relate the structures of collagen molecules and collagen fibres to their function

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology 9700 – Proteins: Collagen Structure & Function

Proteins – Collagen

Learning Objective

Relate the structures of collagen molecules and collagen fibres to their biological function.

Key Features of Collagen

  • Most abundant protein in mammals (≈30 % of total body protein).

  • Provides tensile strength to connective tissues such as skin, tendon, bone and cartilage.

  • Forms a hierarchical structure: amino‑acid sequence → triple‑helical molecule → fibril → fibre.

1. Primary Structure

The primary structure of collagen is a repetitive tripeptide motif:

\$\text{(Gly‑X‑Y)}_n\$

where Gly = glycine, X is often proline, and Y is frequently hydroxyproline. The high glycine content (every third residue) is essential because the small side chain of glycine allows the three polypeptide chains to pack tightly together in the centre of the triple helix.

2. Secondary Structure – Triple Helix

Three left‑handed poly‑proline II helices intertwine to form a right‑handed super‑helix (the collagen molecule). Important stabilising factors:

  1. Hydrogen bonds between the carbonyl oxygen of Gly in one chain and the amide hydrogen of X or Y in an adjacent chain.

  2. Hydroxyproline residues stabilise the helix through additional hydrogen bonding with water.

The resulting molecule is approximately 300 nm long and 1.5 nm in diameter.

3. Tertiary/Quaternary Structure – Fibril Formation

Collagen molecules self‑assemble into staggered arrays called fibrils. Key characteristics:

  • Each molecule is offset by about 67 nm relative to its neighbours, creating a characteristic banding pattern (D‑period).

  • Cross‑linking between lysine‑derived aldehyde groups (via lysyl oxidase) covalently links adjacent molecules, providing mechanical strength.

4. Collagen Fibre

Fibrils aggregate laterally to form collagen fibres, which may be further bundled into fascicles. The hierarchical organisation gives rise to:

  • High tensile strength along the fibre axis.

  • Resistance to stretching and tearing.

  • Flexibility due to the ability of fibrils to slide slightly under load.

Structure–Function Relationship

Structural LevelKey Structural FeatureResulting Functional Property
Primary structureGly‑X‑Y repeat; high glycine contentAllows tight packing of three chains into a compact triple helix.
Triple helix (secondary)Right‑handed super‑helix; intra‑chain hydrogen bonds; hydroxyproline stabilisationProvides rigidity and resistance to enzymatic degradation.
Fibril (quaternary)Staggered, 67 nm D‑period; covalent cross‑linksImparts tensile strength and elasticity to tissues.
FibreAggregation of fibrils; lateral packingEnables load‑bearing capacity of tendons, ligaments and skin.

Clinical Relevance

  • Scurvy: Deficiency of vitamin C prevents hydroxylation of proline and lysine, leading to unstable triple helices and weak connective tissue.

  • Osteogenesis imperfecta: Mutations in collagen‑type I genes disrupt the Gly‑X‑Y motif, producing brittle bones.

  • Collagen cross‑linking disorders: Excessive cross‑linking (e.g., in diabetes) reduces tissue elasticity.

Suggested diagram: Hierarchical structure of collagen – from amino‑acid sequence to triple helix, staggered fibril, and macroscopic fibre with labelled D‑period.

Summary Points

  1. Collagen’s primary structure (Gly‑X‑Y) is essential for the formation of a tight triple helix.

  2. The triple helix provides a rigid, rod‑like molecule that can pack into staggered fibrils.

  3. Covalent cross‑links between fibrils give collagen fibres their remarkable tensile strength.

  4. The hierarchical organisation directly explains collagen’s role in supporting and protecting connective tissues.