5.6.5Physical Chemistry and Transition Elements
The Biochemical Importance of Iron in Haemoglobin — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 4 parts Lesson 38 of 40
Part 1 of 4Structure of Haemoglobin
Haemoglobin is a protein in red blood cells that carries oxygen from the lungs to the tissues.
Four haem groups
- Each haemoglobin molecule contains four haem groups, so it can carry up to four O₂ molecules.
Iron(II) centre
- Each haem has one Fe²⁺ ion at its centre, held in a flat ring called a porphyrin.
Four ligand bonds
- Four nitrogen atoms of the porphyrin ring donate lone pairs to Fe²⁺, so the porphyrin acts as a tetradentate ligand.
Fifth and sixth bonds
- A nitrogen from the globin protein bonds from one side, leaving the sixth octahedral position free for oxygen or water.
Part 2 of 4Oxygen Transport
Oxygen binds and is released by reversible ligand substitution.
1
In the lungs the partial pressure of oxygen is high, so O₂ replaces a weakly bound water ligand at the sixth position of Fe²⁺.
2
The oxygen molecule donates a lone pair in a dative covalent bond, forming oxyhaemoglobin, which is bright red.
3
The overall equilibrium can be written Hb + 4O₂ ⇌ Hb(O₂)₄, where Hb stands for haemoglobin.
4
In tissues the oxygen concentration is low, so the equilibrium shifts to the left and O₂ is released for respiration.
5
The oxygen must be bound strongly enough to be picked up but weakly enough to be released, which is why Fe²⁺ is suitable.
Part 3 of 4Carbon Monoxide Poisoning
Carbon Monoxide Poisoning
Competing ligand
- CO has a lone pair on carbon and also binds to Fe²⁺ at the sixth position, forming carboxyhaemoglobin.
Stronger bond
- CO bonds much more strongly than O₂, so the substitution is very hard to reverse and equilibrium lies far on the CO side.
Effect
- Fewer haem groups are available to carry oxygen, so the tissues are starved of oxygen even at low CO concentrations.
Treatment
- The person is moved into fresh air and given pure oxygen, which slowly displaces CO from the haemoglobin.
Part 4 of 4Exam Checklist & Tips
5.6.5 – Key Points for the Exam
- Haemoglobin The iron-containing protein in red blood cells that transports oxygen.
- Haem group The Fe²⁺ ion bonded to a porphyrin ring within haemoglobin.
- Oxyhaemoglobin Haemoglobin with oxygen bound to the iron(II) centres.
- Carboxyhaemoglobin Haemoglobin with carbon monoxide bound, which cannot carry oxygen.
Common Mistakes
- Stating that the iron in haemoglobin is Fe³⁺, whereas it is Fe²⁺.
- Describing oxygen binding as ionic instead of a reversible dative covalent bond.
- Claiming that CO binds to a different part of the molecule, when it competes for the same site as oxygen.
- Saying that haemoglobin has only one haem group, rather than four.
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