Transport of oxygen and carbon dioxide

Haemoglobin (Hb) is the red, oxygen-carrying protein of red blood cells. It is a quaternary protein: four polypeptide subunits are held together by hydrogen bonds, ionic interactions and hydrophobic forces.

In adult haemoglobin (HbA):

• Two α-globin chains (each ~141 amino acids).
• Two β-globin chains (each ~146 amino acids).

Each subunit folds into a globular tertiary structure (the 'globin fold', containing several α-helices) and cradles a prosthetic group called haem.

A haem group is a porphyrin ring (an organic molecule of four pyrrole rings) containing a central iron(II) ion (Fe²⁺). The Fe²⁺ can reversibly bind one O₂ molecule:

$\text{Hb} + 4\text{O}_2 \rightleftharpoons \text{Hb(O}_2)_4 \quad (\text{oxyhaemoglobin})$

So each Hb molecule has 4 haem groups, 4 Fe²⁺ ions and can carry up to 4 O₂. A single RBC contains about 250 million Hb molecules, so each RBC can carry roughly 10⁹ O₂.

When O₂ leaves Hb at the tissues, the protein returns to its 'deoxy' conformation. The Fe²⁺ remains in the +2 oxidation state throughout — if it were oxidised to Fe³⁺ (forming methaemoglobin), it could no longer carry O₂.

Plotting percentage saturation of haemoglobin against partial pressure of oxygen ($p\text{O}_2$) gives the oxygen dissociation curve.

For adult haemoglobin, the curve is sigmoid (S-shaped). This shape is the direct consequence of cooperative binding:

1. At low $p\text{O}_2$ (start of curve, shallow): the first O₂ molecule binds with difficulty because the binding site is somewhat buried within the Hb molecule. Saturation rises slowly.
2. Conformational change: binding the first O₂ pulls the Fe²⁺ slightly into the plane of the haem ring, which propagates a change of shape through the whole quaternary protein. The other three haem groups are now more exposed and accessible.
3. At intermediate $p\text{O}_2$ (steep middle): the second, third and fourth O₂ molecules bind progressively more easily because the affinity has increased. Saturation rises rapidly.
4. At high $p\text{O}_2$ (plateau, top): most sites are already filled, so saturation can rise only slowly towards 100%. The curve plateaus at about 98% saturation at the alveolar $p\text{O}_2$ (~13 kPa).

Loading at the lungs. The plateau at high $p\text{O}_2$ means Hb leaves the lungs essentially fully saturated, even if alveolar $p\text{O}_2$ drops a little (e.g. at altitude).

Unloading at tissues. The steep middle section overlaps the $p\text{O}_2$ range found in respiring tissues (~2-5 kPa). A small fall in tissue $p\text{O}_2$ causes a large drop in saturation — i.e. a lot of O₂ is released to the tissue cells.

The sigmoid shape is therefore the chemical basis for haemoglobin's dual function: pick up nearly all available O₂ in the lungs, and release a large fraction of it at respiring tissues.

The Bohr shift (or Bohr effect) is the rightward shift of the oxygen dissociation curve that occurs when $p\text{CO}_2$ rises or pH falls.

Mechanism. Carbon dioxide produced by respiring tissues enters red blood cells. There, the enzyme carbonic anhydrase catalyses:

$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$

The $\text{H}^+$ ions produced bind to amino acid side chains on haemoglobin (notably histidine residues). CO₂ also binds directly to some terminal amino groups of Hb to form carbaminohaemoglobin.

Both bindings cause a conformational change in Hb that stabilises the deoxy form and lowers its affinity for O₂. At any given $p\text{O}_2$, percentage saturation falls — graphically, the curve shifts to the right.

Why it matters. Tissues with the highest respiration rates (e.g. exercising skeletal muscle, contracting cardiac muscle) generate the most CO₂. The local Bohr shift means Hb passing through those tissues releases even more O₂ than the basic dissociation curve would predict — the system automatically delivers more O₂ to the cells that need it most.

At the lungs. The opposite occurs: CO₂ is exhaled, blood pH rises, the curve shifts left, Hb's affinity for O₂ increases, and loading is maximised. The same molecule does both jobs efficiently because its affinity is responsive to local conditions.

During exercise. The Bohr shift, together with increased blood flow (vasodilation), increased ventilation rate and the release of myoglobin-stored O₂, allows muscles to sustain aerobic respiration during high O₂ demand.

Cambridge expects you to compare adult haemoglobin (HbA), fetal haemoglobin (HbF) and myoglobin on a single dissociation-curve diagram.

Fetal haemoglobin (HbF). Composition: 2α + 2γ chains (instead of 2β in adults). The γ-subunits bind 2,3-BPG much less strongly than β-subunits, so 2,3-BPG cannot lower HbF's affinity as much as it lowers HbA's. Consequently:

• HbF curve lies to the left of HbA — higher O₂ affinity at any given $p\text{O}_2$.
• In the placental capillaries, fetal blood (carrying HbF) flows next to maternal blood (carrying HbA). At the prevailing $p\text{O}_2$, HbF binds more O₂ than HbA, so O₂ diffuses down its concentration gradient from maternal Hb to fetal Hb.
• This ensures the fetus is adequately oxygenated despite the relatively low placental $p\text{O}_2$.

After birth, the γ-globin gene is switched off and β-globin is expressed. By ~6 months HbA predominates, and the dissociation curve shifts right to suit air-breathing.

Myoglobin. A monomer — a single polypeptide chain with one haem group. Found in skeletal and cardiac muscle (the red colour of meat).

• Dissociation curve is hyperbolic (no cooperative binding because there is only one binding site).
• Curve lies far to the left of HbA — myoglobin has a much higher affinity for O₂.
• Function: acts as an O₂ store in muscle. At normal $p\text{O}_2$ in resting muscle, myoglobin is fully saturated and holds onto its O₂ tightly. Only when $p\text{O}_2$ falls to very low values — for example during intense exercise — does myoglobin release its O₂.
• This delays the switch to anaerobic respiration and the build-up of lactic acid.

Diving mammals (seals, whales) have very high myoglobin concentrations in their muscles, storing huge reserves of O₂ for the duration of a dive.

Carbon dioxide is much more soluble in water than oxygen, but the metabolic load is large — adults exhale around 300 ml of CO₂ per minute at rest. The blood transports CO₂ in three forms:

1. Dissolved in plasma (~5%). A small amount of CO₂ simply dissolves in the plasma and is carried in solution. This fraction maintains the $p\text{CO}_2$ that drives diffusion.

2. Carbaminohaemoglobin (~10-20%). Some CO₂ binds reversibly to the terminal amino groups of haemoglobin's globin chains:

$\text{Hb-NH}_2 + \text{CO}_2 \rightleftharpoons \text{Hb-NHCOOH (carbaminohaemoglobin)}$

Note that CO₂ binds to the protein part of Hb, not the haem — so it does not compete directly with O₂ binding. (Carbon monoxide DOES compete with O₂ for the haem itself.)

3. Hydrogencarbonate ions in plasma (~70-85%). Most CO₂ is transported as HCO₃⁻ in the plasma. This works as follows:

(a) CO₂ produced in tissues diffuses into RBCs.

(b) Inside the RBC, carbonic anhydrase catalyses:

$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$

(c) The HCO₃⁻ then diffuses out of the RBC into the plasma. To preserve electrical neutrality, Cl⁻ ions move from plasma into the RBC — the chloride shift.

(d) The H⁺ would lower the pH of the RBC dangerously, but it is buffered by haemoglobin (forming HHb). H⁺ binding to Hb is exactly what causes the Bohr shift — beautifully integrated chemistry: making the HCO₃⁻ to carry CO₂ also helps unload O₂ to the same tissue.

At the lungs the reactions reverse. CO₂ diffuses out of the plasma into the alveolar air; HCO₃⁻ in the RBC recombines with H⁺ to form H₂CO₃, which carbonic anhydrase dehydrates back to CO₂ + H₂O. CO₂ leaves the RBC and is exhaled. Cl⁻ moves back out into the plasma (reverse chloride shift) and HCO₃⁻ moves in to be converted.

Carbon monoxide (CO) is a colourless, odourless gas produced by incomplete combustion (faulty heaters, car exhausts, cigarette smoke). It is acutely toxic because of how it interacts with haemoglobin.

CO binds to the same Fe²⁺ on the haem group that normally binds O₂, but it binds with an affinity about 250 times greater than O₂'s and effectively irreversibly under physiological conditions. The product is called carboxyhaemoglobin (COHb).

Consequences of CO binding:

1. Reduced O₂ carrying capacity — fewer haem sites are available for O₂.
2. Left shift of the dissociation curve for the remaining sites — bound CO induces a conformational change that increases the affinity of the other haem groups for O₂, so even less O₂ is released at the tissues.

A small percentage of COHb is dangerous. Cigarette smokers commonly carry 5-10% COHb in their blood — chronically reducing the effective O₂ carrying capacity and contributing to cardiovascular disease.

Treatment for severe CO poisoning is 100% O₂ (or hyperbaric O₂), which slowly displaces CO from haem and accelerates its excretion via the lungs.