Structure

• Fibrous proteins?are long strands of polypeptide chains that have cross-linkages due to hydrogen bonds
• These proteins have little or no tertiary structure
• Fibrous proteins have a limited number of amino acids with the sequence usually being highly repetitive
• The highly repetitive sequence creates?very organised structures

Function

• Due to a large number of?hydrophobic R groups, fibrous proteins are?insoluble?in water
• Fibrous proteins are?strong?and this, along with their insolubility property, makes fibrous proteins very suitable for?structural roles
• Examples of fibrous proteins:
  • Keratin?makes up hair, nails, horns and feathers (it is a very tough fibrous protein)
  • Elastin?is found in connective tissue, tendons, skin and bone (it can stretch and then return to its original shape)
  • Collagen?is a connective tissue found in skin, tendons and ligaments

Globular and fibrous protein models illustrating the spherical shape of globular proteins and the long, stranded shape of fibrous proteins.

Collagen

• Collagen?is the most common?structural?protein found in?vertebrates
• It provides structural?support
• 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 of collagen

• 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 arrange into collagen fibrils and finally into collagen fibres which have high tensile strength

Function of collagen

• 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 present. These amino acids increase stability as their R groups repel each other
• The length of collagen molecules means they take too long to dissolve in water (making it insoluble in water)

Comparison of Collagen and Haemoglobin Table