue=1|pages= 97–109|pmid=15598494|doi=10.1016/j.jinorgbio.2004.10.035}}</ref> In general, diatomic gases only bind to the reduced heme, as ferrous Fe(II) while most peroxidases cycle between Fe(III) and Fe(IV) and hemeproteins involved in mitochondrial redox, oxidation-reduction, cycle between Fe(II) and Fe(III).

It has been speculated that the original evolutionary function of [[hemoproteins]] was electron transfer in primitive [[sulfur]]-based [[photosynthesis]] pathways in ancestral [[cyanobacteria]]-like [[Living organisms|organisms]] before the appearance of molecular [[oxygen]].<ref>{{cite journal|last=Hardison|first= R.|title=The Evolution of Hemoglobin: Studies of a very ancient protein suggest that changes in gene regulation are an important part of the evolutionary story|journal=American Scientist|year=1999|volume=87|issue=2|pages= 126|doi= 10.1511/1999.20.809|doi-broken-date= 11 March 2025|s2cid= 123532036}}</ref>

Hemoproteins achieve their remarkable functional diversity by modifying the environment of the heme macrocycle within the protein matrix.<ref>{{cite journal|last=Poulos|first= T.|title=Heme Enzyme Structure and Function.|journal=Chem. Rev.|year=2014|volume=114|issue=7|pages=3919–3962|pmid=24400737|doi=10.1021/cr400415k|pmc=3981943}}</ref> For example, the ability of [[hemoglobin]] to effectively deliver [[oxygen]] to [[Tissue (biology)|tissues]] is due to specific [[amino acid]] residues located near the heme molecule.<ref>{{cite journal|last=Thom|first= C. S.|title=Hemoglobin Variants: Biochemical Properties and Clinical Correlates.|journal=Cold Spring Harbor Perspectives in Medicine|year=2013|volume=3|issue=3|pages=a011858|pmid=23388674|doi=10.1101/cshperspect.a011858|pmc=3579210}}</ref> Hemoglobin reversibly binds to oxygen in the lungs when the [[pH]] is high, and the [[carbon dioxide]] concentration is low. When the situation is reversed (low pH and high carbon dioxide concentrations), hemoglobin will release oxygen into the tissues. This phenomenon, which states that hemoglobin's oxygen [[binding affinity]] is [[inversely proportional]] to both [[acidity]] and concentration of carbon dioxide, is known as the [[Bohr effect]].<ref name=":2">{{cite journal|last=Bohr|author2=Hasselbalch, Krogh|title=Concerning a Biologically Important Relationship - The Influence of the Carbon Dioxide Content of Blood on its Oxygen Binding|url=http://www.udel.edu/chem/white/C342/Bohr%281904%29.html|url-status=live|archive-url=https://web.archive.org/web/20170418183908/http://www1.udel.edu/chem/white/C342/Bohr(1904).html|archive-date=2017-04-18}}</ref> The molecular [[Chemical mechanism|mechanism]] behind this effect is the [[steric]] organization of the [[globin]] chain; a [[histidine]] residue, located adjacent to the heme group, becomes positively charged under acidic conditions (which are caused by [[Carbonic acid|dissolved CO<sub>2</sub>]] in working muscles, etc.), releasing oxygen from the heme group.<ref>{{cite journal|last1=Ackers|first1= G. K.|last2=Holt|first2=J. M.|title=Asymmetric cooperativity in a symmetric tetramer: human hemoglobin.|journal=J. Biol. Chem.|year=2006|volume=281|issue=17|pages=11441–3|pmid=16423822|doi=10.1074/jbc.r500019200|s2cid= 6696041|doi-access=free}}</ref>