Bohr effect is a property of
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
first described in 1904 by the Danish physiologist
Christian BohrChristian Harald Lauritz Peter Emil Bohr was a Danish physician, father of the physicist and Nobel laureate Niels Bohr, as well as the mathematician Harald Bohr and grandfather of another physicist and nobel laureate Aage Bohr...
(father of physicist
Niels BohrNiels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...
), which states that an increasing concentration of protons and/or
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
will reduce the oxygen
affinityAffinity is a word used in a variety of fields, usually to indicate some kind of preference, relationship, or a potential or actual closeness between two entities.Articles dealing with various usages of the word: affinity include:-Commerce and law:...
of
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
. Increasing blood carbon dioxide levels can lead to a decrease in
pHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
because of the
chemical equilibriumIn a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
between protons and carbon dioxide.
Proton and oxygen coupling
In deoxyhemoglobin, the N-terminal amino groups of the α-subunits and the C-terminal
histidineHistidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
of the β-subunits participate in
ion pairsIon-association is a chemical reaction whereby ions of opposite electrical charge come together in solution to form a distinct chemical entity. Ion-associates are classified according to the number of ions that associate with each other, and the nature of the interaction. The most important factor...
. The formation of ion pairs causes them to decrease in acidity. Thus, deoxyhemoglobin binds one proton for every two O
2 released. In oxyhemoglobin, these ion pairings are absent and these groups increase in acidity. Consequentially, a proton is released for every two O
2 bound. Specifically, this reciprocal coupling of protons and oxygen is the Bohr effect.
Effect of carbondioxide and hydrogen ions on the oxygen affinity of haemoglobin is known as Bohr effect.
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
bicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
through the following reaction:
- CO2 + H2O
H2CO3 
H+ + HCO3−
Although the reaction usually proceeds very slowly, the enzyme family,
carbonic anhydraseThe carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
in red blood cells accelerates the formation of bicarbonate and protons. 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
exhalationExhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....
. Since these two reactions are closely matched, there is little change in blood pH.
The
dissociation curveDissociation curve may refer to:*Oxygen-haemoglobin dissociation curve, a graphical representation of oxygen release from haemoglobin*Melting curve analysis, a biochemical technique relying on heat-dependent dissociation between two DNA strands...
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 CO
2 and
lactic acidLactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
as products of
cellular respirationCellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
and
lactic acid fermentationLactic acid fermentation is a biological process by which sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic byproduct lactate. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells, in the...
. 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.
Carbamates
Carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
modulates O
2 binding to hemoglobin directly by combining reversibly to N-terminal amino groups of blood proteins to form carbamates:
- R−NH2 + CO2
R−NH−COO- + H+
Deoxyhemoglobin binds to CO
2 more readily to form a
carbamateCarbamates are organic compounds derived from carbamic acid . A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.-Synthesis:Carbamic acids are derived...
than oxyhemoglobin. When CO
2 concentration is high (as in the capillaries), the protons released by carbamate formation further promotes oxygen release.
Although the difference in CO
2 binding between the oxy and deoxy states of hemoglobin accounts for only 5% of the total blood CO
2, it is responsible for half of the CO
2 transported by blood. This is because 10% of the total blood CO
2 is lost through the lungs in each circulatory cycle.
Effects of cooperativity
The Bohr effect is dependent on cooperative interactions between the
hemeA heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
s of the hemoglobin tetramer. This is evidenced by the fact that
myoglobinMyoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
, a
monomerA monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
with no
cooperativityCooperativity is a phenomenon displayed by enzymes or receptors that have multiple binding sites where the affinity of the binding sites for a ligand is increased, positive cooperativity, or decreased, negative cooperativity, upon the binding of a ligand to a binding site...
, does not exhibit the Bohr effect. Hemoglobin mutants with weaker cooperativity may exhibit a reduced Bohr effect.
In the Hiroshima variant
hemoglobinopathyHemoglobinopathy is a kind of genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule. Hemoglobinopathies are inherited single-gene disorders; in most cases, they are inherited as autosomal co-dominant traits. Common hemoglobinopathies include...
, cooperativity 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.
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