Globular & Fibrous Proteins (CIE A Level 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
  • 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; globular proteins 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 e.g. haemoglobin which contains the prosthetic group called haem

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, e.g. keratin that makes up hair, nails, horns and feathers and collagen which is a connective tissue found in skin, tendons and ligaments

Fibrous vs Globular 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

	Globular	Fibrous
Shape	Roughly spherical	Long strands
Amino Acid Sequence	Irregular with a wide range of R groups	Repetitive with a limited range of R groups
Function	Physiological/functional	Structural
Examples	Haemoglobin, insulin, immunoglobulins	Collagen, keratin, myosin, actin, fibrin
Solubility	Generally soluble in water	Generally insoluble in water