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Lesson 7 of 7 / Biomolecules: structure explains function

Haemoglobin carries oxygen reversibly

How does a four-subunit protein serve transport?

I can relate haemoglobin molecular structure to oxygen transport.

  • I can relate haemoglobin molecular structure to transport.Syllabus

    Syllabus 8876, Core 1(m). Relate haemoglobin molecular structure to transport.

About 5 min

Make a guess. You are not marked.

How many oxygen molecules can one haemoglobin molecule carry?

  1. Hundreds, packed inside the molecule.
  2. Four, one at each haem group.
  3. One, held permanently.
Show the answer

Four, one at each haem group.

Haemoglobin has four subunits, each with an iron-containing haem that binds one O2 reversibly.

The key idea

Adult 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

alphaFebetaFealphaFebetaFe

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 is a globular protein, and the way its chains fold makes it soluble. Amino acids with hydrophobic R groups point into the centre of each subunit, and amino acids with hydrophilic R groups face outwards. The outer R groups form hydrogen bonds with water, so haemoglobin dissolves in the cytoplasm of red blood cells. Each haem also sits in a hydrophobic pocket, which helps keep its iron as Fe2+, the form that binds oxygen.

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.

Exam questions often give the haemoglobin sequence of two species and ask how one amino acid difference changes oxygen transport. Build the answer as a chain. (1) A different amino acid has a different R group. (2) The new R group forms different bonds: hydrogen bonds, ionic bonds, disulfide bonds or hydrophobic interactions. (3) The polypeptide folds differently, changing its tertiary structure. (4) The way the four subunits fit together changes, so the quaternary structure changes. (5) The shape around the haem groups, and how easily the molecule changes shape when oxygen binds, changes, so its affinity for oxygen changes.

Sickle-cell anaemia shows the chain in a real case. In the beta chain, the amino acid at position 6 is glutamic acid, whose R group is charged and hydrophilic. In sickle-cell haemoglobin it is valine, whose R group is hydrophobic. When oxygen is low, the exposed valine of one molecule sticks to a hydrophobic patch on another. The molecules join into long fibres that distort the red blood cell into a sickle shape, so it carries less oxygen and can block capillaries.

Step by step
  1. 1

    Count binding sites

    Four subunits each carry one haem group with an oxygen-binding iron ion.

  2. 2

    Explain loading

    Oxygen can bind where availability is high, with cooperative changes in affinity.

  3. 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.

Watch out for this

A student says: "Each haemoglobin molecule binds one oxygen atom permanently." What is wrong with this?

Show the answer

It can bind up to four oxygen molecules, one at each haem group, and binding must be reversible for transport.

Check your understanding

Why is reversible binding essential?

  1. It keeps all four oxygen sites occupied even in oxygen-poor tissue.
  2. It allows haemoglobin to store oxygen inside red cells for later use by the cell itself.
  3. It allows oxygen loading and later unloading.
  4. It means oxygen forms permanent bonds with the polypeptide backbone.
Show the answer

It allows oxygen loading and later unloading.

A transporter must deliver the substance it picks up.

Exam-style questions

Check your understanding

Which feature of haemoglobin does NOT help it transport oxygen?

  1. Each subunit has a haem group with Fe2+
  2. Hydrophilic R groups face outwards
  3. Peptide bonds join its amino acids
  4. Oxygen binds reversibly
Show the answer

Peptide bonds join its amino acids

Every protein has peptide bonds. They do not explain oxygen transport, unlike haem, reversibility and solubility.

Quick recall

Card 1 of 3. Answer in your head, then check.

How does a four-subunit protein serve transport?

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