2.3.b Explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins and describe the types of bonding (hydrogen, ionic, disulfide and hydrophobic interactions) that hold these molecules in shape. 

• Primary Structure: sequence of amino acids in a polypeptide/protein chain.
• Secondary Structure: folded structure of a protein molecule in a repeated pattern, resulting in either regular coiling or folding of the amino acid chain (either α-helix or β-pleated sheet). 
  • α-helix: coiled into corkscrew shape and is held together by hydrogen bonds between -NH group and the -CO group. 
  • β-pleated sheet: looser and straighter than α-helix structure. 
• Tertiary Structure: compact structure of protein molecule, where protein coils up into a three-dimensional shape. 
• Quaternary Structure: the three-dimensional association of two or more polypeptide chains, or of a polypeptide and a non-protein component such as haem, in a protein molecule.

Bonds play an important role in maintaining the precise shape of a protein. There are four types of bonding that are important in protein structure: 

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Written by Aayushi | GCE CIE Biology Specialist @ TUTTEE

References:

• Jones, M., & Parkin, M. (2018). Cambridge International AS and A Level Biology. Cambridge: Cambridge University Press.
• Protein structure: Primary, secondary, tertiary & quatrenary (article). Retrieved from https://www.khanacademy.org/science/biology/macromolecules/proteins-and-amino-acids/a/orders-of-protein-structure