Hemoglobin Structure and Oxygen Transport

Introduction

Oxygen is poorly soluble in water: plasma alone could carry only about 3 mL of O₂ per litre of blood at normal lung pressures. Haemoglobin raises this to around 200 mL per litre and, just as importantly, releases a large share of that oxygen in the tissues. A single myoglobin-like heme could load oxygen well but would release very little. Haemoglobin solves this by assembling four heme subunits that communicate with each other, producing a binding curve suited to picking up oxygen in the lungs and delivering it where it is needed.

Core explanation

Structure. Adult haemoglobin (HbA) is a tetramer of two α chains (141 residues) and two β chains (146 residues), molar mass about 64.5 kDa. Each chain has the globin fold seen in myoglobin, with eight helices wrapped around one heme and a proximal histidine (His F8) bound to iron. The four chains pack as two αβ dimers, α₁β₁ and α₂β₂. The interfaces between the dimers, especially α₁β₂, change significantly during oxygen binding.

Capacity. A red blood cell contains about 280 million haemoglobin molecules, and blood contains around 150 g of haemoglobin per litre. Each gram of fully saturated haemoglobin carries about 1.34 mL of O₂, so arterial blood holds roughly 200 mL O₂ per litre, almost all bound to haemoglobin.

The binding curve. Unlike myoglobin's hyperbola, haemoglobin shows a sigmoidal (S-shaped) saturation curve with P₅₀ of about 3.5 kPa (26 mmHg) under physiological conditions. In the lungs, where pO₂ is about 13 kPa (100 mmHg), haemoglobin is about 97-98% saturated. In resting tissues, where venous pO₂ is about 5.3 kPa (40 mmHg), saturation falls to about 75%. In working muscle, where pO₂ may fall to around 2.7 kPa (20 mmHg), saturation drops to around 30%. The steep middle portion of the curve lies precisely in the range of tissue oxygen pressures, so small falls in pO₂ release large amounts of oxygen.

Comparison with myoglobin. Myoglobin, with P₅₀ of about 0.3 kPa, would be almost fully saturated at all these pressures and would unload very little. Haemoglobin therefore delivers oxygen to myoglobin in muscle, which in turn passes it to mitochondria — a sequence of increasing affinity.

Regulation. The position of haemoglobin's curve is not fixed. Protons, carbon dioxide and 2,3-bisphosphoglycerate (2,3-BPG) bind to the protein away from the heme and lower its O₂ affinity, shifting the curve to the right and enhancing release. These allosteric effects are developed on later pages.

Fetal haemoglobin. Before birth, the fetus uses HbF (α₂γ₂). The γ chain binds 2,3-BPG more weakly, so HbF has higher O₂ affinity than maternal HbA, allowing oxygen to pass across the placenta from mother to fetus.

Other ligands and variants. Carbon monoxide, nitric oxide and oxidising agents all interact with the heme sites. Mutations such as the β-chain Glu6→Val substitution in sickle-cell haemoglobin show that single changes on the protein surface can alter behaviour dramatically, by causing deoxy-Hb molecules to polymerise.

Formulae

O₂ content of blood ≈ 1.34 × [Hb] × SaO₂ + 0.003 × pO₂(mmHg), in mL O₂ per 100 mL of blood, with [Hb] in g per 100 mL. O₂ delivered = (Y lungs − Y tissue) × total O₂-binding capacity.

Step-by-step reasoning

1. Read the saturation Y at the pulmonary pO₂ from the binding curve. 2. Read Y at the tissue pO₂. 3. Subtract to find the fraction of sites that unload. 4. Multiply by the total O₂ capacity of the blood to find the volume delivered. 5. Repeat with myoglobin's curve to show the contrast.

Visual explanation

On one set of axes, plot myoglobin's hyperbola rising steeply near the origin and haemoglobin's sigmoid crossing Y = 0.5 at about 3.5 kPa. Draw vertical lines at 13 kPa (lungs) and 5 kPa (tissue). Mark the drop in saturation between these lines for each protein: large for haemoglobin, small for myoglobin.

Real-world analogy

Haemoglobin works like a delivery lorry that loads fully at the warehouse and unloads generously at shops, while myoglobin is a shop's storeroom that accepts stock easily but rarely gives it back. A lorry that clung to its cargo, however well loaded, would be useless for delivery. Unlike a lorry, haemoglobin's "unloading" is an equilibrium response to local oxygen pressure.

Real-world example

At high altitude, the pO₂ in the lungs falls. People who live at altitude for weeks increase their red blood cell count and 2,3-BPG levels. The extra 2,3-BPG shifts the curve right, sacrificing a little loading in the lungs in exchange for better unloading in tissues. The simulation for this unit lets you compare adult haemoglobin, fetal haemoglobin and myoglobin under such conditions.

Why?

Why does haemoglobin need four subunits rather than one? A single-site protein has a hyperbolic curve, which cannot be both nearly full at 13 kPa and substantially empty at 5 kPa. Communication between four sites produces the steep sigmoid that combines high loading with efficient unloading.

Common misconception

"Haemoglobin releases all its oxygen in the tissues." At rest, venous blood still carries about 75% saturated haemoglobin; only about a quarter of the load is delivered. This reserve allows rapid extra delivery during exercise. Another error is thinking O₂ is carried mainly dissolved in plasma; about 98% is bound to haemoglobin.

Worked example

Question: Blood contains 150 g Hb per litre. Saturation falls from 0.97 in the lungs to 0.75 in resting tissue. How much O₂ is delivered per litre of blood?

Reasoning: Capacity = 150 × 1.34 = 201 mL O₂ per litre. Fraction released = 0.97 − 0.75 = 0.22. O₂ delivered = 0.22 × 201 ≈ 44 mL.

Answer: About 44 mL of O₂ per litre of blood.

Quick check

1. Why does fetal haemoglobin have a higher O₂ affinity than adult haemoglobin? Answer: Its γ chains bind 2,3-BPG less strongly, so the allosteric lowering of affinity is weaker.

Exam focus

Describe HbA as an α₂β₂ tetramer with four hemes. Compare the sigmoidal Hb curve with the hyperbolic Mb curve and use saturation differences to calculate O₂ delivery. Explain the function of fetal haemoglobin and name factors that shift the curve.

Advanced insight

Haemoglobin also transports nitric oxide-related species, and reactions of NO with oxy-Hb (forming nitrate and met-Hb) and deoxy-Hb (forming nitrosyl-Hb) influence blood vessel tone. Some researchers propose that S-nitrosylation of a β-chain cysteine allows Hb to deliver NO bioactivity where oxygen is low, although the physiological importance remains debated.

Summary

Adult haemoglobin is an α₂β₂ tetramer whose four globin chains each hold a heme with a proximal histidine. It binds O₂ with a sigmoidal curve, P₅₀ ≈ 3.5 kPa, loading almost fully in the lungs and releasing a large share at tissue pressures. Allosteric effectors adjust its affinity, and fetal haemoglobin's higher affinity ensures oxygen transfer across the placenta.

Practice questions

1. How many O₂ molecules can one haemoglobin tetramer carry, and why? Answer: Four, because each of the four subunits holds one heme iron that binds one O₂. 2. Calculate the O₂ capacity of blood containing 140 g Hb per litre. Answer: 140 × 1.34 ≈ 188 mL O₂ per litre. 3. Explain why myoglobin would be a poor oxygen carrier in blood. Answer: Its very low P₅₀ means it stays nearly saturated at tissue pO₂, so it would release very little oxygen. 4. During exercise, tissue pO₂ falls from 5.3 kPa to 2.7 kPa. How does haemoglobin's saturation change, and what is the effect? Answer: It falls from about 75% to about 30%, so much more oxygen is released where it is needed.