Globular & Fibrous Proteins (HL) (HL IB Biology)

Globular

• Globular proteins are compact, roughly spherical (circular) in shape and soluble in water
• Globular proteins form a spherical shape when folding into their tertiary structure because:
  • Their non-polar hydrophobic R-groups are orientated towards the centre of the protein away from the aqueous surroundings and
  • Their polar hydrophilic R-groups orientate themselves on the outside of the protein
• This orientation enables globular proteins to be (generally) soluble in water as the water molecules can surround the polar hydrophilic R-groups
• The solubility of globular proteins in water means they play important physiological roles as they can be easily transported around organisms and be involved in metabolic reactions
• The folding of the protein due to the interactions between the R-groups results in globular proteins having specific shapes. This also enables globular proteins to play physiological roles, for example, enzymes can catalyse specific reactions and immunoglobulins can respond to specific antigens
• Some globular proteins are conjugated proteins that contain a prosthetic group eg. haemoglobin which contains the prosthetic group called haem

Insulin

• The first protein to have its sequence determined by scientists was the hormone insulin
• Insulin is a globular protein produced in the pancreas. It plays an important role in the control of blood glucose concentration
• It consists of two polypeptide chains
  • Polypeptide A has 21 amino acid residues
  • Polypeptide B has 30 amino acid residues
• The two polypeptide chains are held together by three disulfide bridges

Fibrous

• Fibrous proteins are long strands of polypeptide chains that have cross-linkages due to hydrogen bonds
• They have little or no tertiary structure
• Due to the 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, for example, keratin that makes up hair, nails, horns and feathers and collagen which is a connective tissue found in skin, tendons and ligaments

Collagen

• Collagen is the most common structural protein found in vertebrates
• It has a flexible structure, forming connective tissues:
  
  • Tendons
  • Cartilage
  • Ligaments
  • Bones
  • Teeth
  • Skin
  • Walls of blood vessels
  • Cornea of the eye
• Collagen is an insoluble fibrous protein
• Collagen is formed from three polypeptide chains closely held together by hydrogen bonds to form a triple helix; the hydrogen bonds give great tensile strength
• Each polypeptide chain is a helix shape and contains about 1000 amino acids with glycine, proline and hydroxyproline being the most common
• 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 which provide strength
• 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

Globular and fibrous proteins diagram

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