Lesson 6 of 6 / Biological molecules
Haemoglobin and collagen: shape for a job
Why is one protein soluble and the other a strong fibre?
In this lesson: Relate globular haemoglobin and fibrous collagen structure to function.
About 6 min
The key ideaHaemoglobin uses a compact multi-subunit structure for reversible oxygen binding; collagen uses long associated chains and cross-links for tensile strength.
Structure for a function
Compare a globular transporter with a fibre
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
Haemoglobin is a globular protein with four polypeptide subunits. Each subunit carries a haem group containing an iron ion that binds oxygen reversibly. Its compact arrangement and surface chemistry suit transport within red blood cells; its quaternary structure enables interaction between subunits.
Oxygen binding promotes a conformational change that increases the affinity of remaining sites, producing cooperative binding. Loading where oxygen partial pressure is high and unloading where it is lower supports transport. Do not confuse oxygen binding to haem with synthesis of a new polypeptide.
Collagen is fibrous: three polypeptide chains wind around one another into a triple helix. Repeated small glycine residues permit close packing, and hydrogen bonding stabilises association. Collagen molecules align in fibrils with covalent cross-links between molecules, producing strong fibres.
The resulting tensile strength suits connective tissues such as tendons and skin. A complete structure-function explanation links a named structural feature to a physical property, then to the role; stating merely that collagen is strong or haemoglobin carries oxygen omits the mechanism.
Step by step
- 1
Identify the architecture
Contrast compact globular organisation with extended fibres.
- 2
Name the working feature
Use haem groups or intermolecular cross-links.
- 3
Explain the role
Connect reversible binding to transport and tensile strength to support.
Worked example
Work through the evidence
A mutation reduces collagen cross-linking. Predict an effect on connective tissue.
One way to explain it
Fibrils are less effectively tied together, reducing tensile strength, so tissue is less able to resist pulling forces.
Why this answer works
- Cross-links connect collagen molecules.
- The predicted effect concerns mechanical strength rather than oxygen binding.
Is this true? "Collagen is a single chain held together only by peptide bonds."
Collagen has associated chains and higher-order fibrils; hydrogen bonds and intermolecular covalent cross-links contribute to strength.