Editing Tyrosine (section)

==Functions==
Aside from being a [[proteinogenic amino acid]], tyrosine has a special role by virtue of the [[phenol]] functionality. Its hydroxy group is able to form the [[ester linkage]], with phosphate in particular. Phosphate groups are transferred to tyrosine residues by way of [[protein kinase]]s. This is one of the [[post-translational modification]]s. Phosphorylated tyrosine occurs in proteins that are part of [[signal transduction]] processes.

Similar functionality is also presented in [[serine]] and [[threonine]], whose side chains have a hydroxy group, but are [[Alcohol (chemistry)|alcohols]]. Phosphorylation of these three amino acids' moieties (including tyrosine) creates a negative charge on their ends, that is greater than the negative charge of the only negatively charged [[Aspartic acid|aspartic]] and [[Glutamic acid|glutamic]] acids. Phosphorylated proteins keep these same properties—which are useful for more reliable protein-protein interactions—by means of phosphotyrosine, phosphoserine and phosphothreonine.<ref>{{Cite journal |last=Hunter |first=Tony |date=2012-09-19 |title=Why nature chose phosphate to modify proteins |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=367 |issue=1602 |pages=2513–2516 |doi=10.1098/rstb.2012.0013 |issn=0962-8436 |pmc=3415839 |pmid=22889903}}</ref>

Binding sites for a signalling phosphoprotein may be diverse in their chemical structure.<ref>{{Cite journal |last1=Lu |first1=Zheng-Chang |last2=Jiang |first2=Fan |last3=Wu |first3=Yun-Dong |date=2021-12-11 |title=Phosphate binding sites prediction in phosphorylation-dependent protein-protein interactions |url=https://pubmed.ncbi.nlm.nih.gov/34270697/ |journal=Bioinformatics |volume=37 |issue=24 |pages=4712–4718 |doi=10.1093/bioinformatics/btab525 |issn=1367-4811 |pmid=34270697}}</ref>

Phosphorylation of the hydroxyl group can change the activity of the target protein, or may form part of a signaling cascade via [[SH2 domain]] binding.<ref>{{Cite journal |last1=Liu |first1=Bernard A. |last2=Nash |first2=Piers D. |date=2012-09-19 |title=Evolution of SH2 domains and phosphotyrosine signalling networks |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=367 |issue=1602 |pages=2556–2573 |doi=10.1098/rstb.2012.0107 |issn=0962-8436 |pmc=3415846 |pmid=22889907}}</ref>

A tyrosine residue also plays an important role in [[photosynthesis]]. In [[chloroplast]]s ([[photosystem II]]), it acts as an electron donor in the [[Redox|reduction]] of oxidized [[chlorophyll]].  In this process, it loses the hydrogen atom of its phenolic OH-group.  This radical is subsequently reduced in the photosystem II by the four core [[Oxygen-evolving complex|manganese clusters]].<ref>{{Cite journal |last=Barry |first=Bridgette A. |date=January 2015 |title=Reaction dynamics and proton coupled electron transfer: studies of tyrosine-based charge transfer in natural and biomimetic systems |url=https://pubmed.ncbi.nlm.nih.gov/25260243/ |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |volume=1847 |issue=1 |pages=46–54 |doi=10.1016/j.bbabio.2014.09.003 |issn=0006-3002 |pmid=25260243}}</ref>