describe the structure of a molecule of collagen as an example of a fibrous protein, and the arrangement of collagen molecules to form collagen fibres

Proteins – Collagen (Fibrous Protein)

Learning Objective

Describe the structure of a molecule of collagen as an example of a fibrous protein, and explain how collagen molecules are arranged to form collagen fibres.

Key Features of Fibrous Proteins

• Predominantly structural role
• Long, repetitive polypeptide chains
• High tensile strength, low solubility
• Examples: keratin, elastin, collagen

Collagen – Molecular Structure

1. Primary structure

   Repeating tripeptide sequence: Gly‑X‑Y, where X is often proline and Y is hydroxyproline.

2. Secondary structure

   Each polypeptide chain forms a left‑handed poly‑proline II helix (extended, no α‑helix).

3. Supramolecular arrangement

   Three individual chains intertwine to form a right‑handed triple helix (the collagen molecule).

4. Stabilising interactions

   • Hydrogen bonds between the carbonyl of Gly and the amide of the adjacent chain.
   • Hydroxyproline residues increase hydrogen‑bond stability.
   • Inter‑chain covalent cross‑links (lysine‑derived) form after secretion.

Dimensions of a Collagen Molecule

From Molecules to Fibres

The hierarchical assembly of collagen can be summarised in four levels:

1. Collagen molecule (tropocollagen) – the triple‑helical unit described above.
2. Fibril – several tropocollagen molecules staggered in a quarter‑stagger arrangement, creating a characteristic 67 nm periodicity (the “D‑period”).
3. Fiber – bundles of fibrils aligned parallel, bound together by intermolecular cross‑links and associated proteoglycans.
4. Fascicle / tissue – groups of fibres embedded in a ground substance, forming tendons, ligaments, skin, bone matrix, etc.

Quarter‑Stagger Model (Fibril Formation)

In the fibril, each tropocollagen molecule overlaps the next by approximately one‑quarter of its length. This produces a repeating banding pattern visible under electron microscopy.

Cross‑linking and Mechanical Strength

• Lysyl oxidase converts lysine residues to aldehydes, which form covalent bonds between adjacent molecules.
• Cross‑links increase with age, contributing to the increased rigidity of tissues.
• Defects in cross‑linking (e.g., in scurvy) lead to weakened collagen fibres.

Summary Checklist

• Collagen primary structure: repeating Gly‑X‑Y tripeptide.
• Three left‑handed helices → one right‑handed triple helix.
• Triple helix ≈ 300 nm long, 1.5 nm diameter.
• Quarter‑stagger arrangement → 67 nm D‑period in fibrils.
• Cross‑linking stabilises fibrils into strong fibres.

Potential Exam Questions

1. Explain why glycine is essential at every third position in the collagen primary structure.
2. Describe how the triple‑helix structure of collagen contributes to its tensile strength.
3. Using a labelled diagram, illustrate the hierarchical organisation of collagen from molecule to tissue.