Editing Cysteine (section)

== Evolutionary role of cysteine ==
Cysteine is considered a "newcomer" amino acid, being the 17th amino acid incorporated into the [[genetic code]].<ref>{{Cite journal |last1=Osawa |first1=S |last2=Jukes |first2=T H |last3=Watanabe |first3=K |last4=Muto |first4=A |date=March 1992 |title=Recent evidence for evolution of the genetic code |journal=Microbiological Reviews |language=en |volume=56 |issue=1 |pages=229–264 |doi=10.1128/mr.56.1.229-264.1992 |issn=0146-0749 |pmc=372862 |pmid=1579111}}</ref><ref>{{Cite journal |last=Trifonov |first=Edward N. |date=September 2009 |title=The origin of the genetic code and of the earliest oligopeptides |url=https://linkinghub.elsevier.com/retrieve/pii/S0923250809000576 |journal=Research in Microbiology |language=en |volume=160 |issue=7 |pages=481–486 |doi=10.1016/j.resmic.2009.05.004|pmid=19524038 }}</ref> Similar to other later-added amino acids such as [[methionine]], [[tyrosine]], and [[tryptophan]], cysteine exhibits strong nucleophilic and redox-active properties.<ref>{{Cite journal |last1=Paulsen |first1=Candice E. |last2=Carroll |first2=Kate S. |date=2013-07-10 |title=Cysteine-Mediated Redox Signaling: Chemistry, Biology, and Tools for Discovery |journal=Chemical Reviews |language=en |volume=113 |issue=7 |pages=4633–4679 |doi=10.1021/cr300163e |issn=0009-2665 |pmc=4303468 |pmid=23514336}}</ref><ref>{{Cite journal |last1=Giles |first1=Niroshini M |last2=Watts |first2=Aaron B |last3=Giles |first3=Gregory I |last4=Fry |first4=Fiona H |last5=Littlechild |first5=Jennifer A |last6=Jacob |first6=Claus |date=August 2003 |title=Metal and Redox Modulation of Cysteine Protein Function |url=https://doi.org/10.1016/S1074-5521(03)00174-1 |journal=Chemistry & Biology |volume=10 |issue=8 |pages=677–693 |doi=10.1016/s1074-5521(03)00174-1 |pmid=12954327 |issn=1074-5521}}</ref> These properties contribute to the depletion of cysteine from [[Electron transport chain|respiratory chain]] complexes, such as [[Respiratory complex I|Complexes I]] and [[Complex IV|IV]],<ref>{{Cite journal |last1=Moosmann |first1=Bernd |last2=Behl |first2=Christian |date=February 2008 |title=Mitochondrially encoded cysteine predicts animal lifespan |journal=Aging Cell |language=en |volume=7 |issue=1 |pages=32–46 |doi=10.1111/j.1474-9726.2007.00349.x |pmid=18028257 |issn=1474-9718|doi-access=free }}</ref> since reactive oxygen species ([[Reactive oxygen species|ROS]]) produced by the respiratory chain can react with the cysteine residues in these complexes, leading to dysfunctional proteins and potentially contributing to aging. The primary response of a protein to ROS is the oxidation of cysteine and the loss of free thiol groups,<ref>{{Cite journal |last=Sohal |first=Rajindar S |date=2002-07-01 |title=Role of oxidative stress and protein oxidation in the aging process1, 2 |url=https://www.sciencedirect.com/science/article/pii/S0891584902008560 |journal=Free Radical Biology and Medicine |volume=33 |issue=1 |pages=37–44 |doi=10.1016/S0891-5849(02)00856-0 |pmid=12086680 |issn=0891-5849}}</ref> resulting in increased [[Thiyl radical|thiyl radicals]] and associated protein cross-linking.<ref>{{Cite journal |last1=Jacob |first1=Claus |last2=Giles |first2=Gregory I. |last3=Giles |first3=Niroshini M. |last4=Sies |first4=Helmut |date=2003-10-13 |title=Sulfur and Selenium: The Role of Oxidation State in Protein Structure and Function |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200300573 |journal=Angewandte Chemie International Edition |language=en |volume=42 |issue=39 |pages=4742–4758 |doi=10.1002/anie.200300573 |pmid=14562341 |issn=1433-7851}}</ref><ref>{{Cite journal |last1=Nauser |first1=Thomas |last2=Pelling |first2=Jill |last3=Schöneich |first3=Christian |date=2004-10-01 |title=Thiyl Radical Reaction with Amino Acid Side Chains: Rate Constants for Hydrogen Transfer and Relevance for Posttranslational Protein Modification |url=https://pubs.acs.org/doi/10.1021/tx049856y |journal=Chemical Research in Toxicology |language=en |volume=17 |issue=10 |pages=1323–1328 |doi=10.1021/tx049856y |pmid=15487892 |issn=0893-228X}}</ref> In contrast, another sulfur-containing, redox-active amino acid, methionine, does not exhibit these biochemical properties and its content is relatively upregulated in [[Mitochondrion|mitochondrially]] encoded proteins.<ref>{{citation |last1=Moosmann |first1=Bernd |first2=Parvana |last2=Hajieva |first3=Christian |last3=Behl |title=The antioxidant function of protein methionine explains about the evolution of a non-standard genetic code in mitochondria |journal=Free Radical Biology and Medicine |volume=41 |article-number=402 |doi=10.1016/j.freeradbiomed.2006.10.015 |page=S149–S150 |date=2006}}</ref>