Protein structure

Protein structure may be considered in terms of orders of complexity:

• primary structure: this is the sequence of amino acids along a polypeptide chain
• secondary structure: the manner in which the primary structure folds upon itself to give:
  • alpha-helices
  • beta-pleated sheets
  • crystals
  • fibres
• tertiary structure: the manner in which several alpha-helices or beta-pleated sheets are organized together into domains
• quaternary structure: the way in which subunits containing domains interact in larger proteins

The key determinant of all levels of structure is the sequence of amino acids in individual polypeptides. The side groups on the amino acids tend to form higher levels of structure spontaneously by their interaction with neighbouring groups. Additionally, prosthetic groups or co-factors may be necessary for the activity of individual proteins.