Editing Protein biosynthesis (section)

===Addition of chemical groups===
[[File:Post-translational modification through the addition of small chemical groups.png|400px|thumb|alt= Three polypeptide chains with one amino acid side chain showing, two have a lysine and one has a serine. Three arrows indicating different post-translational modifications with the new chemical group added to each side chain. The first is methylation then acetylation followed by phosphorylation.| Shows the post-translational modification of protein by methylation, acetylation and phosphorylation]]
Following translation, small chemical groups can be added onto amino acids within the mature protein structure.<ref name="Brenner 2001">{{Cite book |title=Encyclopedia of genetics |vauthors=Brenner S, Miller JH |date=2001 |publisher=Elsevier Science Inc |isbn=978-0-12-227080-2 |pages=2800}}</ref> Examples of processes which add chemical groups to the target protein include methylation, [[Protein acetylation|acetylation]] and [[Protein phosphorylation|phosphorylation]].<ref>{{Cite journal |last=Zhong |first=Qian |last2=Xiao |first2=Xina |last3=Qiu |first3=Yijie |last4=Xu |first4=Zhiqiang |last5=Chen |first5=Chunyu |last6=Chong |first6=Baochen |last7=Zhao |first7=Xinjun |last8=Hai |first8=Shan |last9=Li |first9=Shuangqing |last10=An |first10=Zhenmei |last11=Dai |first11=Lunzhi |date=2023-05-02 |title=Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications |url=https://pmc.ncbi.nlm.nih.gov/articles/PMC10152985/ |journal=MedComm |language=en |volume=4 |issue=3 |doi=10.1002/mco2.261 |issn=2688-2663 |archive-url=http://web.archive.org/web/20250211232904/https://pmc.ncbi.nlm.nih.gov/articles/PMC10152985/ |archive-date=2025-02-11|pmc=10152985 }}</ref>

Methylation is the reversible addition of a [[methyl group]] onto an amino acid catalyzed by [[methyltransferase]] enzymes. Methylation occurs on at least 9 of the 20 common amino acids, however, it mainly occurs on the amino acids [[lysine]] and [[arginine]]. One example of a protein which is commonly methylated is a [[histone]]. Histones are proteins found in the nucleus of the cell. DNA is tightly wrapped round histones and held in place by other proteins and interactions between negative charges in the DNA and positive charges on the histone. A highly specific pattern of [[histone methylation|amino acid methylation]] on the histone proteins is used to determine which regions of DNA are tightly wound and unable to be transcribed and which regions are loosely wound and able to be transcribed.<ref name="Murn 2017">{{Cite journal |vauthors=Murn J, Shi Y |date=August 2017 |title=The winding path of protein methylation research: milestones and new frontiers |journal=Nature Reviews. Molecular Cell Biology |volume=18 |issue=8 |pages=517–527 |doi=10.1038/nrm.2017.35 |pmid=28512349 |s2cid=3917753}}</ref>

Histone-based regulation of DNA transcription is also modified by acetylation. Acetylation is the reversible covalent addition of an [[acetyl group]] onto a lysine amino acid by the enzyme [[acetyltransferase]]. The acetyl group is removed from a donor molecule known as [[Acetyl-CoA|acetyl coenzyme A]] and transferred onto the target protein.<ref name="Drazic 2016">{{Cite journal |vauthors=Drazic A, Myklebust LM, Ree R, Arnesen T |date=October 2016 |title=The world of protein acetylation |journal=Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics |volume=1864 |issue=10 |pages=1372–1401 |doi=10.1016/j.bbapap.2016.06.007 |pmid=27296530 |doi-access=free}}</ref> [[Histone acetylation and deacetylation|Histones undergo acetylation]] on their lysine residues by enzymes known as [[histone acetyltransferase]]. The effect of acetylation is to weaken the charge interactions between the histone and DNA, thereby making more genes in the DNA accessible for transcription.<ref name="Bannister 2011">{{Cite journal |vauthors=Bannister AJ, Kouzarides T |date=March 2011 |title=Regulation of chromatin by histone modifications |journal=Cell Research |volume=21 |issue=3 |pages=381–395 |doi=10.1038/cr.2011.22 |pmc=3193420 |pmid=21321607 |doi-access=free}}</ref>

The final, prevalent post-translational chemical group modification is phosphorylation. Phosphorylation is the reversible, covalent addition of a [[phosphate]] group to specific amino acids ([[serine]], [[threonine]] and [[tyrosine]]) within the protein. The phosphate group is removed from the donor molecule [[Adenosine triphosphate|ATP]] by a protein [[kinase]] and transferred onto the [[hydroxyl]] group of the target amino acid, this produces [[adenosine diphosphate]] as a byproduct. This process can be reversed and the phosphate group removed by the enzyme protein [[phosphatase]]. Phosphorylation can create a binding site on the phosphorylated protein which enables it to interact with other proteins and generate large, multi-protein complexes. Alternatively, phosphorylation can change the level of protein activity by altering the ability of the protein to bind its substrate.<ref name="Alberts 2015" />