Drugs & Medication

Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis for many proteins.

A protein (also called a polypeptide) is a chain of amino acids. During protein synthesis, 20 different amino acids can be incorporated in proteins. After translation, the posttranslational modification (PTM) of amino acids extends the range of functions of the protein by attaching to it other biochemical functional groups such as acetate, phosphate, various lipids and carbohydrates, by changing the chemical nature of an amino acid (e.g. citrullination) or by making structural changes, like the formation of disulfide bridges.

Also, enzymes may remove amino acids from the amino end of the protein, or cut the peptide chain in the middle. For instance, the peptide hormone insulin is cut twice after disulfide bonds are formed, and a propeptide is removed from the middle of the chain; the resulting protein consists of two polypeptide chains connected by disulfide bonds.

Other modifications, like phosphorylation, are part of common mechanisms for controlling the behavior of a protein, for instance activating or inactivating an enzyme.

Contents 1 PTMs involving addition of functional groups 2 PTMs involving addition of other proteins or peptides 3 PTMs involving changing the chemical nature of amino acids 4 PTMs involving structural changes 5 Case examples 6 References

PTMs involving addition of functional groups

PTMs involving addition include:

• acetylation, the addition of an acetyl group, usually at the N-terminus of the protein
• alkylation, the addition of an alkyl group (e.g. methyl, ethyl)
  • methylation the addition of a methyl group, usually at lysine or arginine residues. (This is a type of alkylation.)
• biotinylation, acylation of conserved lysine residues with a biotin appendage
  glutamylation, covalent linkage of glutamic acid residues to tubulin and some other proteins.
  glycylation, covalent linkage of one to more than 40 glycine residues to the tubulin C-terminal tail
  glycosylation, the addition of a glycosyl group to either asparagine, hydroxylysine, serine, or threonine, resulting in a glycoprotein
  isoprenylation, the addition of an isoprenoid group (e.g. farnesol and geranylgeraniol)
  lipoylation, attachment of a lipoate functionality
  phosphopantetheinylation, the addition of a 4'-phosphopantetheinyl moiety from coenzyme A, as in fatty acid, polyketide, non-ribosomal peptide and leucine biosynthesis
  phosphorylation, the addition of a phosphate group, usually to serine, tyrosine, threonine or histidine
  sulfation, the addition of a sulfate group to a tyrosine.
  Selenation
  C-terminal amidation

PTMs involving addition of other proteins or peptides

• ISGylation, the covalent linkage to the ISG15 protein (Interferon-Stimulated Gene 15) (2)
  SUMOylation, the covalent linkage to the SUMO protein (Small Ubiquitin-related MOdifier) (1)
  ubiquitination, the covalent linkage to the protein ubiquitin.

PTMs involving changing the chemical nature of amino acids

• citrullination, or deimination the conversion of arginine to citrulline
  deamidation, the conversion of glutamine to glutamic acid or asparagine to aspartic acid

PTMs involving structural changes

• disulfide bridges, the covalent linkage of two cysteine amino acids
  proteolytic cleavage, cleavage of a protein at a peptide bond

Case examples

• cleavage and formation of disulfide bridges during the production of insulin
  PTM of histones as regulation of transcription: RNA polymerase control by chromatin structure
  PTM of RNA polymerase II as regulation of transcription: RNA polymerase II

References

1. Van G. Wilson (Ed.) (2004). Sumoylation: Molecular Biology and Biochemistry. Horizon Bioscience. ISBN 0-9545232-8-8.

2. Malakhova, Oxana A.; Yan, Ming; Malakhov, Michael P.; Yuan, Youzhong; Ritchie, Kenneth J.; Kim, Keun Il; Peterson, Luke F.; Shuai, Ke; and Dong-Er Zhang. (2003). Protein ISGylation modulates the JAK-STAT signaling pathway. Genes & Development 17 (4), 455-460.