Haemoglobin: How Blood Carries Oxygen

Oxygen is not very soluble in water. If your blood had to carry oxygen simply dissolved in plasma, it could carry only about 3 mL of O₂ per litre, roughly one-seventieth of what your body needs. Haemoglobin solves this problem. This remarkable protein packs red blood cells so tightly that each cell contains about 270 million haemoglobin molecules, and each molecule can carry four oxygen molecules. More impressively, it grabs oxygen where it’s plentiful and lets it go where it’s needed, adjusting automatically to exercise, altitude and the chemistry of the tissues.

Structure

Haemoglobin (Hb) is a globular protein with quaternary structure (see protein structure levels):

• Four polypeptide chains: in adults, two α chains (141 amino acids each) and two β chains (146 amino acids each).
• Each chain holds one haem group, so each haemoglobin has four haem groups and can carry four O₂ molecules.
• The whole molecule has a molar mass of about 64,500 g mol⁻¹.

Each chain is folded mostly into α-helices, forming a pocket for its haem, much like the single chain of myoglobin, the related oxygen-storage protein in muscle.

The haem group and its iron

Haem is a flat porphyrin ring with an iron(II) ion (Fe²⁺) at its centre (see iron in the body):

• the iron is held by four nitrogen atoms of the porphyrin;
• a fifth bond links it to a histidine side chain of the protein (the “proximal histidine”);
• the sixth position is where O₂ binds.

The porphyrin’s large conjugated ring gives haem its colour. Oxygenated haemoglobin is bright red; deoxygenated haemoglobin is darker, purplish red. (Veins aren’t actually blue; they only look blue through skin because of how light scatters.)

Only Fe²⁺ binds oxygen. The protein surrounding the haem stops oxygen from oxidising the iron to Fe³⁺, which would form methaemoglobin, unable to carry oxygen.

Cooperative binding: the S-shaped curve

Plot the percentage saturation of haemoglobin against the partial pressure of oxygen, and you get an S-shaped (sigmoidal) curve, the oxygen dissociation curve. Myoglobin, with a single chain, gives a simple hyperbola instead.

The S shape comes from cooperative binding: the four haem groups influence each other.

• Deoxygenated haemoglobin is in a tense (T) state, with a relatively low affinity for oxygen. Binding the first O₂ is hard.
• When one haem binds O₂, the iron moves slightly into the plane of the ring, pulling the histidine and shifting the whole subunit. This change is passed to the other subunits, which switch towards a relaxed (R) state with a higher affinity.
• So each O₂ that binds makes the next one bind more easily.

Why cooperativity matters

• In the lungs, where oxygen partial pressure is high (about 13 kPa, or 100 mmHg), haemoglobin becomes almost fully saturated (about 97–98%).
• In active tissues, where oxygen partial pressure is low (perhaps 2–5 kPa), the steep part of the curve means a small drop in oxygen causes a large release. Haemoglobin can unload much of its oxygen exactly where it’s needed.

The partial pressure at which haemoglobin is half saturated, the P₅₀, is about 3.5 kPa (26–27 mmHg) for normal adult blood.

A simple protein that bound oxygen with a normal hyperbolic curve would face a dilemma: tight binding would load well but release poorly; weak binding would release well but load poorly. Cooperativity gives haemoglobin the best of both.

The Bohr effect: acid and CO₂ release oxygen

In 1904, Christian Bohr (father of the physicist Niels Bohr) found that haemoglobin’s affinity for oxygen falls when pH falls or CO₂ rises. This shifts the dissociation curve to the right, so more oxygen is released.

This is perfectly suited to the body’s needs. Actively respiring tissues produce CO₂ and acids (such as lactic acid), lowering the local pH (see the pH scale explained). There, haemoglobin gives up more oxygen. In the lungs, CO₂ is breathed out and pH rises slightly, so haemoglobin loads oxygen more easily.

Chemically, H⁺ ions and CO₂ bind to specific sites on haemoglobin, stabilising the T state. It’s a classic example of allosteric regulation, and of Le Chatelier’s principle in a living system.

Other factors that shift the curve right:

• higher temperature (working muscles are warmer);
• 2,3-bisphosphoglycerate (2,3-BPG), a small molecule in red cells that binds in the central cavity of deoxyhaemoglobin. Its level rises at high altitude and in anaemia, helping release more oxygen to tissues.

Carrying carbon dioxide back

Haemoglobin also helps carry CO₂ from tissues to the lungs:

• About 70% of CO₂ travels as hydrogencarbonate ions (HCO₃⁻) in plasma. In red cells, the zinc enzyme carbonic anhydrase rapidly converts CO₂ and water into carbonic acid, which splits into H⁺ and HCO₃⁻ (see the blood buffer system). Haemoglobin mops up the H⁺, acting as a buffer.
• About 20–25% binds directly to amino groups on haemoglobin as carbaminohaemoglobin.
• The rest travels dissolved in plasma.

Fetal haemoglobin

A baby in the womb must take oxygen from its mother’s blood. Fetal haemoglobin (HbF) has γ chains instead of β chains (α₂γ₂), which bind 2,3-BPG less strongly. As a result, HbF has a higher affinity for oxygen: its curve lies to the left of adult haemoglobin’s. In the placenta, oxygen is transferred from the mother’s haemoglobin to the baby’s. After birth, HbF is gradually replaced by adult haemoglobin over the first months of life.

Carbon monoxide

Carbon monoxide binds to the same site on the haem iron as oxygen, but about 200–250 times more strongly. It blocks oxygen transport and also makes the remaining sites hold their oxygen more tightly, making things even worse. See carbon monoxide poisoning: the chemistry.

Sickle cell disease: one amino acid

In sickle cell haemoglobin (HbS), the sixth amino acid of each β chain is valine instead of glutamic acid. Glutamic acid has a charged, water-loving side chain; valine’s is non-polar. When HbS is deoxygenated, this new sticky patch fits into a pocket on a neighbouring haemoglobin molecule, and the molecules link into long fibres. These distort red blood cells into rigid sickle shapes that block small blood vessels and break down easily.

It was the first disease traced to a specific change in a protein’s structure (by Linus Pauling and colleagues in 1949), and a powerful demonstration that a single change in primary structure can alter function (see proteins: chains of amino acids). Carrying one copy of the sickle gene gives some protection against severe malaria, which explains why the gene is common in regions where malaria is widespread.

Some numbers

• Normal blood haemoglobin: about 130–170 g L⁻¹ in adult men and 120–150 g L⁻¹ in women.
• Each gram of haemoglobin can carry about 1.34 mL of O₂ when fully saturated.
• So one litre of blood carries about 200 mL of O₂ bound to haemoglobin, compared with about 3 mL dissolved.

Key takeaways

• Haemoglobin has four chains, each with a haem group containing Fe²⁺, so it carries up to four O₂.
• Cooperative binding gives an S-shaped dissociation curve, allowing efficient loading in the lungs and unloading in tissues.
• The Bohr effect (lower pH, more CO₂), higher temperature and 2,3-BPG shift the curve right, releasing more oxygen.
• Fetal haemoglobin has a higher oxygen affinity; CO binds 200–250 times more strongly than O₂.
• A single amino acid change causes sickle cell disease. For the iron chemistry behind it all, see iron in the body.