Lesson 7 of 7 / Biomolecules: structure explains function
Haemoglobin carries oxygen reversibly
How does a four-subunit protein serve transport?
In this lesson: Relate haemoglobin molecular structure to oxygen transport.
About 5 min
The key ideaAdult haemoglobin has four polypeptide subunits, each with an iron-containing haem group that can bind one oxygen molecule reversibly.
Explore the idea
Binding sites and their occupancy
0 sites occupied; the molecule still has four subunits and four haem groups. Oxygen binds at the iron in each haem group and can later be released.
The orange site includes iron even when its label shows O2. This model demonstrates capacity and occupancy, not a quantitative oxygen-dissociation curve.
Explanation
A typical adult haemoglobin molecule has two alpha and two beta polypeptide subunits. Each folded subunit contains a haem prosthetic group with an Fe2+ ion. Oxygen binds reversibly to the haem, so one haemoglobin molecule can carry up to four oxygen molecules.
Its multi-subunit organisation is quaternary structure. Binding oxygen influences the conformation of the protein and can increase the affinity of the remaining sites. This cooperative behaviour helps haemoglobin load oxygen where oxygen is abundant.
Reversible binding is equally important: oxygen must be released where its concentration is lower and cells are consuming it. A protein that bound oxygen permanently would be a poor transporter. Haemoglobin's globular, soluble structure allows it to function in the aqueous interior of red blood cells.
A complete structure-function answer connects subunits, haem groups, iron and reversible binding. H1 does not require memorising the number of amino acids or the detailed secondary-structure composition of the chains.
Step by step
- 1
Count binding sites
Four subunits each carry one haem group with an oxygen-binding iron ion.
- 2
Explain loading
Oxygen can bind where availability is high, with cooperative changes in affinity.
- 3
Explain unloading
Reversible binding permits release where oxygen availability is lower.
Worked example
Capacity is not saturation
A haemoglobin molecule currently carries two oxygen molecules. Has it lost two polypeptide chains?
One way to explain it
No. It still has four subunits and four haem groups, but only two sites are occupied. Occupancy can change through reversible oxygen binding without loss of subunits.
Why this answer works
- Distinguish the number of binding sites from their current occupancy.
- Keep reversible transport separate from protein breakdown.
Is this true? "Each haemoglobin molecule binds one oxygen atom permanently."
It can bind up to four oxygen molecules, one at each haem group, and binding must be reversible for transport.