Post-translational modification

Post-translational modification of insulin. At the top, the ribosome translates a mRNA sequence into a protein, insulin, and passes the protein through the endoplasmic reticulum, where it is cut, folded and held in shape by disulfide (-S-S-) bonds. Then the protein passes through the golgi apparatus, where it is packaged into a vesicle. In the vesicle, more parts are cut off, and it turns into mature insulin.

Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins during or after protein biosynthesis. Proteins are synthesized by ribosomes translating mRNA into polypeptide chains, which may then undergo PTM to form the mature protein product. PTMs are important components in cell signaling.

Post-translational modifications can occur on the amino acid side chains or at the protein's C- or N- termini.^[1] They can extend the chemical repertoire of the 20 standard amino acids by introducing new functional groups such as phosphate, acetate, amide groups, or methyl groups. Phosphorylation is a very common mechanism for regulating the activity of enzymes and is the most common post-translational modification.^[2] Many eukaryotic proteins also have carbohydrate molecules attached to them in a process called glycosylation, which can promote protein folding and improve stability as well as serving regulatory functions. Attachment of lipid molecules, known as lipidation, often targets a protein or part of a protein to the cell membrane.

Other forms of post-translational modification consist of cleaving peptide bonds, as in processing a propeptide to a mature form or removing the initiator methionine residue. The formation of disulfide bonds from cysteine residues may also be referred to as a post-translational modification.^[3]^:17.6 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.

Some types of post-translational modification are consequences of oxidative stress. Carbonylation is one example that targets the modified protein for degradation and can result in the formation of protein aggregates.^[4]^[5] Specific amino acid modifications can be used as biomarkers indicating oxidative damage.^[6]

Sites that often undergo post-translational modification are those that have a functional group that can serve as a nucleophile in the reaction: the hydroxyl groups of serine, threonine, and tyrosine; the amine forms of lysine, arginine, and histidine; the thiolate anion of cysteine; the carboxylates of aspartate and glutamate; and the N- and C-termini. In addition, although the amides of asparagine and glutamine are weak nucleophiles, both can serve as attachment points for glycans. Rarer modifications can occur at oxidized methionines and at some methylenes in side chains.^[7]^:12–14

Post-translational modification of proteins can be experimentally detected by a variety of techniques, including mass spectrometry, Eastern blotting, and Western blotting.

PTMs involving addition of functional groups

The genetic code diagram^[8] showing the amino acid residues as target of modification.

Addition by an enzyme in vivo

Hydrophobic groups for membrane localization

myristoylation, attachment of myristate, a C₁₄ saturated acid
palmitoylation, attachment of palmitate, a C₁₆ saturated acid
isoprenylation or prenylation, the addition of an isoprenoid group (e.g. farnesol and geranylgeraniol)
- farnesylation
- geranylgeranylation
glypiation, glycosylphosphatidylinositol (GPI) anchor formation via an amide bond to C-terminal tail

Cofactors for enhanced enzymatic activity

lipoylation, attachment of a lipoate (C₈) functional group
flavin moiety (FMN or FAD) may be covalently attached
heme C attachment via thioether bonds with cysteins
phosphopantetheinylation, the addition of a 4'-phosphopantetheinyl moiety from coenzyme A, as in fatty acid, polyketide, non-ribosomal peptide and leucine biosynthesis
retinylidene Schiff base formation

Modifications of translation factors

diphthamide formation (on a histidine found in eEF2)
ethanolamine phosphoglycerol attachment (on glutamate found in eEF1α)^[9]
hypusine formation (on conserved lysine of eIF5A (eukaryotic) and aIF5A (archaeal))

Smaller chemical groups

acylation, e.g. O-acylation (esters), N-acylation (amides), S-acylation (thioesters)
- acetylation, the addition of an acetyl group, either at the N-terminus ^[10] of the protein or at lysine residues.^[11] See also histone acetylation.^[12]^[13] The reverse is called deacetylation.
- formylation
alkylation, the addition of an alkyl group, e.g. methyl, ethyl
- methylation the addition of a methyl group, usually at lysine or arginine residues. The reverse is called demethylation.
amide bond formation
- amidation at C-terminus
- amino acid addition
  - arginylation, a tRNA-mediation addition
  - polyglutamylation, covalent linkage of glutamic acid residues to the N-terminus of tubulin and some other proteins.^[14] (See tubulin polyglutamylase)
  - polyglycylation, covalent linkage of one to more than 40 glycine residues to the tubulin C-terminal tail
butyrylation
gamma-carboxylation dependent on Vitamin K^[15]
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.
- polysialylation, addition of polysialic acid, PSA, to NCAM
malonylation
hydroxylation
iodination (e.g. of thyroglobulin)
nucleotide addition such as ADP-ribosylation
oxidation
phosphate ester (O-linked) or phosphoramidate (N-linked) formation
- phosphorylation, the addition of a phosphate group, usually to serine, threonine, and tyrosine (O-linked), or histidine (N-linked)
- adenylylation, the addition of an adenylyl moiety, usually to tyrosine (O-linked), or histidine and lysine (N-linked)
propionylation
pyroglutamate formation
S-glutathionylation
S-nitrosylation
S-sulfenylation (aka S-sulphenylation), reversible covalent attachment of hydroxide to the thiol group of cysteine residues^[16]
succinylation addition of a succinyl group to lysine
sulfation, the addition of a sulfate group to a tyrosine.

Non-enzymatic additions in vivo

glycation, the addition of a sugar molecule to a protein without the controlling action of an enzyme.

Non-enzymatic additions in vitro

biotinylation, acylation of conserved lysine residues with a biotin appendage
pegylation

Other proteins or peptides

ISGylation, the covalent linkage to the ISG15 protein (Interferon-Stimulated Gene 15)^[17]
SUMOylation, the covalent linkage to the SUMO protein (Small Ubiquitin-related MOdifier)^[18]
ubiquitination, the covalent linkage to the protein ubiquitin.
Neddylation, the covalent linkage to Nedd
Pupylation, the covalent linkage to the Prokaryotic ubiquitin-like protein

Chemical modification 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
eliminylation, the conversion to an alkene by beta-elimination of phosphothreonine and phosphoserine, or dehydration of threonine and serine, as well as by decarboxylation of cysteine ^[19]
carbamylation, the conversion of lysine to homocitrulline ^[20]

Structural changes

disulfide bridges, the covalent linkage of two cysteine amino acids
proteolytic cleavage, cleavage of a protein at a peptide bond
racemization
- of proline by prolyl isomerase
- of serine by protein-serine epimerase
- of alanine in dermorphin, a frog opioid peptide
- of methionine in deltorphin, also a frog opioid peptide
protein splicing, self-catalytic removal of inteins analogous to mRNA processing