relate the structure of haemoglobin to its function, including the importance of iron in the haem group

Proteins – Haemoglobin

Learning Objective

Relate the structure of haemoglobin to its function, with particular emphasis on the role of iron in the heme (haem) group.

1. Classification of Haemoglobin

• Protein type: globular, soluble protein (contrast with fibrous proteins such as collagen that form long, structural fibres).
• Syllabus link (AO1): Identify haemoglobin as a globular protein.

2. Quaternary Structure and Sub‑unit Composition

• Haemoglobin is a tetramer (quaternary structure) composed of four polypeptide chains.
• Adult human haemoglobin (HbA): α₂β₂ (two α‑chains, two β‑chains).
• Fetal haemoglobin (HbF): α₂γ₂ – the γ‑chains replace the β‑chains, giving a higher affinity for O₂ (useful for A‑Level depth).

3. The Heme (Haem) Group

• Each heme consists of a planar protoporphyrin IX ring that chelates a central iron ion (Fe²⁺).
• Coordination geometry (octahedral):

  • Proximal histidine (His F8): side‑chain nitrogen occupies one coordination site, anchoring the iron to the globin chain.
  • Distal histidine (His E7): lies opposite the proximal histidine, orients incoming O₂ and protects Fe²⁺ from oxidation to Fe³⁺.
  • One site is free for binding a single O₂ molecule; a second site can bind other ligands (CO, NO, etc.).

• The planar nature of the porphyrin allows tight packing inside the protein matrix and efficient electronic communication between the iron centre and the surrounding globin.

4. Importance of Iron (Fe²⁺)

1. Oxidation state: Only the ferrous ion (Fe²⁺) can bind O₂ reversibly. Oxidation to ferric (Fe³⁺) produces met‑haemoglobin, which cannot carry O₂.
2. Reversible coordination: The Fe²⁺–O₂ bond is a weak, reversible coordination bond, enabling uptake in the lungs and release in tissues.
3. Spin‑state change: Binding O₂ changes the iron’s electronic spin state, triggering the conformational shift that underlies cooperative binding.

5. Structure ↔ Function Relationships

5.1 Quaternary Structure and Cooperative O₂ Binding

• Tense (T) state: All four sub‑units have relatively low affinity for O₂.
• When the first O₂ molecule binds, the iron moves into the plane of the porphyrin and pulls on the proximal histidine. This motion shifts the relative positions of the α‑ and β‑chains, converting the tetramer to the relaxed (R) state.
• The R state has higher affinity for O₂, so the remaining haem sites bind O₂ more readily – the basis of positive cooperativity.
• This T ↔ R transition explains the sigmoidal O₂‑dissociation curve seen in the lungs‑to‑tissues gas‑exchange diagram (Syllabus 9.1).

5.2 Allosteric Effectors

• 2,3‑Bisphosphoglycerate (2,3‑BPG): Binds in the central cavity of the T state, stabilising it and lowering O₂ affinity. This promotes O₂ release in peripheral tissues.
• Bohr Effect: Lower pH (high H⁺) and higher CO₂ concentrations stabilise the T state, shifting the O₂‑dissociation curve to the right. This enhances O₂ delivery where metabolism is high (exam‑type graph interpretation – AO2).
• Other effectors such as temperature and ATP act in a similar fashion, modulating the T ↔ R equilibrium.

5.3 Summary Table – Structural Feature → Functional Consequence

6. Quantitative Description of Cooperative Binding

The relationship between fractional saturation (Y) and the partial pressure of oxygen (pO₂) is described by the Hill equation:

\( Y = \dfrac{(pO2)^{\,n}}{P{50}^{\,n} + (pO_2)^{\,n}} \)

• Hill coefficient (n): ≈ 2.8 for adult HbA, indicating strong positive cooperativity.
• P₅₀: pO₂ at 50 % saturation (≈ 26 mm Hg in normal adult blood). A right‑shift (higher P₅₀) reflects reduced affinity (e.g., in the presence of 2,3‑BPG or low pH).

7. Link to the Gas‑Exchange System (Syllabus 9.1)

• In the pulmonary capillaries (high pO₂, alkaline pH) haemoglobin is driven into the R state, becoming fully saturated.
• In systemic capillaries (lower pO₂, acidic pH, higher CO₂, more 2,3‑BPG) the T state is favoured, allowing O₂ to dissociate and diffuse into tissues.
• This dynamic matches the oxygen‑delivery requirements of active cells and underpins the overall efficiency of the respiratory system.

8. Clinical & Experimental Relevance (AO3)

• Iron‑deficiency anaemia: Insufficient iron limits heme synthesis → ↓ haemoglobin concentration → reduced O₂‑carrying capacity.
• Sickle‑cell disease: A single β‑chain point mutation (Glu⁶ → Val) alters the quaternary interface, causing polymerisation of deoxy‑HbS and distorted red cells.
• Carbon monoxide poisoning: CO binds Fe²⁺ with ~250‑fold higher affinity than O₂, forming carboxyhaemoglobin and preventing O₂ transport.
• Met‑haemoglobin formation: Oxidation of Fe²⁺ to Fe³⁺ (e.g., by certain drugs or oxidative stress) yields a form that cannot bind O₂, leading to cyanosis.
• Laboratory investigation: The Hill plot (log[Y/(1‑Y)] vs. log pO₂) is used to determine the Hill coefficient and assess cooperativity – a typical A‑Level practical.