Structure and Functions of Haemoglobin

Overview

Structure

1. Primary Structure (Amino Acid Sequence)

• HbA consists of four polypeptide (globin) chains: 2 α-chains (141 amino acids each) and 2 β-chains (146 amino acids each).
• Each chain is encoded by a separate gene; mutations in these genes cause haemoglobinopathies (e.g., HbS in sickle cell disease, where Glu→Val at position 6 of the β-chain).

2. Secondary Structure

• Each globin chain contains stretches of α-helical segments (conventionally labeled A through H from the amino terminus).
• These helices fold to create a hydrophobic heme-binding pocket near the surface of each subunit.

3. Tertiary Structure (Individual Subunit)

• Each globin chain folds around a single heme group — a protoporphyrin IX ring containing a central ferrous iron (Fe²⁺) atom.
• The Fe²⁺ forms six coordination bonds:
  • Four bonds to the pyrrole nitrogens of the porphyrin ring
  • One bond to the imidazole nitrogen of the proximal histidine (His F8 — 87th residue in α-chain, 92nd in β-chain) of the globin chain
  • One bond reversibly available for O₂ binding
• A distal histidine (His E7) lies on the opposite side of the heme, stabilising the bound O₂ and preventing Fe²⁺ oxidation to Fe³⁺ (methaemoglobin).

4. Quaternary Structure (Tetramer)

The haemoglobin tetramer (α₁β₁α₂β₂). Each black disk represents a heme group nestled in a hydrophobic pocket of its globin chain. Labels A and H indicate the α-helix segments. — Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 660

T form (Taut / Deoxy form)

• The two αβ dimers are constrained by an extensive network of ionic bonds and hydrogen bonds.
• Fe²⁺ is displaced slightly out of the plane of the heme ring.
• Low O₂ affinity — stabilised by H⁺, CO₂, 2,3-BPG, and Cl⁻.

R form (Relaxed / Oxy form)

• Binding of O₂ pulls Fe²⁺ into the plane of the heme; this movement pulls the proximal histidine and the attached helix, breaking some of the inter-dimer polar bonds.
• High O₂ affinity — once one O₂ binds, subsequent binding is facilitated (cooperativity).

Structural changes on oxygenation/deoxygenation: The T form is stabilised by weak ionic/H-bonds between dimers; the R form results from rupture of these bonds after O₂ binding. — Lippincott's Illustrated Reviews: Biochemistry, 8th ed., p. 95

Heme Group

• Heme = iron protoporphyrin IX: a porphyrin ring (four pyrrole units linked by methine bridges) with a central Fe²⁺.
• Synthesised from succinyl-CoA + glycine → δ-aminolevulinic acid (ALA) → porphobilinogen → protoporphyrin IX → heme.
• Fe must remain as Fe²⁺ (ferrous) to bind O₂ reversibly. Oxidation to Fe³⁺ produces methaemoglobin, which cannot carry O₂.

Developmental Variants

Functions

1. Oxygen Transport

• Each Hb molecule carries up to 4 O₂ molecules (one per heme).
• O₂ binding is cooperative (sigmoidal dissociation curve), meaning the affinity increases as each successive O₂ binds — the basis of efficient loading in the lungs and unloading in the tissues.
• Without haemoglobin, plasma can carry only ~0.3 mL O₂ per 100 mL blood; with HbA, this rises to ~20 mL/100 mL.

2. Carbon Dioxide Transport

• ~20–23% of CO₂ is carried as carbamino-haemoglobin (CO₂ bound to the N-terminal amino groups of globin chains as carbamate).
• CO₂ binding to the T form stabilises deoxyhaemoglobin and promotes O₂ release (Bohr effect).

3. Buffering (Bohr Effect)

• Hb is one of the most important blood buffers.
• In tissues, rising CO₂ → ↑ H⁺ (via carbonic anhydrase) → H⁺ binds to specific histidine residues on globin chains → T form stabilised → O₂ released.
• In the lungs, CO₂ is expelled → ↓ H⁺ → R form favoured → O₂ loaded.
• This reciprocal relationship between H⁺/CO₂ and O₂ affinity is the Bohr effect.

4. Regulation via 2,3-Bisphosphoglycerate (2,3-BPG)

• 2,3-BPG is a product of glycolysis in RBCs (Rapoport–Luebering shunt).
• It binds in the central cavity between the β-chains of the T form, forming ionic bonds and reducing O₂ affinity (right-shifts the O₂ dissociation curve).
• Elevated at high altitude and in chronic anaemia → enhances O₂ delivery to tissues.
• HbF binds 2,3-BPG poorly → higher O₂ affinity (important for placental gas exchange).

5. Nitric Oxide (NO) Transport

• Haemoglobin can bind NO at the Fe²⁺ of heme (forming nitrosyl-Hb) or at cysteine β-93 (forming S-nitrosyl-Hb), modulating vascular tone.

6. Proton Transport (Haldane Effect)

• Deoxyhaemoglobin has a higher affinity for H⁺ than oxyhaemoglobin; this helps carry H⁺ from tissues to the lungs for excretion as H₂O after combining with HCO₃⁻.

Summary Table