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
Racemization
- 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
