collagen

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

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

• The length of collagen molecules means they take too long to dissolve in water, so collagen is therefore insoluble in water