Bohr effect

There is some more debate over whether Bohr was actually the first to discover the relationship between CO₂ and oxygen affinity, or whether the Russian physiologist Bronislav Verigo beat him to it, allegedly discovering the effect in 1898, six years before Bohr.[6] While this has never been proven, Verigo did in fact publish a paper on the haemoglobin-CO₂ relationship in 1892.[7] His proposed model was flawed, and Bohr harshly criticized it in his own publications.[1]

Another challenge to Bohr's discovery comes from within his lab. Though Bohr was quick to take full credit, his associate Krogh, who invented the apparatus used to measure gas concentrations in the experiments,[8] maintained throughout his life that he himself had actually been the first to demonstrate the effect. Though there is some evidence to support this, retroactively changing the name of a well-known phenomenon would be extremely impractical, so it remains known as the Bohr effect.[4]

The magnitude of the Bohr effect is usually given by the slope of the ${\textstyle \log(P_{50})}$ vs ${\textstyle {\text{pH}}}$ curve where, P₅₀ refers to the partial pressure of oxygen when 50% of haemoglobin's binding sites are occupied. The slope is denoted: ${\textstyle {\scriptstyle \Delta \log(P_{50}) \over \Delta {\text{pH}}}}$ where ${\textstyle \Delta }$ denotes change. That is, ${\textstyle \Delta \log(P_{50})}$ denotes the change in ${\textstyle \log(P_{50})}$ and ${\textstyle \Delta {\text{pH}}}$ the change in ${\textstyle {\text{pH}}}$. Bohr effect strength exhibits an inverse relationship with the size of an organism: the magnitude increases as size and weight decreases. For example, mice possess a very strong Bohr effect, with a ${\textstyle {\scriptstyle \Delta \log(P_{50}) \over \Delta {\text{pH}}}}$ value of -0.96, which requires relatively minor changes in H⁺ or CO₂ concentrations, while elephants require much larger changes in concentration to achieve a much weaker effect ${\textstyle \left({\scriptstyle \Delta \log(P_{50}) \over \Delta {\text{pH}}}=-0.38\right)}$.[9]

Haemoglobin changes conformation from a high-affinity R state (oxygenated) to a low-affinity T state (deoxygenated) to improve oxygen uptake and delivery.

The Bohr effect hinges around allosteric interactions between the hemes of the haemoglobin tetramer, a mechanism first proposed by Max Perutz in 1970.[10] Haemoglobin exists in two conformations: a high-affinity R state and a low-affinity T state. When oxygen concentration levels are high, as in the lungs, the R state is favored, enabling the maximum amount of oxygen to be bound to the hemes. In the capillaries, where oxygen concentration levels are lower, the T state is favored, in order to facilitate the delivery of oxygen to the tissues. The Bohr effect is dependent on this allostery, as increases in CO₂ and H⁺ help stabilize the T state and ensure greater oxygen delivery to muscles during periods of elevated cellular respiration. This is evidenced by the fact that myoglobin, a monomer with no allostery, does not exhibit the Bohr effect.[2] Haemoglobin mutants with weaker allostery may exhibit a reduced Bohr effect. For example, in Hiroshima variant haemoglobinopathy, allostery in haemoglobin is reduced, and the Bohr effect is diminished. As a result, during periods of exercise, the mutant haemoglobin has a higher affinity for oxygen and tissue may suffer minor oxygen starvation.[11]

When hemoglobin is in its T state, the N-terminal amino groups of the α-subunits and the C-terminal histidine of the β-subunits are protonated, giving them a positive charge and allowing these residues to participate in ionic interactions with carboxyl groups on nearby residues. These interactions help hold the haemoglobin in the T state. Decreases in pH (increases in acidity) stabilize this state even more, since a decrease in pH makes these residues even more likely to be protonated, strengthening the ionic interactions. In the R state, the ionic pairings are absent, meaning that the R state's stability increases when the pH increases, as these residues are less likely to stay protonated in a more basic environment. The Bohr effect works by simultaneously destabilizing the high-affinity R state and stabilizing the low-affinity T state, which leads to an overall decrease in oxygen affinity.[2] This can be visualized on an oxygen-haemoglobin dissociation curve by shifting the whole curve to the right.

Carbon dioxide can also react directly with the N-terminal amino groups to form carbamates, according to the following reaction:

${\displaystyle {\ce {R-NH2 + CO2 <=> R-NH-COO^- + H+}}}$

CO₂ forms carbamates more frequently with the T state, which helps to stabilize this conformation. The process also creates protons, meaning that the formation of carbamates also contributes to the strengthening of ionic interactions, further stabilizing the T state.[2]

An exception to the otherwise well-supported link between animal body size and the sensitivity of its haemoglobin to changes in pH was discovered in 1961.[12] Based on their size and weight, many marine mammals were hypothesized to have a very low, almost negligible Bohr effect.[9] However, when their blood was examined, this was not the case. Humpback whales weighing 41,000 kilograms had an observed ${\textstyle {\scriptstyle \Delta \log(P_{50}) \over \Delta {\text{pH}}}}$ value of 0.82, which is roughly equivalent to the Bohr effect magnitude in a 0.57 kg guinea pig.[9] This extremely strong Bohr effect is hypothesized to be one of marine mammals' many adaptations for deep, long dives, as it allows for virtually all of the bound oxygen on haemoglobin to dissociate and supply the whale's body while it is underwater.[12] Examination of other marine mammal species supports this. In pilot whales and porpoises, which are primarily surface feeders and seldom dive for more than a few minutes, the ${\textstyle {\scriptstyle \Delta \log(P_{50}) \over \Delta {\text{pH}}}}$was 0.52, comparable to a cow,[9] which is much closer to the expected Bohr effect magnitude for animals of their size.[12]

Another special case of the Bohr effect occurs when carbon monoxide is present. This molecule serves as a competitive inhibitor for oxygen, and binds to haemoglobin to form carboxyhaemoglobin.[13] Haemoglobin's affinity for CO is about 210 times stronger than its affinity for O₂,[14] meaning that it is very unlikely to dissociate, and once bound, it blocks the binding of O₂ to that subunit. At the same time, CO is structurally similar enough to O₂ to cause carboxyhemoglobin to favor the R state, raising the oxygen affinity of the remaining unoccupied subunits. This combination significantly reduces the delivery of oxygen to the tissues of the body, which is what makes carbon monoxide so toxic. This toxicity is reduced slightly by an increase in the strength of the Bohr effect in the presence of carboxyhemoglobin. This increase is ultimately due to differences in interactions between heme groups in carboxyhemoglobin relative to oxygenated hemoglobin. It is most pronounced when the oxygen concentration is extremely low, as a last-ditch effort when the need for oxygen delivery becomes critical. However, the physiological implications of this phenomenon remain unclear.[13]