Collagen (Cambridge International AS Biology)

The Molecular Structure of Collagen

Spec point — 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. relate the structures of collagen molecules and collagen fibres to their function

The molecular structure of collagen

• 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)
  • 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

• Collagen is a 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

  • This results in more stability as their R groups repel each other

• Collagen molecules are very large and are cross-linked into fibrils and fibres, so collagen is insoluble in water

Exam Hint Use the acronym NOT MAPS(number, outline, type etc.) to help you revise how the function relates to the structure of collagen and know the difference between collagen and haemoglobin.