Not to be confused with the Bohr Equation
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. Id est, an increase in blood CO2 concentration which leads to a
decrease in blood pH will result in hemoglobin proteins releasing their load
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 pairs. The formation
of ion pairs causes them to decrease in acidity. Thus, deoxyhemoglobin binds one
proton 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.
Additionally, carbon dioxide reacts with the N-terminal amino groups of
α-subunits to form carbamates: R−NH2 + CO2 R−NH−COO− + H+
Deoxyhemoglobin binds to CO2 more readily to form a carbamate than
oxyhemoglobin. When CO2 concentration is high, 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. 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 H2CO3 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. 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 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. Effects of allostery
The Bohr effect is dependent on allosteric interactions between the
hemes of the hemoglobin 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. See also
Allosteric regulation Haldane Effect
Root effect References
External links Impact of training