• Fibrous proteins?are long strands of polypeptide chains that have cross-linkages due to hydrogen bonds
• These proteins have little or no tertiary structure
• Due to a large number of?hydrophobic R groups, fibrous proteins are?insoluble?in water
• Fibrous proteins have a limited number of amino acids with the sequence usually being highly repetitive
• The highly repetitive sequence creates?very organised structures?that 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 roughly spherical shape of globular proteins and the long, stranded shape of fibrous proteins

Comparison of Globular & Fibrous Tertiary Proteins Table

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 arranging 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 between Collagen & Haemoglobin Table