Post-translational modification

After a protein is synthesized and released from a ribosome, it can undergo covalent changes known as post-translational modifications (PTMs). These modifications are often reversible and are part of post-translational regulation, which controls the levels of active protein. An irreversible example is proteolysis, or protein degradation. PTMs allow a protein’s function to become more varied and extended than what is directly specified by transcription. More than 650 types of PTM have been identified as of 2023, and they occur in both eukaryotes and prokaryotes.

PTMs can be carried out by enzymes or happen spontaneously. They are important in cell signaling—for instance, when prohormones are converted into hormones. Modifications can take place on amino acid side chains or at the protein’s C- or N-termini. They expand the chemical repertoire of the 22 standard amino acids by altering an existing functional group or adding a new one, such as a phosphate. Phosphorylation, the most common PTM, is highly effective at controlling enzyme activity. Many proteins in eukaryotes and prokaryotes also receive carbohydrate attachments through glycosylation, which can promote folding, improve stability, and serve regulatory roles. Lipidation, the attachment of lipid molecules, often targets a protein or part of it to the cell membrane.

Other PTMs involve cleaving peptide bonds, such as processing a propeptide into its mature form or removing the initiator methionine. The formation of disulfide bonds from cysteine residues is also considered a PTM. For example, the hormone insulin is cut twice after disulfide bonds form, and a propeptide is removed from the middle of the chain, leaving two polypeptide chains connected by disulfide bonds.

Some PTMs result from oxidative stress. Carbonylation, for instance, targets the modified protein for degradation and can lead to protein aggregates. Specific amino acid modifications can serve as biomarkers for oxidative damage. PTMs and metal ions have a crucial, reciprocal role in regulating protein function, influencing processes like signal transduction and gene expression. Dysregulated interactions between them are linked to diseases such as cancer and neurodegenerative disorders.

Sites that frequently undergo PTM are those with functional groups that can act as nucleophiles: 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. Although the amide of asparagine is a weak nucleophile, it can serve as an attachment point for glycans. Rarer modifications occur at oxidized methionines and some methylene groups in side chains. PTMs can be detected experimentally using techniques such as mass spectrometry, Eastern blotting, and Western blotting.

PTMs that add functional groups include the attachment of hydrophobic groups for membrane localization, such as myristoylation (addition of myristate, a C14 saturated acid), palmitoylation (addition of palmitate, a C16 saturated acid), isoprenylation or prenylation (addition of isoprenoid groups like farnesol and geranylgeraniol), farnesylation, geranylgeranylation, and glypiation (formation of a glycosylphosphatidylinositol anchor via an amide bond to the C-terminal tail). Cofactors for enhanced enzymatic activity can also be added, including lipoylation (attachment of a lipoate group), covalent attachment of flavin mononucleotide or flavin adenine dinucleotide, heme C attachment via thioether bonds with cysteines, phosphopantetheinylation (addition of a 4'-phosphopantetheinyl moiety from coenzyme A), and retinylidene Schiff base formation.

Modifications of translation factors include diphthamide formation (on a histidine in eEF2), ethanolamine phosphoglycerol attachment (on glutamate in eEF1α), hypusine formation (on a conserved lysine of eIF5A in eukaryotes and aIF5A in archaea), and beta-lysine addition on a conserved lysine of elongation factor P in most bacteria. Smaller chemical groups can also be added, such as acylation (including O-acylation, N-acylation, and S-acylation), acetylation (addition of an acetyl group at the N-terminus or lysine residues, reversed by deacetylation), formylation, alkylation (e.g., methyl or ethyl groups), methylation (usually at lysine or arginine, reversed by demethylation), amidation at the C-terminus (formed by oxidative dissociation of a C-terminal glycine), monoaminylation (addition of monoamines like dopamine, histamine, or serotonin to glutamine residues via transamidation), amide bond formation, and amino acid addition (including arginylation via tRNA, polyglutamylation of tubulin and other proteins, and polyglycylation).