Posttranslational modification (PTM) is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, 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 to become a protein. After translation, the posttranslational modification of amino acids extends the range of functions of the protein by attaching it to other biochemical functional groups (such as acetate, phosphate, various lipids and carbohydrates), changing the chemical nature of an amino acid (e.g. citrullination), or making structural changes (e.g. formation of disulfide bridges).
The bottom of this diagram shows the modification of primary structure
of insulin, as described.
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. Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid. This amino acid is usually taken off during post-translational modification.
Other modifications, like phosphorylation, are part of common mechanisms for controlling the behavior of a protein, for instance activating or inactivating an enzyme.
Post-translational modification of proteins is detected by mass spectrometry or Eastern blotting.
PTMs involving addition of functional groups
The genetic code diagram
showing the amino acid residues as target of modification.
PTMs involving addition by an enzyme in vivo
PTMs involving addition of hydrophobic groups for membrane localization
PTMs involving addition of cofactors for enhanced enzymatic activity
PTMs involving unique modifications of translation factors
PTMs involving addition of smaller chemical groups
acylation, e.g. O-acylation (esters), N-acylation (amides), S-acylation (thioesters)
alkylation, the addition of an alkyl group, e.g. methyl, ethyl
amide bond formation
gamma-carboxylation dependent on Vitamin K
glycosylation, the addition of a glycosyl group to either arginine, asparagine, cysteine, hydroxylysine, serine, threonine, tyrosine, or tryptophan resulting in a glycoprotein. Distinct from glycation, which is regarded as a nonenzymatic attachment of sugars.
iodination (e.g. of thyroglobulin)
nucleotide addition such as ADP-ribosylation
phosphate ester (O-linked) or phosphoramidate (N-linked) formation
succinylation addition of a succinyl group to lysine
sulfation, the addition of a sulfate group to a tyrosine.
selenoylation (co-translational incorporation of selenium in selenoproteins)
PTMs involving non-enzymatic additions in vivo
glycation, the addition of a sugar molecule to a protein without the controlling action of an enzyme.
PTMs involving non-enzymatic additions in vitro
PTMs involving addition of other proteins or peptides
PTMs involving changing the chemical nature of amino acids
PTMs involving structural changes
Post-translational modification statistics
Recently statistics of each post-translational modification experimentally and putatively detected have been compiled using proteome-wide information from the Swiss-Prot database. These statistics can be found at http://selene.princeton.edu/PTMCuration/.
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