The Bohr Effect and Oxygen Release

Introduction

Active tissues produce acid and carbon dioxide. It would be ideal if haemoglobin released more oxygen in exactly those regions. That is what the Bohr effect achieves: falling pH and rising CO₂ lower haemoglobin's affinity for oxygen, so more O₂ is released where metabolism is fastest. In the lungs, the reverse happens. This page explains the chemistry of these allosteric effects and how they fit together with 2,3-bisphosphoglycerate and temperature.

Core explanation

Protons stabilise the T state. Some groups in haemoglobin have higher pKa values in the T state than in the R state. The most important is His146 at the C-terminus of each β chain. In deoxy-Hb, its protonated imidazolium forms a salt bridge with Asp94 on the same chain, and the nearby environment raises its pKa. Protonation therefore favours T. The α-chain N-terminal amino groups also contribute. Conversely, when O₂ binds and the protein switches to R, these groups release protons. Overall, about 0.5 H⁺ is released per O₂ bound near physiological pH.

Consequences. Because deoxy-Hb binds H⁺ more strongly than oxy-Hb, the equilibrium Hb(O₂)₄ + nH⁺ ⇌ Hb·Hₙ + 4O₂ shifts right as acid increases. Lower pH therefore lowers O₂ affinity and shifts the dissociation curve to the right. Quantitatively, P₅₀ rises by roughly 0.2 kPa for a fall of 0.1 pH unit in the physiological range.

Carbon dioxide. CO₂ acts in two ways. First, carbonic anhydrase in red blood cells converts CO₂ and water to H⁺ and HCO₃⁻, lowering pH and producing a Bohr shift. Second, CO₂ reacts directly with uncharged N-terminal amino groups of haemoglobin to form carbamates, R−NH₂ + CO₂ ⇌ R−NH−COO⁻ + H⁺. The negatively charged carbamates form extra salt bridges that stabilise T, and the reaction also releases a proton. Deoxy-Hb forms carbamates more readily than oxy-Hb.

The Haldane effect. The coupling works in both directions. Deoxygenated haemoglobin binds more H⁺ and CO₂, so blood takes up CO₂ more easily in tissues where O₂ is released. In the lungs, O₂ binding drives off H⁺ and carbamate CO₂, helping to release CO₂ for exhalation.

2,3-BPG. This highly negatively charged molecule binds in the central cavity between the two β chains of deoxy-Hb, interacting with positively charged residues such as β-His2, Lys82 and His143. The cavity narrows in the R state, expelling 2,3-BPG. By stabilising T, it raises P₅₀ from about 1.5 kPa for stripped haemoglobin to about 3.5 kPa in red cells. Fetal γ chains replace His143 with serine, weakening BPG binding.

Temperature. O₂ binding is exothermic, so higher temperature in working muscle lowers affinity and shifts the curve right, reinforcing the Bohr effect.

Step-by-step reasoning

1. Identify the change in conditions: more H⁺, more CO₂, more 2,3-BPG or higher temperature. 2. Decide which quaternary state the effector favours; all four favour T. 3. Conclude that O₂ affinity falls and P₅₀ rises. 4. Compare saturations at lung and tissue pressures before and after the shift. 5. Interpret the change in O₂ delivered.

Visual explanation

Plot two sigmoidal dissociation curves. The left curve is labelled pH 7.4 and the right curve pH 7.2 with higher CO₂. At a tissue pO₂ of 5 kPa, draw vertical arrows showing lower saturation on the shifted curve. At 13 kPa, show both curves close to full saturation, emphasising that loading in the lungs is barely affected.

Real-world analogy

The Bohr effect resembles a delivery lorry that automatically unloads more goods at shops with long queues. The queue — acid and CO₂ from metabolism — signals demand, and the lorry responds without a driver deciding. The analogy is imperfect, since the "signal" works by shifting a chemical equilibrium, not by any decision.

Real-world example

During intense exercise, muscle pH may fall to around 7.2 and temperature rise by a few degrees. Both factors shift haemoglobin's curve right. Combined with the lower tissue pO₂, this increases O₂ extraction from about 25% of the load at rest to 70% or more, greatly improving oxygen supply. The simulation for this unit lets you set working-muscle conditions and measure the extra delivery.

Why?

Why does a shift that lowers O₂ affinity not seriously impair loading in the lungs? The upper part of the sigmoid curve is flat: at 13 kPa haemoglobin remains nearly saturated even when the curve moves right. The shift matters most on the steep middle portion corresponding to tissue pressures.

Common misconception

"The Bohr effect means CO₂ competes with O₂ for the heme iron." CO₂ does not bind to iron; it acts through pH and carbamate formation at sites away from the heme. Carbon monoxide, not carbon dioxide, competes at the iron. Another error is to think a right shift always reduces oxygen supply; it improves delivery to tissues.

Worked example

Question: At pH 7.4, haemoglobin in muscle capillaries at pO₂ = 4.0 kPa is 60% saturated. At pH 7.2 it is 45% saturated. Arterial saturation is 97% in both cases. With an O₂ capacity of 200 mL per litre, how much extra O₂ is delivered per litre at pH 7.2?

Reasoning: At pH 7.4: released = (0.97 − 0.60) × 200 = 74 mL. At pH 7.2: released = (0.97 − 0.45) × 200 = 104 mL. Difference = 30 mL.

Answer: About 30 mL extra O₂ per litre of blood.

Quick check

1. Does a fall in blood pH shift the O₂ dissociation curve left or right, and why? Answer: Right, because protonation stabilises the low-affinity T state, raising P₅₀.

Exam focus

State the Bohr effect precisely and explain it using protonation of His146β and T-state stabilisation. Describe the two actions of CO₂ and the role of 2,3-BPG in the central cavity. Link right shifts to increased O₂ delivery with calculations.

Advanced insight

The Bohr effect is thermodynamically linked to O₂ binding: the number of protons released per O₂ equals the change in log P₅₀ per unit pH (Wyman linkage). Studies with site-directed mutants have shown that many residues each contribute a small part, and the classic His146β–Asp94β salt bridge accounts for a substantial but not complete share of the alkaline Bohr effect.

Summary

Protons, CO₂, 2,3-BPG and heat all stabilise haemoglobin's T state, lowering O₂ affinity and shifting the dissociation curve to the right. Protonation of His146β and N-terminal groups underlies the Bohr effect; CO₂ acts through pH change and carbamate formation; 2,3-BPG binds in the central cavity of deoxy-Hb. These effects increase O₂ release in active tissues with little effect on loading in the lungs.

Practice questions

1. Explain how carbamate formation lowers haemoglobin's O₂ affinity. Answer: Carbamates add negative charge at the N-termini that forms salt bridges stabilising the T state, and their formation releases protons that also favour T. 2. Why does fetal haemoglobin have a lower P₅₀ than adult haemoglobin? Answer: Its γ chains lack a positively charged residue that binds 2,3-BPG, so BPG stabilises T less and affinity stays higher. 3. What is the Haldane effect? Answer: Deoxygenated haemoglobin binds more H⁺ and CO₂ than oxygenated haemoglobin, aiding CO₂ uptake in tissues and its release in the lungs. 4. Why does increased temperature reduce haemoglobin's O₂ affinity? Answer: O₂ binding is exothermic, so raising the temperature shifts the equilibrium towards release by Le Chatelier's principle.