Protein Modification

Post-translational modifications refer to any change in the chemical composition of proteins following translation

  • These modifications may be vital to the formation of a mature, functional protein (e.g. common in cell signalling components)

Addition of new functional groups

  • Enzymes may modify protein structure via the introduction of a new chemical group to specific amino acids in the molecule
  • This can include phosphorylation, methylation, acetylation, glycosylation, ubiquitination, lipidation, biotination, etc.
  • The addition of chemical groups may alter the properties of the chain and induce conformational changes – affecting activity

Proteolytic cleavage of existing elements

  • Proteins may also be modified via the removal of specific amino acid segments from a propeptide
  • This occurs in zymogens, where the active site is occluded and inactive until proteolytic cleavage occurs
  • The hormone insulin also requires the separation of a middle segment to form two polypeptides chains linked by disulfide bridges
  • Another common form of proteolytic cleavage is the removal of the initiator methionine residue


  • Amino acids can exist as chiral enantiomers ('mirror image' stereoisomers)
  • Racemization involves converting proteins from one enantiomeric arrangement to a different enantiomeric arrangement
  • Different enantiomers may have distinct chemical properties due to the ability to form different intermolecular interactions

Example of Protein Modification – Proteolytic Cleavage