The Molecular Structure of Collagen (AQA A Level Biology)

• Collagen is the most common structural protein found in vertebrates
• In vertebrates it is the component of connective tissue which forms:
  • Tendons
  • Cartilage
  • Ligaments
  • Bones
  • Teeth
  • Skin
  • Walls of blood vessels
  • Cornea of the eye

• Collagen is an insoluble fibrous protein

Structure

• Collagen is formed from three polypeptide chains closely held together by hydrogen bonds to form a triple helix (known as tropocollagen)
• Each polypeptide chain is a helix shape (but not α-helix as the chain is not as tightly wound) and contains about 1000 amino acids with glycine, proline and hydroxyproline being the most common
• In the primary structure of collagen almost every third amino acid is glycine
  • This is the smallest amino acid with a R group that contains a single hydrogen atom
  • Glycine tends to be found on the inside of the polypeptide chains allowing the three chains to be arranged closely together forming a tight triple helix structure

• Along with hydrogen bonds forming between the three chains there are also covalent bonds present
• Covalent bonds also form cross-links between R groups of amino acids in interacting triple helices when they are arranged parallel to each other. The cross-links hold the collagen molecules together to form fibrils
• The collagen molecules are positioned in the fibrils so that there are staggered ends (this gives the striated effect seen in electron micrographs)
• When many fibrils are arranged together they form collagen fibres
• Collagen fibres are positioned so that they are lined up with the forces they are withstanding

Collagen is a fibrous structural protein that is formed by triple helices collagen molecules arranging into collagen fibrils and finally into collagen fibres which have high tensile strength

Function

• Flexible structural protein forming connective tissues
• The presence of the many hydrogen bonds within the triple helix structure of collagen results in great tensile strength. This enables collagen to be able to withstand large pulling forces without stretching or breaking
• The staggered ends of the collagen molecules within the fibrils provide strength
• Collagen is a stable protein due to the high proportion of proline and hydroxyproline amino acids result in more stability as their R groups repel each other
• Length of collagen molecules means they take too long to dissolve in water (collagen is therefore insoluble in water)

Comparison between Collagen & Haemoglobin Table