Lesson 7 of 7 / Biological molecules
Collagen: a fibrous protein built for strength
What makes a collagen fibre strong enough to hold a tendon together?
In this lesson: Describe the molecular structure of collagen and relate it to its structural role.
About 7 min
The key ideaThree chains wind into a triple helix (tropocollagen) held by hydrogen bonds. Staggered molecules joined by covalent cross-links form fibrils and fibres with high tensile strength.
Structure for a function
Compare a globular transporter with a fibre
The triple helix and many intermolecular cross-links help the fibril resist pulling forces. Small glycine residues permit close packing of the chains.
Original schematic of a small part of a fibril, not an atomic structure. Hydrogen bonding within the triple helix is not individually drawn. Molecular alignment, stagger and dimensions are simplified.
Explanation
Collagen gives strength to connective tissues such as tendons, ligaments, skin, cartilage and bone. It is a fibrous protein: long and rope-like, not compact like haemoglobin. Its job is to resist pulling forces without breaking or stretching much. This property is called high tensile strength. Collagen is also insoluble in water, so it stays in place in the tissue.
One collagen molecule is called tropocollagen. It is made of three polypeptide chains wound around one another to form a triple helix. In each chain, every third amino acid is glycine, so the sequence repeats as glycine-X-Y. X and Y are often proline and hydroxyproline. Glycine has the smallest R group, a single hydrogen atom. It is the only amino acid small enough to fit in the crowded centre of the triple helix, so the three chains can pack very tightly.
The three chains are held together by many hydrogen bonds. Each one forms between an N-H group in one chain and a C=O group in a neighbouring chain. One hydrogen bond is weak, but there are a great many along the length of the helix. Together they hold the chains firmly in place.
Tropocollagen molecules then line up side by side to form a fibril. They are staggered, so the ends of neighbouring molecules do not line up. Covalent cross-links form between lysine residues on neighbouring tropocollagen molecules. Many fibrils bundle together to make a collagen fibre.
Each level adds strength. Hydrogen bonds hold each triple helix together. Covalent cross-links tie the molecules to each other, and covalent bonds are much stronger than hydrogen bonds. Because the molecules are staggered, there is no single weak line where all the ends meet, so a pulling force is shared along the fibre. Collagen is insoluble because it is a very large molecule with many non-polar R groups, and its chains are bonded to each other rather than to water.
Step by step
- 1
Start with one molecule
Three chains form a triple helix, with glycine as every third amino acid.
- 2
Name the bonds
Hydrogen bonds within the helix; covalent cross-links between molecules.
- 3
Build up the levels
Staggered molecules form fibrils; fibrils form fibres with high tensile strength.
Worked example
Work through the evidence
A mutation replaces one glycine in a collagen chain with an amino acid that has a much larger R group. Predict the effect on the tissue.
One way to explain it
The larger R group cannot fit in the centre of the triple helix. The three chains cannot pack tightly, so fewer hydrogen bonds form and the helix is weaker. Fibrils and fibres built from these molecules have lower tensile strength, so tissues such as bone and skin are weaker and break or tear more easily.
Why this answer works
- Locate glycine in the centre of the triple helix.
- Link a larger R group to looser packing and fewer hydrogen bonds.
- Follow the effect up the levels to the fibre and the tissue.
Is this true? "Collagen is a single chain held together only by peptide bonds."
Peptide bonds join amino acids within each chain. Three chains form each tropocollagen molecule, held by hydrogen bonds, and covalent cross-links join neighbouring molecules.