Unit 2 Biology Cape notes on the bohr effect

1. Bohr effect
From Wikipedia, the free encyclopedia
Hemoglobin Dissociation Curve. Dotted red line corresponds with shift to the right caused by
Bohr effect
The Bohr effect is a microbiological phenomena first described in 1904 by the Danish
physiologist Christian Bohr (father of physicist Niels Bohr), hemoglobin's oxygen binding
affinity is inversely related to acidity and the concentration of carbon dioxide.[1]
That is to say, a
decrease in blood pH or an increase in blood CO2 concentration will result in hemoglobin
proteins releasing their loads of oxygen and a decrease in carbon dioxide or increase in pH will
result 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 ß-subunits participate in 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.

2. Consequentially, a proton is released for every two O2 bound. Specifically, this reciprocal
coupling of protons and oxygen is the Bohr effect. [2]
Additionally, carbon dioxide reacts with the N-terminal amino groups of α-subunits to form
carbamates[3]
:
R−NH2 + CO2 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.[4]
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, carbonic anhydrase 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. [4]
Effects of cooperativity
The Bohr effect is dependent on cooperative interactions between the hemes of the hemoglobin
tetramer. This is evidenced by the fact that myoglobin, a monomer with no cooperativity, does
not exhibit the Bohr effect. Hemoglobin mutants with weaker cooperativity may exhibit a
reduced Bohr effect.
In the Hiroshima variant hemoglobinopathy, 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.[5]