Post-translational Modification

Post-translational modification is the chemical modification of a protein following its translation. It might be considered one of the last stages of gene expression. Various modifications undergone by the protein may completely change its function. For instance, the addition of lipid or carbohydrate side groups to certain amino acids might enable the protein's integration into the plasma membrane as a cellular recognition molecule.

Other modifications include:

  • Acetylation or deacetylation, the addition/removal of an acetyl group. See also: histone acetylation and deacetylation
  • Alkylation, the addition of an alkyl group; for instance methylation is the addition of a methyl group
  • Cleavage, the removal of certain amino acid residues in order to activate a protein (for instance converting a prohormone/proenzyme to its active form), or perhaps the removal of a signal peptide once a protein has reached its destination in the cell
  • Glycosylation, the addition of a glycosyl (sugar) group, usually to a serine or threonine residue
  • Hydroxylation, the addition of hydroxyl (OH) groups
  • Phosphorylation (reverse= dephosphorylation) to activate or deactivate a protein, especially an enzyme
  • The formation of disulphide bridges between two cysteine residues to make structural changes to a protein (again, often in the case of enzymes)

Modifications are usually (though not always) enzyme-directed and are often localized to a particular organelle within the cell: for instance, glycosylation is catalysed by an enzyme complex found in the Golgi apparatus.