This essay is an adjunct to GeneGuessr, and uses the principle "learning more than you need to makes learning easier". Oxygen transport provides a context for understanding of conformational changes in proteins.

Oxygen Transport

The Problem of Solubility

Oxygen dissolves poorly in water. At body temperature, blood plasma carries only about 0.3 mL of dissolved O₂ per 100 mL - nowhere near enough to sustain a metabolically active organism. Without a carrier molecule, diffusion alone would limit animal body plans to millimeter scales. Insects manage with tracheal tubes that deliver air directly to tissues. Anything larger needs a different solution.

Hemoglobin is that solution. By binding oxygen reversibly, hemoglobin increases the oxygen-carrying capacity of blood roughly 70-fold. But binding alone is not enough. A carrier that grabs oxygen and never lets go is useless. The challenge is tunable affinity : high affinity in the lungs where oxygen is abundant, lower affinity in the tissues where oxygen must be released. This is the story of allostery.

Carrying Oxygen from the lungs to the organs and muscles is one half of the hemoglobin solution. The fuller picture involves how it hands off to other molecules, and how CO₂ is carried back from tissues to the lungs.

Myoglobin and Hemoglobin: Storage vs Transport

Myoglobin and hemoglobin both bind oxygen using heme, yet their binding curves reveal fundamentally different designs. Hemoglobin hands transported oxygen over to myoglobin.

Myoglobin produces a hyperbolic binding curve. Affinity is high and constant - each myoglobin molecule binds oxygen independently, with no communication between molecules. This makes myoglobin a storage protein. It sits in muscle tissue, grabs oxygen released by hemoglobin, and holds it until cellular pO₂ drops low enough to force release. The high affinity ensures myoglobin remains oxygen saturated across a wide range of oxygen tensions.

Hemoglobin produces a sigmoidal curve. This S-shape is the signature of cooperativity: binding of the first oxygen molecule increases the affinity for subsequent molecules. The curve is shallow at low pO₂, steep in the middle range, and flattens again at high pO₂. This creates a system exquisitely tuned for transport. In the lungs, where pO₂ is high, hemoglobin loads fully. In the tissues, where pO₂ is lower, the steep part of the curve means small drops in oxygen tension produce large amounts of oxygen release.

The affinity gap between the two proteins drives oxygen transfer. Hemoglobin arrives in the capillaries and begins releasing oxygen as tissue pO₂ falls. Myoglobin, with its higher affinity, accepts this oxygen and buffers it within the muscle cell. One protein hands off to the other.

The T-R Transition

Cooperativity arises from hemoglobin's quaternary structure. The protein exists in two conformational states: T (tense) and R (relaxed).

In the T-state, the four subunits are locked together by salt bridges and hydrogen bonds that constrain the heme pockets. Iron sits slightly out of the porphyrin plane. Oxygen affinity is low. This is the predominant form of deoxyhemoglobin.

When oxygen binds to one subunit, iron moves into the porphyrin plane, pulling the proximal histidine with it. This small displacement - a fraction of an angstrom - propagates through the subunit interface, destabilizing the T-state contacts. As more oxygen binds, the equilibrium shifts toward the R-state, where constraints are relaxed, heme pockets are more accessible, and affinity is higher.

The transition is concerted: intermediate states are unstable, so the molecule tends to flip between fully T and fully R. This all-or-nothing character amplifies the cooperative effect. The first oxygen to bind pays the energetic cost of loosening the T-state; subsequent oxygens bind to an already-primed molecule.

The Bohr Effect

Active tissues need more oxygen. They also produce more carbon dioxide and, through carbonic anhydrase, more protons. The Bohr effect links these facts: lower pH reduces hemoglobin's oxygen affinity, right-shifting the binding curve.

The molecular mechanism involves protonation of specific residues, particularly His146 on the β-chains. When protonated, this histidine forms a salt bridge with Asp94, stabilizing the T-state. Lower pH means more protonation, more T-state stabilization, more oxygen release.

The effect is reciprocal. Oxygen binding promotes proton release, because the T-to-R transition breaks the salt bridges that depend on protonated histidines. Hemoglobin thereby acts as a buffer, absorbing protons in the tissues (where it releases oxygen) and releasing them in the lungs (where it binds oxygen). The protein does not merely respond to pH; it helps regulate it.

This is physiological elegance: the very byproducts of metabolism - CO₂ and H⁺ - signal hemoglobin to release more oxygen precisely where metabolic demand is highest.

Carbon Dioxide Transport

Hemoglobin carries carbon dioxide as well as oxygen, though by different mechanisms.

Most CO₂ transport (roughly 70%) occurs as bicarbonate. Inside red blood cells, carbonic anhydrase catalyzes the reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. The bicarbonate exits the cell via an anion exchanger, with chloride entering to maintain electroneutrality - the chloride shift. In the lungs, the process reverses: bicarbonate enters, carbonic anhydrase regenerates CO₂, and the gas diffuses into alveoli for exhalation.

A smaller fraction of CO₂ (roughly 23%) binds directly to hemoglobin, forming carbamino compounds. CO₂ reacts with the uncharged N-terminal amino groups of the globin chains, not with the heme iron. Deoxyhemoglobin binds CO₂ more readily than oxyhemoglobin - the Haldane effect, a mirror of the Bohr effect. In the tissues, as oxygen leaves, CO₂ binding increases. In the lungs, as oxygen binds, CO₂ is displaced.

The remaining CO₂ (roughly 7%) simply dissolves in plasma.

Hemoglobin thus functions as a bidirectional gas exchanger. It loads oxygen and releases CO₂ in the lungs; it releases oxygen and loads CO₂ in the tissues. The allosteric linkage between the two processes ensures they occur together.

Carbon Monoxide Poisoning

Carbon monoxide binds to the same heme site as oxygen, but with approximately 200-fold higher affinity. This alone would be problematic - CO progressively displaces O₂ from hemoglobin, reducing oxygen-carrying capacity. But the situation is worse than simple competition.

CO binding locks hemoglobin in the R-state. The remaining hemes that still carry oxygen have increased affinity: they bind oxygen tightly and refuse to release it in the tissues. A person with 50% carboxyhemoglobin is not equivalent to someone with half the normal hemoglobin. The functional impairment is greater because the oxygen that is carried cannot be delivered.

The clinical presentation reflects this. Tissue hypoxia is severe. The blood remains bright red - carboxyhemoglobin has a cherry-red color - even as cells starve for oxygen. Treatment requires displacing CO, which means high-flow oxygen and, in severe cases, hyperbaric oxygen to accelerate the off-rate.

Sickle Cell Disease

A single nucleotide change in the β-globin gene replaces glutamate at position 6 with valine. The consequences are profound.

Glutamate is charged and hydrophilic; valine is hydrophobic. In oxyhemoglobin, this substitution has little effect - the site is buried or solvent-exposed in ways that accommodate the change. But in deoxyhemoglobin, the valine creates a hydrophobic patch on the molecular surface that fits into a complementary pocket on an adjacent hemoglobin molecule.

This intermolecular contact nucleates polymerization. DeoxyHbS molecules stack into long fibers that distort the red cell into a rigid, sickle shape. Sickled cells cannot deform to pass through capillaries. They obstruct flow, causing the vaso-occlusive crises that define the disease. They also hemolyze easily, producing chronic anemia.

The mutation persists at high frequency in malaria-endemic regions because heterozygotes have a survival advantage. Sickled cells are cleared more rapidly by the spleen, interrupting the Plasmodium lifecycle before the parasite can complete its development. One copy of HbS confers resistance; two copies cause disease.

Fetal Hemoglobin

In muscles myoglobin is essential to taking up oxygen from hemoglobin, and it has a higher oxygen affinity. Oxygen transport to a fetus has a similar challenge. Oxygen must cross the placenta from maternal to fetal blood. This transfer requires a thermodynamic gradient - fetal hemoglobin must have higher oxygen affinity than maternal hemoglobin, so it can "steal" oxygen even when both are exposed to the same pO₂.

Fetal hemoglobin (HbF) achieves this by swapping the β-chains for γ-chains. The resulting α₂γ₂ tetramer has a subtle but critical difference: γ-chains bind 2,3-bisphosphoglycerate (2,3-BPG) more weakly than β-chains do.

2,3-BPG is a small molecule that occupies the central cavity of deoxyhemoglobin, stabilizing the T-state and reducing oxygen affinity. With weaker 2,3-BPG binding, HbF spends more time in the R-state. Its oxygen affinity is higher. In the placenta, maternal HbA releases oxygen (lower affinity), and fetal HbF captures it (higher affinity). The affinity difference drives net transfer.

After birth, γ-chain synthesis declines and β-chain synthesis rises. By six months, HbA predominates. But the γ-globin genes remain in the genome, silenced but intact. Reactivating HbF expression is now a therapeutic strategy for sickle cell disease: HbF does not polymerize with HbS, so increasing its proportion reduces sickling.

2,3-Bisphosphoglycerate

2,3-BPG deserves closer attention. This glycolytic intermediate binds in the central cavity of the hemoglobin tetramer, a space lined with positively charged residues that complement BPG's negatively charged phosphate groups. Only deoxyhemoglobin has the cavity geometry that accommodates BPG; when hemoglobin transitions to the R-state, the cavity narrows and BPG is expelled.

By stabilizing the T-state, 2,3-BPG reduces oxygen affinity. This allows physiological tuning independent of pH. Red blood cells can adjust 2,3-BPG levels over hours to days in response to chronic hypoxia. At high altitude, 2,3-BPG concentrations rise, right-shifting the oxygen binding curve and improving oxygen delivery to tissues despite lower arterial pO₂.

Stored blood presents the inverse problem. Red blood cells in blood bank conditions gradually lose 2,3-BPG. Freshly transfused cells have abnormally high oxygen affinity; they bind oxygen in the lungs but release it poorly in the tissues. Function recovers over 24-48 hours as the cells regenerate 2,3-BPG, but this delay can matter in critical illness.

Diving Mammals

Whales and seals face an extreme oxygen transport problem: they must sustain aerobic metabolism during dives lasting tens of minutes without breathing. Their adaptations reveal how far the hemoglobin-myoglobin system can be pushed.

Myoglobin concentration in diving mammal muscle is extraordinarily high, roughly ten times that of terrestrial mammals. The muscle appears almost black from the accumulated heme. This myoglobin reservoir holds oxygen that sustains aerobic metabolism as arterial pO₂ falls during the dive.

High myoglobin concentration creates its own problem: at such densities, the protein would normally aggregate. Diving mammals have evolved modified myoglobin with increased surface positive charge. Electrostatic repulsion keeps the molecules dispersed and functional.

The spleen contributes another reservoir. Diving mammals have enlarged spleens packed with red blood cells. During a dive, splenic contraction releases this reserve into circulation, transiently increasing oxygen-carrying capacity. Between dives, the spleen resequests red cells, ready for the next descent.

Myoglobin's high affinity is essential here. It holds oxygen until tissue pO₂ falls very low, extracting every available molecule before anaerobic metabolism must take over. The storage protein becomes a critical buffer against hypoxia.

High-Altitude Adaptation

Humans at high altitude face chronic hypoxia. The acute response is hyperventilation, which improves alveolar pO₂ but induces respiratory alkalosis (elevated blood pH from CO₂ loss). The kidneys compensate over days by excreting bicarbonate.

Longer-term acclimatization involves multiple mechanisms. 2,3-BPG increases, right-shifting the curve to improve tissue oxygen delivery. Erythropoietin (EPO) secretion rises, stimulating red blood cell production. Hemoglobin concentration climbs. Capillary density increases in peripheral tissues.

Populations native to high altitude show genetic adaptations beyond acclimatization. Tibetans carry variants in EPAS1, the gene encoding hypoxia-inducible factor 2α, that blunt the erythropoietic response - paradoxically, they maintain lower hemoglobin than acclimatized lowlanders, avoiding the blood viscosity problems of excessive polycythemia. Andean populations show different adaptations, including hemoglobin variants with altered oxygen affinity.

These population differences illustrate multiple molecular solutions to the same physiological problem, arrived at independently in different mountain ranges over thousands of years.