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[[File:Oxyhaemoglobin dissociation curve.png|thumb|250px|Hemoglobin Dissociation Curve. Dotted red line corresponds with shift to the right caused by Bohr effect]]
The '''Bohr effect''' is a physiological phenomenon first described in 1904 by the Danish physiologist [[Christian Bohr]], stating that [[hemoglobin]]'s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide.<ref>{{cite journal|last=Bohr|coauthors=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}}</ref> That is to say, a decrease in blood [[pH]] which leads to an increase in blood CO2 concentration will result in hemoglobin proteins releasing their loads of oxygen. Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in hemoglobin picking up more oxygen. Since carbon dioxide reacts with water to form [[carbonic acid]], an increase in CO2 results in a decrease in blood pH.

==Mechanism==
In deoxyhemoglobin, the [[N-terminal]] amino groups of the α-subunits and the [[C-terminal]] [[histidine]] of the β-subunits participate in [[ion-association|ion pairs]]. The formation of ion pairs causes them to decrease in acidity. Thus, deoxyhemoglobin binds one proton (H+) for every two O2 released. In oxyhemoglobin, these ion pairings are absent and these groups increase in acidity. Consequentially, a proton is released for every two O2 bound. Specifically, this reciprocal coupling of protons and oxygen is the Bohr effect.<ref name= "Murray">{{cite book
| last=Murray | first=Robert K. | coauthors=Darryl K. Granner, Peter A. Mayes, Victor W. Rodwell
| title=Harper’s Illustrated Biochemistry (LANGE Basic Science)
| publisher=McGraw-Hill Medical
| year=2003
| edition=26th
| pages=44–45
| isbn=0-07-138901-6
}}</ref>

Additionally, carbon dioxide reacts with the N-terminal amino groups of α-subunits to form [[carbamates]]:<ref name="lehninger166">{{Cite book|
title=Principles of Biochemistry|
edition=5th|
first1=Albert L.|
first2=David L.|
first3=Michael M.|
last1=Lehninger|
last2=Nelson|
last3=Cox|
publisher=W.H. Freeman and Company|
location=New York, NY|
year=2008|
isbn=978-0-7167-7108-1|
page=166}}</ref>
:R−NH2 + CO2 <math>\rightleftharpoons</math> R−NH−COO- + H+
Deoxyhemoglobin binds to CO2 more readily to form a [[carbamate]] than oxyhemoglobin. When CO2 concentration is high (as in the [[capillaries]]), the protons released by carbamate formation further promotes oxygen release.
Although the difference in CO2 binding between the oxy and deoxy states of hemoglobin accounts for only 5% of the total blood CO2, it is responsible for half of the CO2 transported by blood. This is because 10% of the total blood CO2 is lost through the lungs in each circulatory cycle.<ref name="Voet"/>

==Physiological role==
This effect facilitates oxygen transport as hemoglobin binds to oxygen in the lungs, but then releases it in the tissues, particularly those tissues in most need of oxygen. When a tissue's metabolic rate increases, its carbon dioxide production increases. Carbon dioxide forms [[bicarbonate]] through the following reaction:
:CO2 + H2O <math>\rightleftharpoons</math> H2CO3 <math>\rightleftharpoons</math> H+ + HCO3−
Although the reaction usually proceeds very slowly, the enzyme family of [[carbonic anhydrase]], which is present in [[red blood cells]], accelerates the formation of bicarbonate and protons.{{Citation needed|date=March 2012}} This causes the pH of tissues to decrease, and so, promotes the dissociation of oxygen from hemoglobin to the tissue, allowing the tissue to obtain enough oxygen to meet its demands. Conversely, in the lungs, where oxygen concentration is high, binding of oxygen causes hemoglobin to release protons, which combine with bicarbonate to drive off carbon dioxide in [[exhalation]]. Since these two reactions are closely matched, there is little change in blood pH.

The [[Oxygen-haemoglobin dissociation curve|dissociation curve]] shifts to the right when carbon dioxide or hydrogen ion concentration is increased. This facilitates increased oxygen dumping. This mechanism allows for the body to adapt the problem of supplying more oxygen to tissues that need it the most. When [[muscles]] are undergoing strenuous activity, they generate CO2 and [[lactic acid]] as products of [[cellular respiration]] and [[lactic acid fermentation]]. In fact, muscles generate lactic acid so quickly that pH of the blood passing through the [[muscles]] will drop to around 7.2. As lactic acid releases its protons, pH decreases, which causes hemoglobin to release ~10% more oxygen.<ref name = "Voet">{{cite book
| last=Voet | first=Donald | coauthors=Judith G. Voet, Charlotte W. Pratt
| title=Fundamentals of Biochemistry: Life at the Molecular Level
| publisher=John Wiley & Sons
| year=2008
| edition=3rd
| pages=189–190
}}</ref>

==Effects of allostery==
The Bohr effect is dependent on allosteric interactions between the [[heme]]s of the hemoglobin [[tetrameric protein|tetramer]]. This is evidenced by the fact that [[myoglobin]], a [[monomer]] with no allostery, does not exhibit the Bohr effect. Hemoglobin mutants with weaker allostery may exhibit a reduced Bohr effect.

In the Hiroshima variant [[hemoglobinopathy]], allostery in hemoglobin is reduced, and the Bohr effect is diminished. During periods of exercise, the mutant hemoglobin has a higher affinity for oxygen and tissue may suffer minor oxygen starvation.<ref>{{cite journal | last=Olson | first=JS | coauthors= Gibson QH, Nagel RL, Hamilton HB| title=The ligand-binding properties of hemoglobin Hiroshima ( 2 2 146asp )| journal=The Journal of Biological Chemistry | volume=247 | issue=23 | pages=7485–93 | date=December 1972 | pmid=4636319}}</ref>

==See also==
*[[Allosteric regulation]]
*[[Haldane Effect]]
*[[Root effect]]

== References ==
{{Reflist}}

==External links==
* [http://jap.physiology.org/cgi/content/abstract/52/6/1524 Impact of training]

{{Respiratory physiology}}

[[Category:Respiratory physiology]]

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en>4bpp: Small wording glitch.

86.132.223.116: /* Books */