Introduction to Biochemistry IV

Levels of protein structure

Primary

  • The sequence of amino acid residues

Secondary

  • Localised conformation of the polypeptide backbone
  • Hydrogen bonded
  • 3 types:
    • ⍺ helix
    • β sheet
    • Triple helix

Tertiary

  • 3D structure of an entire polypeptide
  • Fibrous proteins: mechanically strong e.g. collagen
    • Polypeptide chains parallel, single axis
    • Insoluble in water
  • Globular proteins: spherical e.g. haemoglobin
    • Soluble in water and salt solutions

Quaternary

  • The spatial arrangement of polypeptide chains in a protein with multiple subunits
e.g. haemoglobin
  • 4 subunits - 2 ⍺ and 2 β chains
  • Each subunit contains 1 haem group - binds to oxygen
  • Binding of 1 oxygen changes the affinity of other subunits
e.g. Tropocollagen
  • Structural unit of a collagen fibre
  • Three helical chains twisted around each other to form a right-handed superhelix
  • Interchain H-bonds and intermolecular covalent bonds