Post-translational modification

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Post-translational protein modifications ( PTM ) are changes in proteins that occur after translation . Most are triggered by the organism or by the cells themselves.

Proteins are often involved in these processes, by modifying genes ( modifier genes ) are encoded. The gene products of such modification genes can be formed or functionalized depending on environmental factors and influence proteins accordingly.

While some of the processes take place directly at the point of origin, others take place in certain cell organelles, while others take place outside the producing cell.

In addition to intended protein changes, however, unwanted protein modifications also occur. Assuming that the transcription and translation machinery works when transcribing the genes via the mRNA to the proteins with error rates of 1/1000 nucleotides or 1 / 10,000 amino acids, the incorporation of incorrect amino acids will produce significant amounts of mistranslated polypeptide chains. The proportion of mistranslated proteins that are actually not changed post-translationally, but cotranslationally, can be increased by the presence of streptomycin (disruption of the ribosome ) or by a lack of individual amino acids.

In addition, protein chains can be damaged, changed or denatured by radicals , high-energy radiation or other proteins (see prions ) and form folding isoforms that no longer correspond to the original conformation and cannot fulfill the intended function.

Categories of post-translational modification

Cells have a multitude of possibilities to process and change their proteins. To do this, they have a large number of enzymes that are specially formed by the cell for protein modification. Protein modification processes can take place constitutively or they can be influenced by environmental influences or other parameters. The modification can be the N - or C -terminus or a side chain modification done. About 300 different post-translational modifications have been described. The following processes that lead to new protein species were analyzed:

Spin-offs

  • Cleavage of the N -terminal formyl residue by deformylase . Every newly synthesized protein (in prokaryotes ) initially contains an N -terminal formylmethionine (methionine in eukaryotes ), which is always incorporated first during translation and whose formyl residue is subsequently split off by the deformylase. Any formyl residue that is still present indicates that the synthesis of the protein molecule has just ended.
  • the cleavage of the methionyl residue at the N terminus of newly synthesized proteins by methionylaminopeptidase . In bacteria it was observed that the size of the following amino acid influences the cleavage behavior of the N -terminal methionine. The larger the second amino acid, the less likely it is that the starting methionine will be split off.
  • the targeted splitting off of signal sequences (such as protocolagen to collagen)
  • the selective cutting out of partial sequences ( e.g. proinsulin to insulin, generally precursor proteins )
  • Protein inactivation and fragmentation by proteolysis involving proteases

Inorganic Groups

Organic groups

Organic lipid groups

These lipid anchor modifications cause adsorption to the cell membrane .

Adding bindings

Binding to larger molecules

Change of individual amino acids

Miscellaneous

  • Formation of a stable radical (bacteria)
  • the binding ( complexation ) of ions and low molecular weight substances

literature

Web links

Individual evidence

  1. ^ S. Lee: Post-translational modification of proteins in toxicological research: focus on lysine acylation. In: Toxicological research. Volume 29, number 2, June 2013, pp. 81–86, doi : 10.5487 / TR.2013.29.2.081 , PMID 24278632 , PMC 3834447 (free full text).